{"id":824,"date":"2018-05-03T18:32:06","date_gmt":"2018-05-03T18:32:06","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-osbiology2e\/chapter\/chromosomal-theory-and-genetic-linkage\/"},"modified":"2018-06-12T17:15:10","modified_gmt":"2018-06-12T17:15:10","slug":"chromosomal-theory-and-genetic-linkage","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/chapter\/chromosomal-theory-and-genetic-linkage\/","title":{"raw":"Chromosomal Theory and Genetic Linkage","rendered":"Chromosomal Theory and Genetic Linkage"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to do the following:\r\n<ul>\r\n \t<li>Discuss Sutton\u2019s Chromosomal Theory of Inheritance<\/li>\r\n \t<li>Describe genetic linkage<\/li>\r\n \t<li>Explain the process of homologous recombination, or crossing over<\/li>\r\n \t<li>Describe chromosome creation<\/li>\r\n \t<li>Calculate the distances between three genes on a chromosome using a three-point test cross<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-id2021443\">Long before scientists visualized chromosomes under a microscope, the father of modern genetics, Gregor Mendel, began studying heredity in 1843. With improved microscopic techniques during the late 1800s, cell biologists could stain and visualize subcellular structures with dyes and observe their actions during cell division and meiosis. With each mitotic division, chromosomes replicated, condensed from an amorphous (no constant shape) nuclear mass into distinct X-shaped bodies (pairs of identical sister chromatids), and migrated to separate cellular poles.<\/p>\r\n\r\n<div id=\"fs-id2904526\" class=\"bc-section section\">\r\n<h3>Chromosomal Theory of Inheritance<\/h3>\r\n<p id=\"fs-id1406190\">The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel\u2019s publications and reevaluate his model in terms of chromosome behavior during mitosis and meiosis. In 1902, Theodor Boveri observed that proper sea urchin embryonic development does not occur unless chromosomes are present. That same year, Walter Sutton observed chromosome separation into daughter cells during meiosis (<a class=\"autogenerated-content\" href=\"#fig-ch13_01_01\">(Figure)<\/a>). Together, these observations led to the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.<\/p>\r\n\r\n<div id=\"fig-ch13_01_01\" class=\"wp-caption aligncenter\">\r\n<div class=\"wp-caption-text\">(a) Walter Sutton and (b) Theodor Boveri developed the Chromosomal Theory of Inheritance, which states that chromosomes carry the unit of heredity (genes).<\/div>\r\n<span id=\"fs-id2011864\">\r\n<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03183157\/Figure_13_01_01.jpg\" alt=\"Part a is a photo of Walter Sutton. Part b is a photo of Theodor Boveri.\" width=\"275\" \/><\/span>\r\n\r\n<\/div>\r\n<p id=\"fs-id1236139\">The Chromosomal Theory of Inheritance was consistent with Mendel\u2019s laws, which the following observations supported:<\/p>\r\n\r\n<ul id=\"fs-id1770608\">\r\n \t<li>During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.<\/li>\r\n \t<li>Chromosome sorting from each homologous pair into pre-gametes appears to be random.<\/li>\r\n \t<li>Each parent synthesizes gametes that contain only half their chromosomal complement.<\/li>\r\n \t<li>Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.<\/li>\r\n \t<li>The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.<\/li>\r\n<\/ul>\r\n<p id=\"fs-id2335848\">Despite compelling correlations between chromosome behavior during meiosis and Mendel\u2019s abstract laws, scientists proposed the Chromosomal Theory of Inheritance long before there was any direct evidence that chromosomes carried traits. Critics pointed out that individuals had far more independently segregating traits than they had chromosomes. It was only after several years of carrying out crosses with the fruit fly, <em>Drosophila melanogaster<\/em>, that Thomas Hunt Morgan provided experimental evidence to support the Chromosomal Theory of Inheritance.<\/p>\r\n\r\n<\/div>\r\n<div id=\"fs-id1396027\" class=\"bc-section section\">\r\n<h3>Genetic Linkage and Distances<\/h3>\r\n<p id=\"fs-id1780491\">Mendel\u2019s work suggested that traits are inherited independently of each other. Morgan identified a 1:1 correspondence between a segregating trait and the X chromosome, suggesting that random chromosome segregation was the physical basis of Mendel\u2019s model. This also demonstrated that linked genes disrupt Mendel\u2019s predicted outcomes. That each chromosome can carry many linked genes explains how individuals can have many more traits than they have chromosomes. However, researchers in Morgan\u2019s laboratory suggested that alleles positioned on the same chromosome were not always inherited together. During meiosis, linked genes somehow became unlinked.<\/p>\r\n\r\n<div id=\"fs-id2571351\" class=\"bc-section section\">\r\n<h4>Homologous Recombination<\/h4>\r\n<p id=\"fs-id2047023\">In 1909, Frans Janssen observed chiasmata\u2014the point at which chromatids are in contact with each other and may exchange segments\u2014prior to the first meiosis division. He suggested that alleles become unlinked and chromosomes physically exchange segments. As chromosomes condensed and paired with their homologs, they appeared to interact at distinct points. Janssen suggested that these points corresponded to regions in which chromosome segments exchanged. We now know that the pairing and interaction between homologous chromosomes, or synapsis, does more than simply organize the homologs for migration to separate daughter cells. When synapsed, homologous chromosomes undergo reciprocal physical exchanges at their arms in homologous recombination, or more simply, \u201ccrossing over.\u201d<\/p>\r\n<p id=\"fs-id1468864\">To better understand the type of experimental results that researchers were obtaining at this time, consider a heterozygous individual that inherited dominant maternal alleles for two genes on the same chromosome (such as<em> AB<\/em>) and two recessive paternal alleles for those same genes (such as <em>ab<\/em>). If the genes are linked, one would expect this individual to produce gametes that are either <em>AB<\/em> or <em>ab<\/em> with a 1:1 ratio. If the genes are unlinked, the individual should produce <em>AB<\/em>, <em>Ab<\/em>, <em>aB<\/em>, and <em>ab<\/em> gametes with equal frequencies, according to the Mendelian concept of independent assortment. Because they correspond to new allele combinations, the genotypes Ab and aB are nonparental types that result from homologous recombination during meiosis. Parental types are progeny that exhibit the same allelic combination as their parents. Morgan and his colleagues, however, found that when they test crossed such heterozygous individuals to a homozygous recessive parent (<em>AaBb<\/em> \u00d7 <em>aabb<\/em>), both parental and nonparental cases occurred. For example, 950 offspring might be recovered that were either <em>AaBb<\/em> or <em>aabb<\/em>, but 50 offspring would also result that were either <em>Aabb<\/em> or <em>aaBb<\/em>. These results suggested that linkage occurred most often, but a significant minority of offspring were the products of recombination.