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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)? • Crossing over! Linkage • If genes reside on the SAME chromosome, then how are recombinants generated? • What happens in Meiosis I? • Crossing over recombines (shuffles) alleles 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 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 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% 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 Sex linkage • Morgan & students made true breeding red-eyed female and true-breeding white-eyed males Sex linkage • Made true breeding red-eyed female and true-breeding whiteeyed males • All offspring red-eyed • Now let’s cross those F1 offspring Sex linkage • Cross those F1 offspring • ONLY males have white eyes, and approximately ½ of all males are affected • Why are females never affected? Sex linkage • Red female with white male • We know male can only contribute a white allele • If mother is heterozygous? 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 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 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 Xchromosome
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