{"id":264,"date":"2021-05-28T13:51:02","date_gmt":"2021-05-28T13:51:02","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/child\/?post_type=chapter&p=264"},"modified":"2024-07-11T18:35:32","modified_gmt":"2024-07-11T18:35:32","slug":"heredity-and-chromosomes-2","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/child\/chapter\/heredity-and-chromosomes-2\/","title":{"raw":"Heredity and Chromosomes","rendered":"Heredity and Chromosomes"},"content":{"raw":"Heredity<\/b>\u00a0is the passing on of\u00a0traits\u00a0from parents to their offspring. For humans, through\u00a0sexual reproduction, the offspring\u00a0acquire the\u00a0genetic information\u00a0of their parents. Through heredity, variations between individuals can accumulate and cause\u00a0species\u00a0to\u00a0evolve\u00a0by\u00a0natural selection.\r\n\r\nhttps:\/\/youtu.be\/m0T2miip8jo\r\n\r\nVideo 1.\u00a0<\/strong>Heritability\u00a0<\/em>explained.\r\n

Gametes<\/h2>\r\n[caption id=\"attachment_577\" align=\"alignright\" width=\"240\"]\"female<\/a> Figure 4.3.1.<\/strong>\u00a0The Female Reproductive System.[\/caption]\r\n\r\nThere are two types of sex cells or gametes<\/strong> involved in reproduction: the male gametes, or sperm, and female gametes, or ova. The male gametes are produced in the testes through a process called\u00a0spermatogenesis, which begins at about 12 years of age. The female gametes, which are stored in the ovaries, are present at birth but are immature. Each ovary contains about 250,000 ova but only about 400 of these will become mature eggs (Mackon & Fauser, 2000; Rome, 1998). Beginning at puberty, one ovum ripens and is released about every 28 days, a process called\u00a0oogenesis.\r\n\r\nAfter the ovum or egg ripens and is released from the ovary, it is drawn into the fallopian tube and in 3 to 4 days, reaches the uterus. It is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejaculation, millions of sperm are released into the vagina, but only a few reach the egg and typically, only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a\u00a0zygote.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=_5OvgQW6FG4\r\n\r\nVideo 2.<\/strong>\u00a0Fertilization\u00a0<\/em>shows how\u00a0one single sperm survives the long and treacherous journey to fertilize the mother's egg.\r\n\r\n[caption id=\"attachment_3374\" align=\"alignright\" width=\"356\"]\"DNA<\/a> Figure 2.<\/strong>\u00a0Deoxyribonucleic acid (DNA) is a helix-shaped molecule made up of nucleotide base pairs. Sequences of DNA make up genes.<\/span>[\/caption]\r\n

Chromosomes<\/h2>\r\nWhile other normal human cells have 46 chromosomes (or 23 pairs), gametes contain 23 chromosomes.\u00a0Chromosomes<\/strong>\u00a0are long threadlike structures found in a cell nucleus that contains genetic material known as\u00a0deoxyribonucleic acid (DNA)<\/strong>.\u00a0DNA is a helix-shaped molecule made up of nucleotide base pairs [adenine (A), guanine (G), cytosine (C), and thymine (T)]. In each chromosome, sequences of DNA make up\u00a0genes<\/strong>\u00a0that control or partially control a number of visible characteristics, known as traits, such as eye color, hair color, and so on. A single gene may have multiple possible variations or alleles. An\u00a0allele<\/strong>\u00a0is a specific version of a gene. So, a given gene may code for the trait of hair color, and the different alleles of that gene affect which hair color an individual has.\r\n\r\nIn a process called\u00a0meiosis,\u00a0segments of the chromosomes from each parent form pairs, and genetic segments are exchanged as determined by chance.\u00a0Because of the unpredictability of this exchange, the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould & Keeton, 1997).\u00a0Genetic variation is important because it allows a species to adapt so that those who are better suited to the environment will survive and reproduce, which is an important factor in natural selection.\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=nMEyeKQClqI\r\n\r\nVideo 3.\u00a0<\/strong>Meiosis\u00a0<\/em>explained.\r\n

Genotypes and Phenotypes<\/h3>\r\nWhen a sperm and egg fuse, their 23 chromosomes pair up and create a zygote with 23 pairs of chromosomes. Therefore, each parent contributes half the genetic information carried by the offspring; the resulting physical characteristics of the offspring (called the phenotype) are determined by the interaction of genetic material supplied by the parents (called the genotype). A person\u2019s\u00a0genotype<\/strong>\u00a0is the genetic makeup of that individual.\u00a0Phenotype,<\/strong>\u00a0on the other hand, refers to the individual\u2019s inherited physical characteristics.<\/span>\r\n\r\nLook in the mirror. What do you see, your genotype or your phenotype? What determines whether or not genes are expressed? Actually, this is quite complicated. Some features follow the\u00a0additive pattern\u00a0which means that many different genes contribute to a final outcome. Height and skin tone are examples. In other cases, a gene might either be turned on or off depending on several factors, including\u00a0the gene with which it is paired or the inherited epigenetic tags.\r\n