<\/p>\r\n\r\n<div id=\"fs-id1595360\" class=\"art-connection textbox examples\">\r\n<h3>Art Connection<\/h3>\r\n<div id=\"fig-ch13_01_02\">\r\n<div class=\"wp-caption-text\">This figure shows unlinked and linked gene inheritance patterns. In (a), two genes are located on different chromosomes so independent assortment occurs during meiosis. The offspring have an equal chance of being the parental type (inheriting the same combination of traits as the parents) or a nonparental type (inheriting a different combination of traits than the parents). In (b), two genes are very close together on the same chromosome so that no crossing over occurs between them. Therefore, the genes are always inherited together and all the offspring are the parental type. In (c), two genes are far apart on the chromosome such that crossing over occurs during every meiotic event. The recombination frequency will be the same as if the genes were on separate chromosomes. (d) The actual recombination frequency of fruit fly wing length and body color that Thomas Morgan observed in 1912 was 17 percent. A crossover frequency between 0 percent and 50 percent indicates that the genes are on the same chromosome and crossover sometimes occurs.<\/div>\r\n<span id=\"fs-id1262550\">\r\n<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03183201\/Figure_13_01_02.jpg\" alt=\"The illustration shows the possible inheritance patterns of linked and unlinked genes. The example used includes fruit fly body color and wing length. Fruit flies may have a dominant gray color (G) or a recessive black color (g). They may have dominant long wings (L) or recessive short wings (l). Three hypothetical inheritance patterns for a test cross between a heterozygous and a recessive fruit fly are shown, based on gene placement. The actual experimental results published by Thomas Hunt Morgan in 1912 are also shown. In the first hypothetical inheritance pattern in part a, the genes for the two characteristics are on different chromosomes. Independent assortment occurs so that the ratio of genotypes in the offspring is 1 GgLl:1 ggll:1 Ggll:1 ggLl, and 50% of the offspring are nonparental types. In the second hypothetical inheritance pattern in part b, the genes are close together on the same chromosome so that no crossover occurs between them. The ratio of genotypes is 1 GgLl:1 ggll, and none of the offspring are recombinant. In the third hypothetical inheritance pattern in part c, the genes are far apart on the same chromosome so that crossing over occurs 100% of the time. The ratio of genotypes is the same as for genes on two different chromosomes, and 50% of the offspring are recombinant, nonparental types. Part d shows that the number of offspring that Thomas Hunt Morgan actually observed was 965: 944: 206:185 (GgLl:ggll:Ggll:ggLl). Seventeen percent of the offspring were recombinant, indicating that the genes are on the same chromosome and crossing over occurs between them some of the time.\" width=\"370\" \/><\/span>\r\n\r\n<\/div>\r\n<p id=\"fs-id2573656\">In a test cross for two characteristics such as the one here, can the recombinant offspring's predicted frequency be 60 percent? Why or why not?<\/p>\r\n\r\n\r\n[reveal-answer q=\"711355\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"711355\"]\r\n\r\nNo. The predicted frequency of recombinant offspring ranges from 0% (for linked traits) to 50% (for unlinked traits).\r\n\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id1627133\" class=\"bc-section section\">\r\n<h4>Genetic Maps<\/h4>\r\n<p id=\"fs-id1645084\">Janssen did not have the technology to demonstrate crossing over so it remained an abstract idea that scientists did not widely believe. Scientists thought chiasmata were a variation on synapsis and could not understand how chromosomes could break and rejoin. Yet, the data were clear that linkage did not always occur. Ultimately, it took a young undergraduate student and an \u201call-nighter\u201d to mathematically elucidate the linkage and recombination problem.<\/p>\r\n<p id=\"fs-id1684950\">In 1913, Alfred Sturtevant, a student in Morgan\u2019s laboratory, gathered results from researchers in the laboratory, and took them home one night to mull them over. By the next morning, he had created the first \u201cchromosome map,\u201d a linear representation of gene order and relative distance on a chromosome (<a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a>).<\/p>\r\n\r\n<div id=\"fs-id1941826\" class=\"art-connection textbox examples\">\r\n<h3>Art Connection<\/h3>\r\n<div id=\"fig-ch13_01_03\" class=\"wp-caption aligncenter\">\r\n<div class=\"wp-caption-text\">This genetic map orders <em>Drosophila<\/em> genes on the basis of recombination frequency.<\/div>\r\n<span id=\"fs-id1464025\">\r\n<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03183205\/Figure_13_01_03.png\" alt=\"The illustration shows a Drosophila genetic map. The gene for aristae length occurs at 0 centimorgans, or cM. The gene for body color occurs at 48.5 cM. The gene for red versus cinnabar eye color occurs at 57.5 cM. The gene for wing length occurs at 65.5 cM, and the gene for red versus brown eye color occurs at 104.5 cM. One cM is equivalent to a recombination frequency of 0.01.\" width=\"200\" \/><\/span>\r\n\r\n<\/div>\r\n<p id=\"fs-id1785828\">Which of the following statements is true?<\/p>\r\n\r\n<ol id=\"fs-id1511608\" type=\"a\">\r\n \t<li>Recombination of the body color and red\/cinnabar eye alleles will occur more frequently than recombination of the alleles for wing length and aristae length.<\/li>\r\n \t<li>Recombination of the body color and aristae length alleles will occur more frequently than recombination of red\/brown eye alleles and the aristae length alleles.<\/li>\r\n \t<li>Recombination of the gray\/black body color and long\/short aristae alleles will not occur.<\/li>\r\n \t<li>Recombination of the red\/brown eye and long\/short aristae alleles will occur more frequently than recombination of the alleles for wing length and body color.