Weekly Homework 10

Linked Genes

In a dihybrid cross, offspring phenotypes deviate significantly from the expected 9:3:3:1 ratio A dihybrid individual produces four types of gametes in which the two parental types significantly outnumber the two recombinant types The two genes are on a single chromosome A cross between a heterozygote and a homozygous recessive individual yields offspring in four phenotypic classes that deviate significantly from a 1:1:1:1 ratio

Unlinked Genes

In a dihybrid cross, offspring phenotypes occur in a 9:3:3:1 ratio A cross between a heterozygote and a homozygous recessive individual yields offspring in four phenotypic classes exhibiting a 1:1:1:1 ratio A dihybrid individual produces four types of gametes in a 1:1:1:1 ratio The two genes are on different chromosomes, or vary far apart on the same chromosome

Linked

In the offspring of the cross S R / s r x s r / s r, more will be S R / s r and s r / s r, and fewer will be S r / s r and s R / s r In dihybrid test-crosses, the allele combinations in the parents are seen more often than recombinants in the offspring

Not Linked

In the offspring of the cross S R / s r x s r / s r, the expected ratio of genotypes is 1 S R / s r: 1 s r / s r: 1 S r / s r: 1 s r / s r The phenotypic ratios obtained are predicted by Mend's Laws of Independent Assortment and Segregation

In Drosophila, purple eyes, grey body, and short bristles are determined by recessive alleles of three linked autosomal genes called purple (p), grey (g), and short (s). A wild-type female fly is heterozygous for purple eyes, grey body and short bristles. She is crossed to a male fly who is homozygous for purple, grey and short. The following progeny phenotypes were observed

Phenotype Alleles inherited from mother purple, grey, short p g s wild-type p+ g+ s+ purple p g+ s+ grey, short p+ g s purple, short p g+ s grey p+ g s+ short p+ g+ s purple grey p g s+