Determining the Sex of the Child<\/h2>\r\nTwenty-two of those chromosomes from each parent are similar in length to a corresponding chromosome from the other parent. However, the remaining chromosome looks like an X or a Y. Half of the male's sperm contains a Y chromosome and half contain an X. All of the ova contain X chromosomes. If the child receives the combination of XY, the child will be genetically male. If it receives the XX combination, the child will be genetically female.\r\n\r\nMany potential parents have a clear preference for having a boy or a girl and would like to determine the sex of the child. Through the years, a number of tips have been offered for potential parents to maximize their chances of having either a son or daughter as they prefer. For example, it has been suggested that sperm that carry a Y chromosome are more fragile than those carrying an X. So, if a couple desires a male child, they can take measures to maximize the chance that the Y sperm reaches the egg. This involves having intercourse 48 hours after ovulation, which helps the Y sperm have a shorter journey to reach the egg, douching to create a more alkaline environment in the vagina, and having the female reach orgasm first so that sperm are not pushed out of the vagina during orgasm. Today, however, there is new technology available that makes it possible to isolate sperm containing either an X or a Y, depending on the preference, and use that sperm to fertilize a mother's egg.\r\n

Genetic Variation\u00a0and Inheritance<\/h2>\r\nGenetic variation, the genetic difference between individuals, is what contributes to a species\u2019 adaptation to its environment. In humans, genetic variation begins with an egg, several million sperm, and fertilization. The egg and the sperm each contain 23 chromosomes, which make up our genes.\u00a0A single gene may have multiple possible variations or alleles (a specific version of a gene), resulting in a variety of combinations of inherited traits.\r\n\r\nGenetic inheritance of traits for humans is based upon\u00a0Gregor Mendel's model of inheritance. For genes on an\u00a0autosome\u00a0(any\u00a0chromosome\u00a0other than a\u00a0sex chromosome), the alleles and their associated traits are\u00a0autosomal dominant\u00a0or\u00a0autosomal recessive. In this model, some genes are considered dominant because they will be expressed. Others, termed recessive, are only expressed in the absence of a dominant gene. Some characteristics which were once thought of as\u00a0dominant-recessive, such as eye color, are now believed to be a result of the interaction between several genes (McKusick, 1998). Dominant traits include curly hair, facial dimples, normal vision, and dark hair.\u00a0Recessive characteristics include red hair, pattern baldness, and nearsightedness.\r\n
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DOMINANT AND RECESSIVE CHARACTERISTICS<\/strong><\/h3>\r\nTable 1.<\/strong> Characteristics in the left-hand column dominate over those characteristics listed in the right-hand column.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
<\/td>\r\nDOMINANT TRAITS<\/strong><\/td>\r\nRECESSIVE TRAITS<\/strong><\/td>\r\n<\/tr>\r\n
eye coloring<\/td>\r\nbrown eyes<\/td>\r\ngrey, green, hazel, blue eyes<\/td>\r\n<\/tr>\r\n
vision<\/td>\r\nfarsightedness\r\nnormal vision\r\nnormal vision\r\nnormal vision<\/td>\r\nnormal vision\r\nnearsightedness\r\nnight blindness\r\ncolor blindness*<\/td>\r\n<\/tr>\r\n
hair<\/td>\r\ndark hair\r\nnon-red hair\r\ncurly hair\r\nfull head of hair\r\nwidow's peak<\/td>\r\nblonde, light, red hair\r\nred hair\r\nstraight hair\r\nbaldness*\r\nnormal hairline<\/td>\r\n<\/tr>\r\n
facial features<\/td>\r\ndimples\r\nunattached earlobes\r\nfreckles\r\nbroad lips<\/td>\r\nno dimples\r\nattached earlobes\r\nno freckles\r\nthin lips<\/td>\r\n<\/tr>\r\n
appendages<\/td>\r\nextra digits\r\nfused digits\r\nshort digits\r\nfingers lack 1 joint\r\nlimb dwarfing\r\nclubbed thumb\r\ndouble-jointedness<\/td>\r\nnormal number\r\nnormal digits\r\nnormal digits\r\nnormal joints\r\nnormal proportion\r\nnormal thumb\r\nnormal joints<\/td>\r\n<\/tr>\r\n
other<\/td>\r\nimmunity to poison ivy\r\nnormal pigmented skin\r\nnormal blood clotting\r\nnormal hearing\r\nnormal hearing and speaking\r\nnormal- no PKU<\/td>\r\nsusceptibility to poison ivy\r\nalbinism\r\nhemophilia*\r\ncongenital deafness\r\ndeaf mutism\r\nphenylketonuria (PKU)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\nSickle cell anemia is an autosomal recessive disease; Huntington's\u00a0disease is an autosomal dominant disease.\u00a0Other traits are a result of\u00a0partial dominance\u00a0or\u00a0co-dominance\u00a0in which both genes are influential. For example, if a person inherits both recessive genes for cystic fibrosis, the disease will occur.\u00a0But if a person has only one recessive gene for the disease, the person would be a carrier of the disease.\r\n\r\nIn this example, we will call the normal gene \u201cN,\u201d and the gene for cystic fibrosis \u201cc.\u201d The normal gene is dominant, which means that having the\u00a0dominant allele\u00a0either from one parent (Nc) or both parents (NN) will always result in the phenotype associated with the dominant allele. When someone has two copies of the same allele, they are said to be\u00a0homozygous<\/strong>\u00a0for that allele. When someone has a combination of alleles for a given gene, they are said to be\u00a0heterozygous<\/strong>. For example, cystic fibrosis is a recessive disease which means that an individual will only have the disease if they are homozygous for that\u00a0recessive allele\u00a0(cc).<\/span>\r\n\r\nImagine that a woman who is a carrier of the cystic fibrosis gene has a child\u00a0with a man who also is a carrier of the same disease. What are the odds that their child would inherit the disease? Both the woman and the man are heterozygous for this gene (Nc).\u00a0 We can expect the offspring to have a 25% chance of having cystic fibrosis (cc), a 50% chance of being a carrier of the disease (Nc), and a 25% chance of receiving two normal copies of the gene (NN).<\/span>\r\n\r\n[caption id=\"attachment_3373\" align=\"aligncenter\" width=\"390\"]\"Punnett<\/a> Figure 3.<\/strong>\u00a0A Punnett square is a tool used to predict how genes will interact in the production of offspring. The capital N represents the dominant allele, and the lowercase c represents the recessive allele. In the example of cystic fibrosis, where N is the normal gene (dominant allele), wherever a pair contains the dominant allele, N, you can expect a phenotype that does not express the disease. You can expect a cystic fibrosis phenotype only when there are two copies of the c (recessive allele) which contains the gene mutation that causes the disease.[\/caption]\r\n\r\nWhere do harmful genes that contribute to diseases like cystic fibrosis come from? Gene mutations provide one source of harmful genes. A\u00a0mutation<\/strong>\u00a0is a sudden, permanent change in a gene. While many mutations can be harmful or lethal, once in a while a mutation benefits an individual by giving that person an advantage over those who do not have the mutation. Recall that the theory of evolution asserts that individuals best adapted to their particular environments are more likely to reproduce and pass on their genes to future generations. In order for this process to occur, there must be competition\u2014more technically, there must be variability in genes (and resultant traits) that allow for variation in adaptability to the environment. If a population consisted of identical individuals, then any dramatic changes in the environment would affect everyone in the same way, and there would be no variation in selection. In contrast, diversity in genes and associated traits allow some individuals to perform slightly better than others when faced with environmental change. This creates a distinct advantage for individuals best suited for their environments in terms of successful reproduction and genetic transmission.<\/span>\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=NR3779ef9yQ&feature=youtu.be\r\n\r\nVideo 4.<\/strong>\u00a0An Introduction to Mendelian Genetics\u00a0<\/em>demonstrates another example of the interaction of alleles using the Punnett square.\r\n