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"705626\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"705626\"]\r\n\r\nD[\/hidden-answer]\r\n<\/div>\r\n<p id=\"fs-id2137494\">As <a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a> shows, by using recombination frequency to predict genetic distance, we can infer the relative gene order on chromosome 2. The values represent map distances in centimorgans (cM), which correspond to recombination frequencies (in percent). Therefore, the genes for body color and wing size were 65.5 \u2212 48.5 = 17 cM apart, indicating that the maternal and paternal alleles for these genes recombine in 17 percent of offspring, on average.<\/p>\r\n<p id=\"fs-id1951192\">To construct a chromosome map, Sturtevant assumed that genes were ordered serially on threadlike chromosomes. He also assumed that the incidence of recombination between two homologous chromosomes could occur with equal likelihood anywhere along the chromosome's length. Operating under these assumptions, Sturtevant postulated that alleles that were far apart on a chromosome were more likely to dissociate during meiosis simply because there was a larger region over which recombination could occur. Conversely, alleles that were close to each other on the chromosome were likely to be inherited together. The average number of crossovers between two alleles\u2014that is, their recombination frequency\u2014correlated with their genetic distance from each other, relative to the locations of other genes on that chromosome. Considering the example cross between <em>AaBb<\/em> and <em>aabb<\/em> above, we could calculate the recombination's frequency as 50\/1000 = 0.05. That is, the likelihood of a crossover between genes <em>A\/a<\/em> and <em>B\/b<\/em> was 0.05, or 5 percent. Such a result would indicate that the genes were definitively linked, but that they were far enough apart for crossovers to occasionally occur. Sturtevant divided his genetic map into map units, or centimorgans (cM), in which a 0,01 recombination frequency corresponds to 1 cM.<\/p>\r\n<p id=\"fs-id2100648\">By representing alleles in a linear map, Sturtevant suggested that genes can range from linking perfectly (recombination frequency = 0) to unlinking perfectly (recombination frequency = 0.5) when genes are on different chromosomes or genes separate very far apart on the same chromosome. Perfectly unlinked genes correspond to the frequencies Mendel predicted to assort independently in a dihybrid cross. A 0.5 recombination frequency indicates that 50 percent of offspring are recombinants and the other 50 percent are parental types. That is, every type of allele combination is represented with equal frequency. This representation allowed Sturtevant to additively calculate distances between several genes on the same chromosome. However, as the genetic distances approached 0.50, his predictions became less accurate because it was not clear whether the genes were very far apart on the same or on different chromosomes.<\/p>\r\nIn 1931, Barbara McClintock and Harriet Creighton demonstrated the crossover of homologous chromosomes in corn plants. Weeks later, Curt Stern demonstrated microscopically homologous recombination in <em>Drosophila<\/em>. Stern observed several X-linked phenotypes that were associated with a structurally unusual and dissimilar X chromosome pair in which one X was missing a small terminal segment, and the other X was fused to a piece of the Y chromosome. By crossing flies, observing their offspring, and then visualizing the offspring\u2019s chromosomes, Stern demonstrated that every time the offspring allele combination deviated from either of the parental combinations, there was a corresponding exchange of an X chromosome segment. Using mutant flies with structurally distinct X chromosomes was the key to observing the products of recombination because DNA sequencing and other molecular tools were not yet available. We now know that homologous chromosomes regularly exchange segments in meiosis by reciprocally breaking and rejoining their DNA at precise locations.\r\n<div id=\"fs-id2060998\" class=\"interactive textbox tryit\">\r\n<h3>Link to Learning<\/h3>\r\n<p id=\"fs-id2570945\">Review Sturtevant\u2019s process to create a genetic map on the basis of recombination frequencies <a href=\"http:\/\/openstaxcollege.org\/l\/gene_crossover\" target=\"_window\">here<\/a>.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id1961794\" class=\"bc-section section\">\r\n<h4>Mendel\u2019s Mapped Traits<\/h4>\r\n<p id=\"fs-id1802180\">Homologous recombination is a common genetic process, yet Mendel never observed it. Had he investigated both linked and unlinked genes, it would have been much more difficult for him to create a unified model of his data on the basis of probabilistic calculations. Researchers who have since mapped the seven traits that Mendel investigated onto a pea plant genome's seven chromosomes have confirmed that all the genes he examined are either on separate chromosomes or are sufficiently far apart as to be statistically unlinked. Some have suggested that Mendel was enormously lucky to select only unlinked genes; whereas, others question whether Mendel discarded any data suggesting linkage. In any case, Mendel consistently observed independent assortment because he examined genes that were effectively unlinked.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id2336690\" class=\"summary textbox key-takeaways\">\r\n<h3>Section Summary<\/h3>\r\n<p id=\"fs-id1506798\">Sutton and Boveri's Chromosomal Theory of Inheritance states that chromosomes are the vehicles of genetic heredity. Neither Mendelian genetics nor gene linkage is perfectly accurate. Instead, chromosome behavior involves segregation, independent assortment, and occasionally, linkage. Sturtevant devised a method to assess recombination frequency and infer linked genes' relative positions and distances on a chromosome on the basis of the average number of crossovers in the intervening region between the genes. Sturtevant correctly presumed that genes are arranged in serial order on chromosomes and that recombination between homologs can occur anywhere on a chromosome with equal likelihood. Whereas linkage causes alleles on the same chromosome to be inherited together, homologous recombination biases alleles toward an independent inheritance pattern.<\/p>\r\n\r\n<\/div>\r\n<div id=\"fs-idp60956688\" class=\"art-exercise\">\r\n<h3>Art Connections<\/h3>\r\n<div id=\"fs-idp49371568\">\r\n<div id=\"fs-idp83566544\">\r\n<p id=\"fs-idp8315472\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_02\">(Figure)<\/a> In a test cross for two characteristics such as the one shown here, can the predicted frequency of recombinant offspring be 60 percent? Why or why not?