Autosomal linkage and epistasisAQA A-Level Biology: Subtopic test

1. 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).

   (a)

   What is meant by autosomal linkage?

   [1 mark]

   • ATwo genes on the same autosome tend to be inherited together
   • BTwo alleles of the same gene are inherited together
   • CA gene on an autosome is inherited only through the mother
   • DTwo genes on different chromosomes are always inherited together

   (b)

   If linkage were complete, with no crossing over, which offspring would the test cross produce?

   [1 mark]

   • AFour phenotypes in a 1 : 1 : 1 : 1 ratio
   • BGrey long and black vestigial only, in a 1 : 1 ratio
   • CGrey vestigial and black long only, in a 1 : 1 ratio
   • DGrey long only

   (c)

   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]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which genotype gives a yellow Labrador?

   [1 mark]

   • A$BBEe$
   • B$bbEe$
   • C$Bbee$
   • D$BbEE$

   (b)

   What phenotypic ratio of black : brown : yellow is expected from the cross $BbEe$ × $BbEe$?

   [1 mark]

   • A9 : 3 : 3 : 1
   • B12 : 3 : 1
   • C9 : 7
   • D9 : 3 : 4

   (c)

   Explain what is meant by epistasis, using the example of coat colour in these Labradors.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   A $WwYy$ plant is crossed with a green plant. Complete a fully labelled genetic diagram to show the phenotypic ratio expected in the offspring.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two investigations are carried out in plants and insects. In Drosophila, the female (GL/gl) from a test cross with a gl/gl male produced 1000 offspring. In a species of flower, purple pigment is made from a colourless precursor by two enzymes, one coded by gene $A$ and one by gene $B$. Plants need at least one dominant allele of each gene to make purple pigment.

   (a)

   Explain the results of the Drosophila test cross. In the 1000 offspring there were 430 grey long, 430 black vestigial, 70 grey vestigial and 70 black long. Include a calculation of the percentage of recombinant offspring.

   [6 marks]

   (b)

   Two true-breeding white-flowered plants are crossed and all the F1 have purple flowers. The F1 are interbred and the F2 contain purple and white plants in the ratio 9 : 7. Use genetic diagrams to explain these results.

   [6 marks]

   Total for question 4: 12 marks

End of questions