Sex-Linked Traits<\/h3>\r\nSex-linked traits<\/strong>\u00a0are genes located on a sex chromosome (the 23rd pair). In humans, the term generally refers to traits that are influenced by genes on the X chromosome. This is because the X chromosome is large and contains many more genes than the smaller Y chromosome. In a sex-linked disease, it is usually males who are affected because they have a single copy of the X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.\r\n\r\n\r\n\r\nFigure 4.<\/strong> Hemophilia is an example of a sex-linked trait, as are color blindness, congenital night blindness, some high blood pressure genes, Duchenne muscular dystrophy, and also Fragile X syndrome. This exemplifies how the gene for hemophilia may be passed from one parent to their sons and daughters.\r\n\r\nhttps:\/\/youtu.be\/-ROhfKyxgCo\r\n\r\nVideo 5.<\/strong>\u00a0Sex-Linked Traits\u00a0<\/em>explains the traits carried on the 23rd pair of chromosomes and how these\r\n

Chromosomal Abnormalities and Genetic Testing<\/h2>\r\n[caption id=\"attachment_3903\" align=\"alignright\" width=\"331\"]\"PIcture Figure 5.<\/strong>\u00a0The three major single-chromosome mutations: deletion (1), duplication (2), and inversion (3).[\/caption]\r\n\r\nA\u00a0chromosomal abnormality\u00a0occurs when a child inherits too many or too few chromosomes. The most common cause of chromosomal abnormalities is the age of the mother. A 20-year-old woman has a 1 in 800 chance of having a child with a common chromosomal abnormality. A woman of 44, however, has a one in\u00a016 chance. It is believed that the problem occurs when the ovum is ripening prior to ovulation each month. As the mother ages, the ovum is more likely to suffer abnormalities at this time.\r\n\r\nAnother common cause of chromosomal abnormalities occurs because the gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop and are spontaneously aborted by the body. If the abnormal number occurs on pair # 21 or # 23, however, the individual may have certain physical or other abnormalities.\r\n\r\nAn altered chromosome structure may take several different forms, and result in various disorders or malignancies:\r\n