<\/p>\r\n\r\n<\/div>\r\n[reveal-answer q=\"fs-idp38697712\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-idp38697712\"]\r\n<div id=\"fs-idp38697712\">\r\n<p id=\"fs-idm9214224\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_02\">(Figure)<\/a> No. The predicted frequency of recombinant offspring ranges from 0% (for linked traits) to 50% (for unlinked traits).<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div id=\"fs-idp13107328\">\r\n<div id=\"fs-idp196286160\">\r\n<p id=\"fs-idp48650576\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a> Which of the following statements is true?<\/p>\r\n\r\n<ol id=\"fs-idp149339680\" type=\"a\">\r\n \t<li>Recombination of the body color and red\/cinnabar eye alleles will occur more frequently than recombination of the alleles for wing length and aristae length.<\/li>\r\n \t<li>Recombination of the body color and aristae length alleles will occur more frequently than recombination of red\/brown eye alleles and the aristae length alleles.<\/li>\r\n \t<li>Recombination of the gray\/black body color and long\/short aristae alleles will not occur.<\/li>\r\n \t<li>Recombination of the red\/brown eye and long\/short aristae alleles will occur more frequently than recombination of the alleles for wing length and body color.<\/li>\r\n<\/ol>\r\n<\/div>\r\n[reveal-answer q=\"fs-idp119823280\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-idp119823280\"]\r\n<div id=\"fs-idp119823280\">\r\n<p id=\"fs-idm2755664\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a> D<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id2321141\" class=\"multiple-choice textbox exercises\">\r\n<h3>Review Questions<\/h3>\r\n<div id=\"fs-id1596619\">\r\n<div id=\"fs-id1480619\">\r\n<p id=\"fs-id1723705\">X-linked recessive traits in humans (or in <em>Drosophila<\/em>) are observed ________.<\/p>\r\n\r\n<ol id=\"fs-id1894764\" type=\"a\">\r\n \t<li>in more males than females<\/li>\r\n \t<li>in more females than males<\/li>\r\n \t<li>in males and females equally<\/li>\r\n \t<li>in different distributions depending on the trait<\/li>\r\n<\/ol>\r\n<\/div>\r\n[reveal-answer q=\"fs-id2570734\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-id2570734\"]\r\n<div id=\"fs-id2570734\">\r\n<p id=\"fs-id2075720\">A<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div id=\"fs-id2165395\">\r\n<div id=\"fs-id1803123\">\r\n<p id=\"fs-id2190439\">The first suggestion that chromosomes may physically exchange segments came from the microscopic identification of ________.<\/p>\r\n\r\n<ol id=\"fs-id2574822\" type=\"a\">\r\n \t<li>synapsis<\/li>\r\n \t<li>sister chromatids<\/li>\r\n \t<li>chiasmata<\/li>\r\n \t<li>alleles<\/li>\r\n<\/ol>\r\n<\/div>\r\n[reveal-answer q=\"fs-id1780811\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-id1780811\"]\r\n<div id=\"fs-id1780811\">\r\n<p id=\"fs-id1695753\">C<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div id=\"fs-id2914799\">\r\n<div id=\"fs-id1988877\">\r\n<p id=\"fs-id2110663\">Which recombination frequency corresponds to independent assortment and the absence of linkage?<\/p>\r\n\r\n<ol id=\"fs-id2315538\" type=\"a\">\r\n \t<li>0<\/li>\r\n \t<li>0.25<\/li>\r\n \t<li>0.50<\/li>\r\n \t<li>0.75<\/li>\r\n<\/ol>\r\n<\/div>\r\n[reveal-answer q=\"fs-id2155997\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-id2155997\"]\r\n<div id=\"fs-id2155997\">\r\n<p id=\"fs-id1236538\">C<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div id=\"fs-id2310144\">\r\n<div id=\"fs-id2897234\">\r\n<p id=\"fs-id1720799\">Which recombination frequency corresponds to perfect linkage and violates the law of independent assortment?<\/p>\r\n\r\n<ol id=\"fs-id1798480\" type=\"a\">\r\n \t<li>0<\/li>\r\n \t<li>0.25<\/li>\r\n \t<li>0.50<\/li>\r\n \t<li>0.75<\/li>\r\n<\/ol>\r\n<\/div>\r\n[reveal-answer q=\"fs-id2914634\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-id2914634\"]\r\n<div id=\"fs-id2914634\">\r\n<p id=\"fs-id1238553\">A<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-id1384518\" class=\"free-response textbox exercises\">\r\n<h3>Free Response<\/h3>\r\n<div id=\"fs-id2890873\">\r\n<div id=\"fs-id2914426\">\r\n<p id=\"fs-id2317257\">Explain how the Chromosomal Theory of Inheritance helped to advance our understanding of genetics.<\/p>\r\n\r\n<\/div>\r\n[reveal-answer q=\"fs-id2491136\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"fs-id2491136\"]\r\n<div id=\"fs-id2491136\">\r\n<p id=\"fs-id2321542\">The Chromosomal Theory of Inheritance proposed that genes reside on chromosomes. The understanding that chromosomes are linear arrays of genes explained linkage, and crossing over explained recombination.<\/p>\r\n\r\n<\/div>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox shaded\">\r\n<h3>Glossary<\/h3>\r\n<dl id=\"fs-id2200120\">\r\n \t<dt>centimorgan (cM)<\/dt>\r\n \t<dd id=\"fs-id1797882\">(also, map unit) relative distance that corresponds to a 0,01 recombination frequency<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1310227\">\r\n \t<dt>Chromosomal Theory of Inheritance<\/dt>\r\n \t<dd id=\"fs-id1467930\">theory proposing that chromosomes are the genes' vehicles and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id2248874\">\r\n \t<dt>homologous recombination<\/dt>\r\n \t<dd id=\"fs-id1957277\">process by which homologous chromosomes undergo reciprocal physical exchanges at their arms, also crossing over<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id3241178\">\r\n \t<dt>nonparental (recombinant) type<\/dt>\r\n \t<dd id=\"fs-id2013517\">progeny resulting from homologous recombination that exhibits a different allele combination compared with its parents<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id2891394\">\r\n \t<dt>parental types<\/dt>\r\n \t<dd id=\"fs-id1986668\">progeny that exhibits the same allelic combination as its parents<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id2595875\">\r\n \t<dt>recombination frequency<\/dt>\r\n \t<dd id=\"fs-id2026021\">average number of crossovers between two alleles; observed as the number of nonparental types in a progeny's population<\/dd>\r\n<\/dl>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to do the following:<\/p>\n<ul>\n<li>Discuss Sutton\u2019s Chromosomal Theory of Inheritance<\/li>\n<li>Describe genetic linkage<\/li>\n<li>Explain the process of homologous recombination, or crossing over<\/li>\n<li>Describe chromosome creation<\/li>\n<li>Calculate the distances between three genes on a chromosome using a three-point test cross<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-id2021443\">Long before scientists visualized chromosomes under a microscope, the father of modern genetics, Gregor Mendel, began studying heredity in 1843. With improved microscopic techniques during the late 1800s, cell biologists could stain and visualize subcellular structures with dyes and observe their actions during cell division and meiosis. With each mitotic division, chromosomes replicated, condensed from an amorphous (no constant shape) nuclear mass into distinct X-shaped bodies (pairs of identical sister chromatids), and migrated to separate cellular poles.