Autosomal linkage and epistasisAQA A-Level Biology: Revision notes

Section 1

Autosomal linkage

Autosomal linkage is when two or more genes are on the same autosome. They are inherited together because they are on the same chromosome, so they do not assort independently.

The alleles on one chromosome are shown with a slash: GL/gl means alleles $G$ and $L$ are on one chromosome and $g$ and $l$ on the homologue. With complete linkage, a test cross of GL/gl × gl/gl gives only two phenotypes, grey long and black vestigial, in a 1 : 1 ratio, not the 1:1:1:1 expected if the genes were independent.

Key termsautosomelinkageautosomal linkage

Common mistake

Linked genes are on the same chromosome, not the same gene. Do not say the alleles are on the same locus.

Section 2

Crossing over and recombinants

Linkage is not usually complete. During prophase I of meiosis, crossing over occurs between non-sister chromatids of homologous chromosomes. This swaps alleles and makes new combinations, so a few offspring show recombinant phenotypes.

In a test cross, the two parental phenotypes are most common and the two recombinant phenotypes are few. The recombination frequency is the number of recombinants ÷ total offspring × 100. Genes closer together are crossed over less often, so give fewer recombinants.

Key termscrossing overrecombinantparental types

Exam tip

If the data show two large and two small classes, the genes are linked. Four equal classes mean independent assortment.

Section 3

Predicting linked crosses

A fully labelled diagram for a linked cross shows the alleles on each chromosome. For GL/gl × gl/gl without crossing over: gametes from the female GL and gl; from the male gl; offspring GL/gl (grey long) and gl/gl (black vestigial) in a 1:1 ratio.

If crossing over happens, add the recombinant gametes Gl and gL in small numbers. Offspring numbers follow the same proportions as the gametes.

Key termstest cross

Section 4

Epistasis

Epistasis is when the alleles of one gene mask or suppress the expression of the alleles of another gene. The genes are on different chromosomes, so the ratios are altered versions of 9:3:3:1.

Recessive epistasis: homozygous recessive at one gene masks the other, such as the Labrador $ee$ . Ratio 9 : 3 : 4.
Dominant epistasis: a dominant allele at one gene masks the other, such as $W$ in squash. Ratio 12 : 3 : 1.
Complementary (both genes needed): ratio 9 : 7.

Key termsepistasisrecessive epistasisdominant epistasis

Section 5

Interpreting epistatic crosses

Draw the usual 16-square dihybrid grid, then group the genotypes into phenotypes using the rule for the gene interaction. For Labradors (BbEe × BbEe): $B\_E\_$ = 9 black, $bbE\_$ = 3 brown, and $B\_ee$ + $bbee$ = 3 + 1 = 4 yellow.

If an exam gives a ratio such as 9:3:4, 12:3:1 or 9:7, recognise that the four classes of a dihybrid are grouped, and name the epistasis. The dihybrid ratio only changes because two genotypes look the same.

Key termsphenotypic ratio

Exam tip

The total of the ratio is still 16. Add up the groups to check, for example 9 + 3 + 4 = 16.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Autosomal linkage and epistasis

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
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
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

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).