{"id":2762,"date":"2016-06-13T17:28:59","date_gmt":"2016-06-13T17:28:59","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/biologyxwaymakerxmaster\/?post_type=chapter&#038;p=2762"},"modified":"2024-04-29T16:33:32","modified_gmt":"2024-04-29T16:33:32","slug":"reading-epistasis-2","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology1\/chapter\/reading-epistasis-2\/","title":{"raw":"Epistasis","rendered":"Epistasis"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Explain how epistasis impacts trait expression<\/li>\r\n<\/ul>\r\n<\/div>\r\nMendel's studies in pea plants implied that the sum of an individual's phenotype was controlled by genes (or as he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.\r\n<div class=\"textbox shaded\">Eye color in humans is determined by multiple genes. Use the\u00a0<a href=\"http:\/\/www.athro.com\/evo\/inherit.html\" target=\"_blank\" rel=\"noopener\">Eye Color Calculator<\/a> to predict the eye color of children from parental eye color.<\/div>\r\nIn some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other, with one gene modifying the expression of another.\r\n\r\nIn epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. \"Epistasis\" is a word composed of Greek roots that mean \"standing upon.\" The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.\r\n\r\nAn example of epistasis is pigmentation in mice. The wild-type coat color, agouti (<em>AA<\/em>), is dominant to solid-colored fur (<em>aa<\/em>). However, a separate gene (<em>C<\/em>) is necessary for pigment production. A mouse with a recessive <em>c<\/em> allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus <em>A<\/em> (Figure 1). Therefore, the genotypes <em>AAcc<\/em>, <em>Aacc<\/em>, and <em>aacc<\/em> all produce the same albino phenotype. A cross between heterozygotes for both genes (<em>AaCc<\/em> x <em>AaCc<\/em>) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino (Figure 1). In this case, the <em>C<\/em> gene is epistatic to the <em>A<\/em> gene.\r\n\r\n[caption id=\"attachment_1416\" align=\"aligncenter\" width=\"800\"]<img class=\"size-full wp-image-1416\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02183244\/Figure_12_03_05.jpg\" alt=\"A cross between two agouti mice with the heterozygous genotype AaCc is shown. Each mouse produces four different kinds of gametes (AC, aC, Ac, and ac). A 4 \u00d7 4 Punnett square is used to determine the genotypic ratio of the offspring. The phenotypic ratio is 9\/16 agouti, 3\/16 black, and 4\/16 white.\" width=\"800\" height=\"948\" \/> Figure 1. In mice, the mottled agouti coat color (<em>A<\/em>) is dominant to a solid coloration, such as black or gray. A gene at a separate locus (<em>C<\/em>) is responsible for pigment production. The recessive <em>c<\/em> allele does not produce pigment, and a mouse with the homozygous recessive <em>cc<\/em> enotype is albino regardless of the allele present at the <em>A<\/em> locus. Thus, the <em>C<\/em> gene is epistatic to the <em>A<\/em> gene.[\/caption]\r\n\r\nEpistasis can also occur when a dominant allele masks expression at a separate gene. Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the\u00a0<em>W<\/em> gene (<em>ww<\/em>) coupled with homozygous dominant or heterozygous expression of the <em>Y<\/em> gene (<em>YY<\/em> or <em>Yy<\/em>) generates yellow fruit, and the <em>wwyy<\/em> genotype produces green fruit. However, if a dominant copy of the <em>W<\/em> gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the <em>Y<\/em> alleles. A cross between white heterozygotes for both genes (<em>WwYy<\/em> \u00d7 <em>WwYy<\/em>) would produce offspring with a phenotypic ratio of 12 white:3 yellow:1 green.\r\n\r\nFinally, epistasis can be reciprocal such that either gene, when present in the dominant (or recessive) form, expresses the same phenotype. In the shepherd's purse plant (<em>Capsella bursa-pastoris<\/em>), the characteristic of seed shape is controlled by two genes in a dominant epistatic relationship. When the genes <em>A<\/em> and <em>B<\/em> are both homozygous recessive (<em>aabb<\/em>), the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. That is, every possible genotype other than <em>aabb<\/em> results in triangular seeds, and a cross between heterozygotes for both genes (<em>AaBb<\/em> x <em>AaBb<\/em>) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.\r\n\r\nAs you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. Recall the phenotypic inheritance pattern for Mendel's dihybrid cross, which considered two non-interacting genes\u20149:3:3:1. Similarly, we would expect interacting gene pairs to also exhibit ratios expressed as 16 parts. Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes.\r\n<div class=\"textbox shaded\">For an excellent review of Mendel's experiments and to perform your own crosses and identify patterns of inheritance, visit the\u00a0<a href=\"http:\/\/www2.edc.org\/weblabs\/mendel\/MendelMenu.html\" target=\"_blank\" rel=\"noopener\">Mendel's Peas<\/a> web lab.<\/div>\r\nWhether or not they are sorting independently, genes may interact at the level of gene products such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.\r\n<div class=\"textbox learning-objectives\">\r\n<h3>In Summary: Epistasis<\/h3>\r\nEpistasis is a form on non-Mendelian inheritance in which one gene is capable of interfering with expression of another. This is often found associated with gene pathways where the expression of one gene is directly dependent on the presence or absence of another gene product within the pathway.\r\n\r\nWatch this video for a quick review of epistasis:\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1280814\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script>\r\n<iframe id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/ctjw-ZijciM?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe>\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/967db7ce-dc9e-4518-814b-a1acbacbc208\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Explain how epistasis impacts trait expression<\/li>\n<\/ul>\n<\/div>\n<p>Mendel&#8217;s studies in pea plants implied that the sum of an individual&#8217;s phenotype was controlled by genes (or as he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.<\/p>\n<div class=\"textbox shaded\">Eye color in humans is determined by multiple genes. Use the\u00a0<a href=\"http:\/\/www.athro.com\/evo\/inherit.html\" target=\"_blank\" rel=\"noopener\">Eye Color Calculator<\/a> to predict the eye color of children from parental eye color.<\/div>\n<p>In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other, with one gene modifying the expression of another.<\/p>\n<p>In epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. &#8220;Epistasis&#8221; is a word composed of Greek roots that mean &#8220;standing upon.&#8221; The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.<\/p>\n<p>An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (<em>AA<\/em>), is dominant to solid-colored fur (<em>aa<\/em>). However, a separate gene (<em>C<\/em>) is necessary for pigment production. A mouse with a recessive <em>c<\/em> allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus <em>A<\/em> (Figure 1). Therefore, the genotypes <em>AAcc<\/em>, <em>Aacc<\/em>, and <em>aacc<\/em> all produce the same albino phenotype. A cross between heterozygotes for both genes (<em>AaCc<\/em> x <em>AaCc<\/em>) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino (Figure 1). In this case, the <em>C<\/em> gene is epistatic to the <em>A<\/em> gene.<\/p>\n<div id=\"attachment_1416\" style=\"width: 810px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1416\" class=\"size-full wp-image-1416\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02183244\/Figure_12_03_05.jpg\" alt=\"A cross between two agouti mice with the heterozygous genotype AaCc is shown. Each mouse produces four different kinds of gametes (AC, aC, Ac, and ac). A 4 \u00d7 4 Punnett square is used to determine the genotypic ratio of the offspring. The phenotypic ratio is 9\/16 agouti, 3\/16 black, and 4\/16 white.\" width=\"800\" height=\"948\" \/><\/p>\n<p id=\"caption-attachment-1416\" class=\"wp-caption-text\">Figure 1. In mice, the mottled agouti coat color (<em>A<\/em>) is dominant to a solid coloration, such as black or gray. A gene at a separate locus (<em>C<\/em>) is responsible for pigment production. The recessive <em>c<\/em> allele does not produce pigment, and a mouse with the homozygous recessive <em>cc<\/em> enotype is albino regardless of the allele present at the <em>A<\/em> locus. Thus, the <em>C<\/em> gene is epistatic to the <em>A<\/em> gene.<\/p>\n<\/div>\n<p>Epistasis can also occur when a dominant allele masks expression at a separate gene. Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the\u00a0<em>W<\/em> gene (<em>ww<\/em>) coupled with homozygous dominant or heterozygous expression of the <em>Y<\/em> gene (<em>YY<\/em> or <em>Yy<\/em>) generates yellow fruit, and the <em>wwyy<\/em> genotype produces green fruit. However, if a dominant copy of the <em>W<\/em> gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the <em>Y<\/em> alleles. A cross between white heterozygotes for both genes (<em>WwYy<\/em> \u00d7 <em>WwYy<\/em>) would produce offspring with a phenotypic ratio of 12 white:3 yellow:1 green.