<\/p>\n<div id=\"fs-id2904526\" class=\"bc-section section\">\n<h3>Chromosomal Theory of Inheritance<\/h3>\n<p id=\"fs-id1406190\">The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel\u2019s publications and reevaluate his model in terms of chromosome behavior during mitosis and meiosis. In 1902, Theodor Boveri observed that proper sea urchin embryonic development does not occur unless chromosomes are present. That same year, Walter Sutton observed chromosome separation into daughter cells during meiosis (<a class=\"autogenerated-content\" href=\"#fig-ch13_01_01\">(Figure)<\/a>). Together, these observations led to the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.<\/p>\n<div id=\"fig-ch13_01_01\" class=\"wp-caption aligncenter\">\n<div class=\"wp-caption-text\">(a) Walter Sutton and (b) Theodor Boveri developed the Chromosomal Theory of Inheritance, which states that chromosomes carry the unit of heredity (genes).<\/div>\n<p><span id=\"fs-id2011864\"><br \/>\n<img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03183157\/Figure_13_01_01.jpg\" alt=\"Part a is a photo of Walter Sutton. Part b is a photo of Theodor Boveri.\" width=\"275\" \/><\/span><\/p>\n<\/div>\n<p id=\"fs-id1236139\">The Chromosomal Theory of Inheritance was consistent with Mendel\u2019s laws, which the following observations supported:<\/p>\n<ul id=\"fs-id1770608\">\n<li>During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.<\/li>\n<li>Chromosome sorting from each homologous pair into pre-gametes appears to be random.<\/li>\n<li>Each parent synthesizes gametes that contain only half their chromosomal complement.<\/li>\n<li>Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.<\/li>\n<li>The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.<\/li>\n<\/ul>\n<p id=\"fs-id2335848\">Despite compelling correlations between chromosome behavior during meiosis and Mendel\u2019s abstract laws, scientists proposed the Chromosomal Theory of Inheritance long before there was any direct evidence that chromosomes carried traits. Critics pointed out that individuals had far more independently segregating traits than they had chromosomes. It was only after several years of carrying out crosses with the fruit fly, <em>Drosophila melanogaster<\/em>, that Thomas Hunt Morgan provided experimental evidence to support the Chromosomal Theory of Inheritance.<\/p>\n<\/div>\n<div id=\"fs-id1396027\" class=\"bc-section section\">\n<h3>Genetic Linkage and Distances<\/h3>\n<p id=\"fs-id1780491\">Mendel\u2019s work suggested that traits are inherited independently of each other. Morgan identified a 1:1 correspondence between a segregating trait and the X chromosome, suggesting that random chromosome segregation was the physical basis of Mendel\u2019s model. This also demonstrated that linked genes disrupt Mendel\u2019s predicted outcomes. That each chromosome can carry many linked genes explains how individuals can have many more traits than they have chromosomes. However, researchers in Morgan\u2019s laboratory suggested that alleles positioned on the same chromosome were not always inherited together. During meiosis, linked genes somehow became unlinked.<\/p>\n<div id=\"fs-id2571351\" class=\"bc-section section\">\n<h4>Homologous Recombination<\/h4>\n<p id=\"fs-id2047023\">In 1909, Frans Janssen observed chiasmata\u2014the point at which chromatids are in contact with each other and may exchange segments\u2014prior to the first meiosis division. He suggested that alleles become unlinked and chromosomes physically exchange segments. As chromosomes condensed and paired with their homologs, they appeared to interact at distinct points. Janssen suggested that these points corresponded to regions in which chromosome segments exchanged. We now know that the pairing and interaction between homologous chromosomes, or synapsis, does more than simply organize the homologs for migration to separate daughter cells. When synapsed, homologous chromosomes undergo reciprocal physical exchanges at their arms in homologous recombination, or more simply, \u201ccrossing over.\u201d<\/p>\n<p id=\"fs-id1468864\">To better understand the type of experimental results that researchers were obtaining at this time, consider a heterozygous individual that inherited dominant maternal alleles for two genes on the same chromosome (such as<em> AB<\/em>) and two recessive paternal alleles for those same genes (such as <em>ab<\/em>). If the genes are linked, one would expect this individual to produce gametes that are either <em>AB<\/em> or <em>ab<\/em> with a 1:1 ratio. If the genes are unlinked, the individual should produce <em>AB<\/em>, <em>Ab<\/em>, <em>aB<\/em>, and <em>ab<\/em> gametes with equal frequencies, according to the Mendelian concept of independent assortment. Because they correspond to new allele combinations, the genotypes Ab and aB are nonparental types that result from homologous recombination during meiosis. Parental types are progeny that exhibit the same allelic combination as their parents. Morgan and his colleagues, however, found that when they test crossed such heterozygous individuals to a homozygous recessive parent (<em>AaBb<\/em> \u00d7 <em>aabb<\/em>), both parental and nonparental cases occurred. For example, 950 offspring might be recovered that were either <em>AaBb<\/em> or <em>aabb<\/em>, but 50 offspring would also result that were either <em>Aabb<\/em> or <em>aaBb<\/em>. These results suggested that linkage occurred most often, but a significant minority of offspring were the products of recombination.