17.8 Epistasis

Question 1

When does epistasis happen

Answer 1

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

Eg if a cat has alleles for black stripes and black fur, so the fur will cover the phenotype of black stripes

Question 2

In mice, gene A controls distribution of melanin in hairs so determines whether they are banded or not, whilst gene B controls the colour of the coat by determining gene A

Gene A has dominant allele A leading to hairs having black bands, and recessive allele a produced black hairs

Mice with black bands are called agouti

Gene B controls colour of coat by controlling expression of gene A,
so dominant allele B leads to the production of melanin whilst recessive allele b leads to no pigment so hairs would be white

If an agouti mouse with genotype AABB is crossed with an albino mouse with aabb what would the genotype and phenotype of the offspring be

Answer 2

AB and ab are the gametes

So AaBb will be the resulting phenotype of all the offspring
So they will all be Agouti
. This is because the expression of gene A (black bands) is affected by the expression of gene B (production of melanin)
. So if gene B is homozygous recessive bb, then no melanin is produced so coat is albino, so in the absence of melanin gene A can’t be expressed so no matter what alleles there are for A, if there is bb the mouse will always be albino

If dominant allele B is present, melanin is produced so if this allele is present with dominant allele A, then banding occurs and agouti coat results.
Where allele B is present with two recessive alleles aa, the hairs will be uniformly black

Question 3

What if two of the offspring are breeded together so F2

Answer 3

In the F2 generation, if those offspring were bred together,
The gametes will be AB, aB, Ab, ab

9 agouti, 4 albino and 3 black would be made
so ratio 9:4:3

Question 4

What is another form of epistasis

Answer 4

Where genes act in sequence by determining the enzymes in a biochemical pathway.

Eg a plant producing red pigment in its petals using the pathway: EA is enzyme A and EB is enzyme B

                      EA Start molecule --> intermediate molecule ---> red pigment
               EB

So production of enzymes A and B is coded for by genes A and B respectively.
Dominant alleles of each gene code for a functional enzyme while recessive alleles code for a non functional enzyme

So if the alleles of either gene are both recessive, the enzyme won’t work so the pathway won’t be completed , so it affects the next gene as even if that one is functional, the effects can’t be expressed because no intermediate molecule is made.