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Linkage, Sex linkage, Pedigrees

Linkage, Sex linkage, Pedigrees. Linked genes. We know two traits should assort independently. Why? So, offspring phenotype ratios should be: 9:3:3:1 But they’re not Why?. Linked genes. The genes reside on the SAME chromosome What produces recombinants (purple, round & red, long)?

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Linkage, Sex linkage, Pedigrees

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  1. Linkage, Sex linkage, Pedigrees

  2. Linked genes • We know two traits should assort independently. Why? • So, offspring phenotype ratios should be: • 9:3:3:1 • But they’re not • Why?

  3. Linked genes • The genes reside on the SAME chromosome • What produces recombinants (purple, round & red, long)? • Crossing over!

  4. Linkage • If genes reside on the SAME chromosome, then how are recombinants generated? • What happens in Meiosis I? • Crossing over recombines (shuffles) alleles

  5. Sex Linkage & Recombination • T. H. Morgan and his students discovered both phenomena in fruit flies • Eye color – sex linkage • Body color, wing type, other eye color – autosomal linkage

  6. Recombination • G = gray body (dom) • g = black body (rec) • L = long wings (dom) • l = short wings (rec) • GgLl x ggll  • GgLl ALL Gray long • Few Ggll & ggLl

  7. Recombination • G = gray body (dom) • g = black body (rec) • L = long wings (dom) • l = short wings (rec) • All on same chromosome • GgLl x ggll  • Few Ggll & ggLl • Recombination frequency = 17%

  8. Recombination freq & Genetic map • Recombination frequency = 17% • If possibility of crossing over is = at all points along chromosome, then the farther apart two genes are, the greater the chance of recombination between them

  9. Sex linkage • Morgan & students made true breeding red-eyed female and true-breeding white-eyed males

  10. Sex linkage • Made true breeding red-eyed female and true-breeding white-eyed males • All offspring red-eyed • Now let’s cross those F1 offspring

  11. Sex linkage • Cross those F1 offspring • ONLY males have white eyes, and approximately ½ of all males are affected • Why are females never affected?

  12. Sex linkage • Red female with white male • We know male can only contribute a white allele • If mother is heterozygous?

  13. Sex linkage • Red female with white male • We know male can only contribute a white allele • If mother is heterozygous? • 1:1 white-eyed : red-eyed, irrespective of sex

  14. Common sex-linked disorders • Red-green color blindness • Lack either red sensitive or green-sensitive cone cells in retina • Hemophilia • Lack one or more blood-clotting proteins • Duchenne muscular dystrophy • Progressive muscle wasting

  15. Conclusions • Appearance of few recombinants signals some degree of linkage • For any X-linked gene, males with a recessive allele will show the recessive phenotype. • When looking at pedigrees of disease phenotypes, an excess of affected males suggests that disease gene lies on X-chromosome

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