<\/p>\n<div id=\"fs-id1595360\" class=\"art-connection textbox examples\">\n<h3>Art Connection<\/h3>\n<div id=\"fig-ch13_01_02\">\n<div class=\"wp-caption-text\">This figure shows unlinked and linked gene inheritance patterns. In (a), two genes are located on different chromosomes so independent assortment occurs during meiosis. The offspring have an equal chance of being the parental type (inheriting the same combination of traits as the parents) or a nonparental type (inheriting a different combination of traits than the parents). In (b), two genes are very close together on the same chromosome so that no crossing over occurs between them. Therefore, the genes are always inherited together and all the offspring are the parental type. In (c), two genes are far apart on the chromosome such that crossing over occurs during every meiotic event. The recombination frequency will be the same as if the genes were on separate chromosomes. (d) The actual recombination frequency of fruit fly wing length and body color that Thomas Morgan observed in 1912 was 17 percent. A crossover frequency between 0 percent and 50 percent indicates that the genes are on the same chromosome and crossover sometimes occurs.<\/div>\n<p><span id=\"fs-id1262550\"><br \/>\n<img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03183201\/Figure_13_01_02.jpg\" alt=\"The illustration shows the possible inheritance patterns of linked and unlinked genes. The example used includes fruit fly body color and wing length. Fruit flies may have a dominant gray color (G) or a recessive black color (g). They may have dominant long wings (L) or recessive short wings (l). Three hypothetical inheritance patterns for a test cross between a heterozygous and a recessive fruit fly are shown, based on gene placement. The actual experimental results published by Thomas Hunt Morgan in 1912 are also shown. In the first hypothetical inheritance pattern in part a, the genes for the two characteristics are on different chromosomes. Independent assortment occurs so that the ratio of genotypes in the offspring is 1 GgLl:1 ggll:1 Ggll:1 ggLl, and 50% of the offspring are nonparental types. In the second hypothetical inheritance pattern in part b, the genes are close together on the same chromosome so that no crossover occurs between them. The ratio of genotypes is 1 GgLl:1 ggll, and none of the offspring are recombinant. In the third hypothetical inheritance pattern in part c, the genes are far apart on the same chromosome so that crossing over occurs 100% of the time. The ratio of genotypes is the same as for genes on two different chromosomes, and 50% of the offspring are recombinant, nonparental types. Part d shows that the number of offspring that Thomas Hunt Morgan actually observed was 965: 944: 206:185 (GgLl:ggll:Ggll:ggLl). Seventeen percent of the offspring were recombinant, indicating that the genes are on the same chromosome and crossing over occurs between them some of the time.\" width=\"370\" \/><\/span><\/p>\n<\/div>\n<p id=\"fs-id2573656\">In a test cross for two characteristics such as the one here, can the recombinant offspring&#8217;s predicted frequency be 60 percent? Why or why not?<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q711355\">Show Solution<\/span><\/p>\n<div id=\"q711355\" class=\"hidden-answer\" style=\"display: none\">\n<p>No. The predicted frequency of recombinant offspring ranges from 0% (for linked traits) to 50% (for unlinked traits).<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1627133\" class=\"bc-section section\">\n<h4>Genetic Maps<\/h4>\n<p id=\"fs-id1645084\">Janssen did not have the technology to demonstrate crossing over so it remained an abstract idea that scientists did not widely believe. Scientists thought chiasmata were a variation on synapsis and could not understand how chromosomes could break and rejoin. Yet, the data were clear that linkage did not always occur. Ultimately, it took a young undergraduate student and an \u201call-nighter\u201d to mathematically elucidate the linkage and recombination problem.<\/p>\n<p id=\"fs-id1684950\">In 1913, Alfred Sturtevant, a student in Morgan\u2019s laboratory, gathered results from researchers in the laboratory, and took them home one night to mull them over. By the next morning, he had created the first \u201cchromosome map,\u201d a linear representation of gene order and relative distance on a chromosome (<a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a>).<\/p>\n<div id=\"fs-id1941826\" class=\"art-connection textbox examples\">\n<h3>Art Connection<\/h3>\n<div id=\"fig-ch13_01_03\" class=\"wp-caption aligncenter\">\n<div class=\"wp-caption-text\">This genetic map orders <em>Drosophila<\/em> genes on the basis of recombination frequency.<\/div>\n<p><span id=\"fs-id1464025\"><br \/>\n<img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/3206\/2018\/05\/03183205\/Figure_13_01_03.png\" alt=\"The illustration shows a Drosophila genetic map. The gene for aristae length occurs at 0 centimorgans, or cM. The gene for body color occurs at 48.5 cM. The gene for red versus cinnabar eye color occurs at 57.5 cM. The gene for wing length occurs at 65.5 cM, and the gene for red versus brown eye color occurs at 104.5 cM. One cM is equivalent to a recombination frequency of 0.01.\" width=\"200\" \/><\/span><\/p>\n<\/div>\n<p id=\"fs-id1785828\">Which of the following statements is true?<\/p>\n<ol id=\"fs-id1511608\" type=\"a\">\n<li>Recombination of the body color and red\/cinnabar eye alleles will occur more frequently than recombination of the alleles for wing length and aristae length.<\/li>\n<li>Recombination of the body color and aristae length alleles will occur more frequently than recombination of red\/brown eye alleles and the aristae length alleles.<\/li>\n<li>Recombination of the gray\/black body color and long\/short aristae alleles will not occur.<\/li>\n<li>Recombination of the red\/brown eye and long\/short aristae alleles will occur more frequently than recombination of the alleles for wing length and body color.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q705626\">Show Solution<\/span><\/p>\n<div id=\"q705626\" class=\"hidden-answer\" style=\"display: none\">\n<p>D<\/p><\/div>\n<\/div>\n<\/div>\n<p id=\"fs-id2137494\">As <a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a> shows, by using recombination frequency to predict genetic distance, we can infer the relative gene order on chromosome 2. The values represent map distances in centimorgans (cM), which correspond to recombination frequencies (in percent). Therefore, the genes for body color and wing size were 65.5 \u2212 48.5 = 17 cM apart, indicating that the maternal and paternal alleles for these genes recombine in 17 percent of offspring, on average.