{"id":210921,"date":"2025-05-02T08:44:34","date_gmt":"2025-05-02T08:44:34","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=210921"},"modified":"2025-05-02T08:44:36","modified_gmt":"2025-05-02T08:44:36","slug":"skin-color-in-humans-is-determined-by-a-polygenic-inheritance-system","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/05\/02\/skin-color-in-humans-is-determined-by-a-polygenic-inheritance-system\/","title":{"rendered":"Skin color in humans is determined by a polygenic inheritance system"},"content":{"rendered":"\n

Skin color in humans is determined by a polygenic inheritance system, possibly involving as many as 9 genes. For simplicity let\u2019s consider the influence of 3 genes: A, B, and C, which show incomplete dominance for each allele that darkens skin color. Suppose a women who is AABbCc mates with a man who is AaBbcc. Albino individuals fail to make or distribute melanin and therefore lack pigmentation throughout the body. If the couple mentioned were heterozygote for the albino gene, d, what would be their full genotypes and phenotypes?<\/p>\n\n\n\n

a) Individuals who are homozygous recessive (dd) for the albino gene lack pigmentation no matter how many dominant melanin alleles they have. What type of genetic interaction is this called?<\/p>\n\n\n\n

b) Show all the possible genotypes possible for children between the woman and man above, and number each genotype from lightest to darkest skin coloration. Assume each dominant gene gives a value of 1 to the overall skin color<\/p>\n\n\n\n

The correct answer and explanation is :<\/strong><\/mark><\/p>\n\n\n\n

To solve this question, we need to combine knowledge of polygenic inheritance<\/strong> (genes A, B, and C controlling skin color with incomplete dominance) and epistasis<\/strong> (where one gene, d<\/strong>, can mask the effects of others).<\/p>\n\n\n\n

\n\n\n\n

Given Genotypes:<\/h3>\n\n\n\n
    \n
  • Woman<\/strong>: AABbCcDd<\/li>\n\n\n\n
  • Man<\/strong>: AaBbccDd<\/li>\n<\/ul>\n\n\n\n
    \n\n\n\n

    a) What type of genetic interaction is this?<\/h3>\n\n\n\n

    Individuals homozygous recessive for the albino gene (dd)<\/strong> lack pigmentation regardless of their genotypes for skin color genes (A, B, C). This is an example of:<\/p>\n\n\n\n

    \n

    Epistasis<\/strong> \u2013 specifically, recessive epistasis<\/strong>.<\/p>\n<\/blockquote>\n\n\n\n

    In recessive epistasis, the presence of two recessive alleles (dd)<\/strong> masks the expression of other genes, like A, B, and C.<\/p>\n\n\n\n

    \n\n\n\n

    b) Genotypes and Skin Color Phenotypes<\/h3>\n\n\n\n

    Let\u2019s determine the range of dominant alleles (melanin contributors)<\/strong> from A, B, and C.<\/p>\n\n\n\n

    Each dominant allele (A, B, or C) = +1 unit of pigment<\/strong>
    Maximum from both parents:<\/p>\n\n\n\n

      \n
    • A: AA (2 from mom), Aa (1\u20132 from dad) \u2192 range: 1\u20132<\/li>\n\n\n\n
    • B: Bb x Bb \u2192 possible: BB, Bb, bb \u2192 range: 0\u20132<\/li>\n\n\n\n
    • C: Cc x cc \u2192 possible: Cc or cc \u2192 range: 0\u20131<\/li>\n<\/ul>\n\n\n\n

      So the maximum possible melanin score = 6<\/strong> (A=2, B=2, C=2).
      Minimum = 0 (aabbcc)<\/p>\n\n\n\n

      Genotype Combinations:<\/h3>\n\n\n\n

      We will exclude dd<\/strong> individuals first, as they are albino and have no pigmentation<\/strong> (regardless of other genes). All albino phenotypes = \u201calbino\u201d regardless of A, B, C.<\/p>\n\n\n\n

