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 white- eyed males All offspring red-eyed Now let’s cross those F 1 offspring

Sex linkage Cross those F 1 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 X- chromosome