Autosomal linkage and epistasisAQA A-Level Biology: Flashcards

Exam questions on Autosomal linkage and epistasis

1. In Drosophila, a gene for body colour has alleles $G$ (grey, dominant) and $g$ (black), and a gene for wing length has alleles $L$ (long, dominant) and $l$ (vestigial). Both genes are on the same autosome. A female is heterozygous, with the alleles $G$ and $L$ on one chromosome and $g$ and $l$ on its homologue (written GL/gl). She is test crossed with a black, vestigial-winged male (gl/gl).
   The test cross produced a few grey vestigial-winged and black long-winged offspring, as well as the two main types. Explain how these offspring arose.2 marks
2. In Labrador retrievers, one gene controls pigment type: $B$ (black) is dominant to $b$ (brown). A second gene, on a different chromosome, controls whether pigment is deposited in the hair: the dominant allele $E$ allows deposition, but dogs that are $ee$ have no pigment deposited and are yellow whatever their genotype at the first gene. Two black dogs, both $BbEe$, are mated.
   Explain what is meant by epistasis, using the example of coat colour in these Labradors.2 marks
3. In summer squash, fruit colour is controlled by two genes on different chromosomes. The dominant allele $W$ prevents any colour forming, so the fruit is white. In plants that are $ww$, the second gene acts: $Y$ gives yellow fruit and $yy$ gives green fruit. Two plants of genotype $WwYy$ are crossed.
   Explain why the offspring of this cross occur in the phenotypic ratio 12 white : 3 yellow : 1 green, and not 9 : 3 : 3 : 1.3 marks