<\/p>\n<p>Finally, epistasis can be reciprocal such that either gene, when present in the dominant (or recessive) form, expresses the same phenotype. In the shepherd&#8217;s purse plant (<em>Capsella bursa-pastoris<\/em>), the characteristic of seed shape is controlled by two genes in a dominant epistatic relationship. When the genes <em>A<\/em> and <em>B<\/em> are both homozygous recessive (<em>aabb<\/em>), the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. That is, every possible genotype other than <em>aabb<\/em> results in triangular seeds, and a cross between heterozygotes for both genes (<em>AaBb<\/em> x <em>AaBb<\/em>) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.<\/p>\n<p>As you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. Recall the phenotypic inheritance pattern for Mendel&#8217;s dihybrid cross, which considered two non-interacting genes\u20149:3:3:1. Similarly, we would expect interacting gene pairs to also exhibit ratios expressed as 16 parts. Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes.<\/p>\n<div class=\"textbox shaded\">For an excellent review of Mendel&#8217;s experiments and to perform your own crosses and identify patterns of inheritance, visit the\u00a0<a href=\"http:\/\/www2.edc.org\/weblabs\/mendel\/MendelMenu.html\" target=\"_blank\" rel=\"noopener\">Mendel&#8217;s Peas<\/a> web lab.<\/div>\n<p>Whether or not they are sorting independently, genes may interact at the level of gene products such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.<\/p>\n<div class=\"textbox learning-objectives\">\n<h3>In Summary: Epistasis<\/h3>\n<p>Epistasis is a form on non-Mendelian inheritance in which one gene is capable of interfering with expression of another. This is often found associated with gene pathways where the expression of one gene is directly dependent on the presence or absence of another gene product within the pathway.<\/p>\n<p>Watch this video for a quick review of epistasis:<br \/>\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1280814\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script><br \/>\n<iframe loading=\"lazy\" id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/ctjw-ZijciM?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_967db7ce-dc9e-4518-814b-a1acbacbc208\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/967db7ce-dc9e-4518-814b-a1acbacbc208?iframe_resize_id=assessment_practice_id_967db7ce-dc9e-4518-814b-a1acbacbc208\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\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-2762\">\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=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/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>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">All rights reserved content<\/div><ul class=\"citation-list\"><li>Epistasis!. <strong>Authored by<\/strong>: Science Club!. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/ctjw-ZijciM\">https:\/\/youtu.be\/ctjw-ZijciM<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/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":17,"menu_order":17,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"},{\"type\":\"copyrighted_video\",\"description\":\"Epistasis!\",\"author\":\"Science Club!\",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/ctjw-ZijciM\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"}]","CANDELA_OUTCOMES_GUID":"463c4e68-c448-49ab-8747-d1097eea46d4, 8da4ad67-ec54-4c9b-83df-57a17b29914e","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2762","chapter","type-chapter","status-publish","hentry"],"part":258,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/chapters\/2762","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":13,"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/chapters\/2762\/revisions"}],"predecessor-version":[{"id":6014,"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/chapters\/2762\/revisions\/6014"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/parts\/258"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/chapters\/2762\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/wp\/v2\/media?parent=2762"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/pressbooks\/v2\/chapter-type?post=2762"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/wp\/v2\/contributor?post=2762"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology1\/wp-json\/wp\/v2\/license?post=2762"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}