<\/p>\n<p id=\"fs-id1951192\">To construct a chromosome map, Sturtevant assumed that genes were ordered serially on threadlike chromosomes. He also assumed that the incidence of recombination between two homologous chromosomes could occur with equal likelihood anywhere along the chromosome&#8217;s length. Operating under these assumptions, Sturtevant postulated that alleles that were far apart on a chromosome were more likely to dissociate during meiosis simply because there was a larger region over which recombination could occur. Conversely, alleles that were close to each other on the chromosome were likely to be inherited together. The average number of crossovers between two alleles\u2014that is, their recombination frequency\u2014correlated with their genetic distance from each other, relative to the locations of other genes on that chromosome. Considering the example cross between <em>AaBb<\/em> and <em>aabb<\/em> above, we could calculate the recombination&#8217;s frequency as 50\/1000 = 0.05. That is, the likelihood of a crossover between genes <em>A\/a<\/em> and <em>B\/b<\/em> was 0.05, or 5 percent. Such a result would indicate that the genes were definitively linked, but that they were far enough apart for crossovers to occasionally occur. Sturtevant divided his genetic map into map units, or centimorgans (cM), in which a 0,01 recombination frequency corresponds to 1 cM.<\/p>\n<p id=\"fs-id2100648\">By representing alleles in a linear map, Sturtevant suggested that genes can range from linking perfectly (recombination frequency = 0) to unlinking perfectly (recombination frequency = 0.5) when genes are on different chromosomes or genes separate very far apart on the same chromosome. Perfectly unlinked genes correspond to the frequencies Mendel predicted to assort independently in a dihybrid cross. A 0.5 recombination frequency indicates that 50 percent of offspring are recombinants and the other 50 percent are parental types. That is, every type of allele combination is represented with equal frequency. This representation allowed Sturtevant to additively calculate distances between several genes on the same chromosome. However, as the genetic distances approached 0.50, his predictions became less accurate because it was not clear whether the genes were very far apart on the same or on different chromosomes.<\/p>\n<p>In 1931, Barbara McClintock and Harriet Creighton demonstrated the crossover of homologous chromosomes in corn plants. Weeks later, Curt Stern demonstrated microscopically homologous recombination in <em>Drosophila<\/em>. Stern observed several X-linked phenotypes that were associated with a structurally unusual and dissimilar X chromosome pair in which one X was missing a small terminal segment, and the other X was fused to a piece of the Y chromosome. By crossing flies, observing their offspring, and then visualizing the offspring\u2019s chromosomes, Stern demonstrated that every time the offspring allele combination deviated from either of the parental combinations, there was a corresponding exchange of an X chromosome segment. Using mutant flies with structurally distinct X chromosomes was the key to observing the products of recombination because DNA sequencing and other molecular tools were not yet available. We now know that homologous chromosomes regularly exchange segments in meiosis by reciprocally breaking and rejoining their DNA at precise locations.<\/p>\n<div id=\"fs-id2060998\" class=\"interactive textbox tryit\">\n<h3>Link to Learning<\/h3>\n<p id=\"fs-id2570945\">Review Sturtevant\u2019s process to create a genetic map on the basis of recombination frequencies <a href=\"http:\/\/openstaxcollege.org\/l\/gene_crossover\" target=\"_window\">here<\/a>.<\/p>\n<\/div>\n<\/div>\n<div id=\"fs-id1961794\" class=\"bc-section section\">\n<h4>Mendel\u2019s Mapped Traits<\/h4>\n<p id=\"fs-id1802180\">Homologous recombination is a common genetic process, yet Mendel never observed it. Had he investigated both linked and unlinked genes, it would have been much more difficult for him to create a unified model of his data on the basis of probabilistic calculations. Researchers who have since mapped the seven traits that Mendel investigated onto a pea plant genome&#8217;s seven chromosomes have confirmed that all the genes he examined are either on separate chromosomes or are sufficiently far apart as to be statistically unlinked. Some have suggested that Mendel was enormously lucky to select only unlinked genes; whereas, others question whether Mendel discarded any data suggesting linkage. In any case, Mendel consistently observed independent assortment because he examined genes that were effectively unlinked.<\/p>\n<\/div>\n<\/div>\n<div id=\"fs-id2336690\" class=\"summary textbox key-takeaways\">\n<h3>Section Summary<\/h3>\n<p id=\"fs-id1506798\">Sutton and Boveri&#8217;s Chromosomal Theory of Inheritance states that chromosomes are the vehicles of genetic heredity. Neither Mendelian genetics nor gene linkage is perfectly accurate. Instead, chromosome behavior involves segregation, independent assortment, and occasionally, linkage. Sturtevant devised a method to assess recombination frequency and infer linked genes&#8217; relative positions and distances on a chromosome on the basis of the average number of crossovers in the intervening region between the genes. Sturtevant correctly presumed that genes are arranged in serial order on chromosomes and that recombination between homologs can occur anywhere on a chromosome with equal likelihood. Whereas linkage causes alleles on the same chromosome to be inherited together, homologous recombination biases alleles toward an independent inheritance pattern.<\/p>\n<\/div>\n<div id=\"fs-idp60956688\" class=\"art-exercise\">\n<h3>Art Connections<\/h3>\n<div id=\"fs-idp49371568\">\n<div id=\"fs-idp83566544\">\n<p id=\"fs-idp8315472\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_02\">(Figure)<\/a> In a test cross for two characteristics such as the one shown here, can the predicted frequency of recombinant offspring be 60 percent? Why or why not?<\/p>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-idp38697712\">Show Solution<\/span><\/p>\n<div id=\"qfs-idp38697712\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-idp38697712\">\n<p id=\"fs-idm9214224\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_02\">(Figure)<\/a> No. The predicted frequency of recombinant offspring ranges from 0% (for linked traits) to 50% (for unlinked traits).<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-idp13107328\">\n<div id=\"fs-idp196286160\">\n<p id=\"fs-idp48650576\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a> Which of the following statements is true?<\/p>\n<ol id=\"fs-idp149339680\" type=\"a\">\n<li>Recombination of the body color and red\/cinnabar eye alleles will occur more frequently than recombination of the alleles for wing length and aristae length.<\/li>\n<li>Recombination of the body color and aristae length alleles will occur more frequently than recombination of red\/brown eye alleles and the aristae length alleles.