Epistasis Read through the test Jot down answers

Epistasis • Read through the test • Jot down answers to these questions: 1. What does "epistasis" mean? 2. How might epistasis result in unexpected phenotypic ratios from genetic crosses? 1

Epistasis When the allele of one gene affects or masks the expression of another in the phenotype 2

Look at the two boxes: They both have six sections but because the background of the box on the right is black you cannot see the lines.

Tabby Cat This cat has black stripes. We can see these stripes because the rest of its fur is a light colour.

Black Cat This cat could also have stripes but because the rest of its fur is black we cannot tell.

So. . . For a cat to be a tabby cat it needs both the alleles for stripes and also the alleles for a light undercoat so that the stripes can be seen.

Another example: mice • Two genes on different chromosomes • Gene A controls distribution of a black pigment called melanin • Allele A: hairs with black bands • Allele a: uniform black hairs • Gene B controls coat colour • Allele B: melanin • Allele b: no pigment • What possible genotypes could lead to each of the phenotypes? 7

Another example: mice • Black mice: aa. BB or aa. Bb • Agouti mice: AABB, Aa. BB, AABb, or Aa. Bb • White (albino) mice: AAbb, Aabb or aabb 8

Another example: mice • An agouti mouse AABB was crossed with an albino mouse aabb • All offspring in F 1 generation are agouti: Aa. Bb • What phenotypes would we expect from a cross between two F 1 individuals? 9

Gametes AB Ab a. B ab AB AABb Aa. BB Aa. Bb Ab AABb AAbb Aa. Bb Aabb a. B Aa. Bb aa. BB aa. Bb ab Aa. Bb Aabb aa. Bb aabb Agouti Albino 9: 4: 3 Black

In corn plants, a dominant allele I inhibits kernel colour, while the recessive allele i permits colour when homozygous. At a different locus, the dominant gene P causes purple kernel colour, while the homozygous recessive pp causes red kernels. 1. If plants heterozygous for both genes are crossed, what will be the phenotypic ratio for their offspring? 2. If a plant that is heterozygous for both alleles is crossed with a plant that is homozygous recessive for both alleles, what will be the phenotypic ratios of the offspring?

Epistasis - biochemical affect Enzyme B Enzyme A Starting molecule Intermediate molecule Pigment Another type of epistasis occurs when genes act in sequence by determining the enzymes in a biochemical pathway

The yellow coat colour of Labrador dogs is due to the presence of the pigment phaeomelanin in the hairs. The black and chocolate coat colours are due to different amounts of another pigment, eumelanin, deposited in these hairs. The more eumelanin there is, the darker the hair. The diagram shows the action of genes E and B in producing the different coat colours Use this information to explain how (i) the genotype bbee produces a yellow coat colour (ii) the genotype Bb. Ee produces a black coat colour 13

• (i) no enzyme coded for when no dominant / E allele; • phaeomelanin not converted – (remains yellow); • (ii) E allele results in enzyme producing eumelanin; • B allele - more eumelanin deposited in hairs; 14