      For others (with D-<\/strong>), we count the number of dominant alleles.<\/p>\n\n\n\n

      #<\/th>A Alleles<\/th>B Alleles<\/th>C Alleles<\/th>Dominant Total<\/th>Albino Gene<\/th>Phenotype<\/th><\/tr><\/thead>
      1<\/td>aa<\/td>bb<\/td>cc<\/td>0<\/td>D-<\/td>Very Light<\/td><\/tr>
      2<\/td>Aa<\/td>bb<\/td>cc<\/td>1<\/td>D-<\/td>Light<\/td><\/tr>
      3<\/td>aa<\/td>Bb<\/td>cc<\/td>1<\/td>D-<\/td>Light<\/td><\/tr>
      4<\/td>aa<\/td>bb<\/td>Cc<\/td>1<\/td>D-<\/td>Light<\/td><\/tr>
      5<\/td>Aa<\/td>Bb<\/td>cc<\/td>2<\/td>D-<\/td>Light-Medium<\/td><\/tr>
      6<\/td>Aa<\/td>bb<\/td>Cc<\/td>2<\/td>D-<\/td>Light-Medium<\/td><\/tr>
      7<\/td>aa<\/td>Bb<\/td>Cc<\/td>2<\/td>D-<\/td>Light-Medium<\/td><\/tr>
      8<\/td>AA<\/td>bb<\/td>cc<\/td>2<\/td>D-<\/td>Light-Medium<\/td><\/tr>
      9<\/td>AA<\/td>Bb<\/td>cc<\/td>3<\/td>D-<\/td>Medium<\/td><\/tr>
      10<\/td>Aa<\/td>Bb<\/td>Cc<\/td>3<\/td>D-<\/td>Medium<\/td><\/tr>
      11<\/td>AA<\/td>bb<\/td>Cc<\/td>3<\/td>D-<\/td>Medium<\/td><\/tr>
      12<\/td>Aa<\/td>BB<\/td>cc<\/td>3<\/td>D-<\/td>Medium<\/td><\/tr>
      13<\/td>aa<\/td>BB<\/td>Cc<\/td>3<\/td>D-<\/td>Medium<\/td><\/tr>
      14<\/td>AA<\/td>Bb<\/td>Cc<\/td>4<\/td>D-<\/td>Medium-Dark<\/td><\/tr>
      15<\/td>Aa<\/td>BB<\/td>Cc<\/td>4<\/td>D-<\/td>Medium-Dark<\/td><\/tr>
      16<\/td>AA<\/td>BB<\/td>cc<\/td>4<\/td>D-<\/td>Medium-Dark<\/td><\/tr>
      17<\/td>AA<\/td>BB<\/td>Cc<\/td>5<\/td>D-<\/td>Dark<\/td><\/tr>
      18<\/td>Aa<\/td>BB<\/td>Cc<\/td>5<\/td>D-<\/td>Dark<\/td><\/tr>
      19<\/td>AA<\/td>BB<\/td>Cc<\/td>6<\/td>D-<\/td>Very Dark<\/td><\/tr>
      20<\/td>Any<\/td>Any<\/td>Any<\/td>Any<\/td>dd<\/strong><\/td>Albino<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
      \n\n\n\n

      Summary:<\/h3>\n\n\n\n
        \n
      • a) Recessive epistasis<\/strong>: dd masks skin pigmentation genes.<\/li>\n\n\n\n
      • b) Genotype combinations produce a skin tone spectrum from 0\u20136 dominant alleles<\/strong>.<\/li>\n\n\n\n
      • Albino phenotype (dd)<\/strong> overrides all, regardless of melanin genes.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"

        Skin color in humans is determined by a polygenic inheritance system, possibly involving as many as 9 genes. For simplicity let\u2019s consider the influence of 3 genes: A, B, and C, which show incomplete dominance for each allele that darkens skin color. Suppose a women who is AABbCc mates with a man who is AaBbcc. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-210921","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/210921","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=210921"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/210921\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=210921"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=210921"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=210921"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}