<\/li>\n<li>Recombination of the gray\/black body color and long\/short aristae alleles will not occur.<\/li>\n<li>Recombination of the red\/brown eye and long\/short aristae alleles will occur more frequently than recombination of the alleles for wing length and body color.<\/li>\n<\/ol>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-idp119823280\">Show Solution<\/span><\/p>\n<div id=\"qfs-idp119823280\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-idp119823280\">\n<p id=\"fs-idm2755664\"><a class=\"autogenerated-content\" href=\"#fig-ch13_01_03\">(Figure)<\/a> D<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id2321141\" class=\"multiple-choice textbox exercises\">\n<h3>Review Questions<\/h3>\n<div id=\"fs-id1596619\">\n<div id=\"fs-id1480619\">\n<p id=\"fs-id1723705\">X-linked recessive traits in humans (or in <em>Drosophila<\/em>) are observed ________.<\/p>\n<ol id=\"fs-id1894764\" type=\"a\">\n<li>in more males than females<\/li>\n<li>in more females than males<\/li>\n<li>in males and females equally<\/li>\n<li>in different distributions depending on the trait<\/li>\n<\/ol>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-id2570734\">Show Solution<\/span><\/p>\n<div id=\"qfs-id2570734\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-id2570734\">\n<p id=\"fs-id2075720\">A<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id2165395\">\n<div id=\"fs-id1803123\">\n<p id=\"fs-id2190439\">The first suggestion that chromosomes may physically exchange segments came from the microscopic identification of ________.<\/p>\n<ol id=\"fs-id2574822\" type=\"a\">\n<li>synapsis<\/li>\n<li>sister chromatids<\/li>\n<li>chiasmata<\/li>\n<li>alleles<\/li>\n<\/ol>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-id1780811\">Show Solution<\/span><\/p>\n<div id=\"qfs-id1780811\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-id1780811\">\n<p id=\"fs-id1695753\">C<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id2914799\">\n<div id=\"fs-id1988877\">\n<p id=\"fs-id2110663\">Which recombination frequency corresponds to independent assortment and the absence of linkage?<\/p>\n<ol id=\"fs-id2315538\" type=\"a\">\n<li>0<\/li>\n<li>0.25<\/li>\n<li>0.50<\/li>\n<li>0.75<\/li>\n<\/ol>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-id2155997\">Show Solution<\/span><\/p>\n<div id=\"qfs-id2155997\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-id2155997\">\n<p id=\"fs-id1236538\">C<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id2310144\">\n<div id=\"fs-id2897234\">\n<p id=\"fs-id1720799\">Which recombination frequency corresponds to perfect linkage and violates the law of independent assortment?<\/p>\n<ol id=\"fs-id1798480\" type=\"a\">\n<li>0<\/li>\n<li>0.25<\/li>\n<li>0.50<\/li>\n<li>0.75<\/li>\n<\/ol>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-id2914634\">Show Solution<\/span><\/p>\n<div id=\"qfs-id2914634\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-id2914634\">\n<p id=\"fs-id1238553\">A<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1384518\" class=\"free-response textbox exercises\">\n<h3>Free Response<\/h3>\n<div id=\"fs-id2890873\">\n<div id=\"fs-id2914426\">\n<p id=\"fs-id2317257\">Explain how the Chromosomal Theory of Inheritance helped to advance our understanding of genetics.<\/p>\n<\/div>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"qfs-id2491136\">Show Solution<\/span><\/p>\n<div id=\"qfs-id2491136\" class=\"hidden-answer\" style=\"display: none\">\n<div id=\"fs-id2491136\">\n<p id=\"fs-id2321542\">The Chromosomal Theory of Inheritance proposed that genes reside on chromosomes. The understanding that chromosomes are linear arrays of genes explained linkage, and crossing over explained recombination.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox shaded\">\n<h3>Glossary<\/h3>\n<dl id=\"fs-id2200120\">\n<dt>centimorgan (cM)<\/dt>\n<dd id=\"fs-id1797882\">(also, map unit) relative distance that corresponds to a 0,01 recombination frequency<\/dd>\n<\/dl>\n<dl id=\"fs-id1310227\">\n<dt>Chromosomal Theory of Inheritance<\/dt>\n<dd id=\"fs-id1467930\">theory proposing that chromosomes are the genes&#8217; vehicles and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed<\/dd>\n<\/dl>\n<dl id=\"fs-id2248874\">\n<dt>homologous recombination<\/dt>\n<dd id=\"fs-id1957277\">process by which homologous chromosomes undergo reciprocal physical exchanges at their arms, also crossing over<\/dd>\n<\/dl>\n<dl id=\"fs-id3241178\">\n<dt>nonparental (recombinant) type<\/dt>\n<dd id=\"fs-id2013517\">progeny resulting from homologous recombination that exhibits a different allele combination compared with its parents<\/dd>\n<\/dl>\n<dl id=\"fs-id2891394\">\n<dt>parental types<\/dt>\n<dd id=\"fs-id1986668\">progeny that exhibits the same allelic combination as its parents<\/dd>\n<\/dl>\n<dl id=\"fs-id2595875\">\n<dt>recombination frequency<\/dt>\n<dd id=\"fs-id2026021\">average number of crossovers between two alleles; observed as the number of nonparental types in a progeny&#8217;s population<\/dd>\n<\/dl>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-824\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/openstax.org\/details\/books\/biology-2e\">https:\/\/openstax.org\/details\/books\/biology-2e<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/8d50a0af-948b-4204-a71d-4826cba765b8@8.19<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":311,"menu_order":2,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"https:\/\/openstax.org\/details\/books\/biology-2e\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/8d50a0af-948b-4204-a71d-4826cba765b8@8.19\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-824","chapter","type-chapter","status-publish","hentry"],"part":818,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/824","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/users\/311"}],"version-history":[{"count":2,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/824\/revisions"}],"predecessor-version":[{"id":2095,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/824\/revisions\/2095"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/parts\/818"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapters\/824\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/media?parent=824"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/pressbooks\/v2\/chapter-type?post=824"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/contributor?post=824"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-oneonta-osbiology2e-1\/wp-json\/wp\/v2\/license?post=824"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}