Epistasis TES2

How Genes interact

By the end you should be able to:

Define epistasis

Describe the way in which genes interact

Determine the phenotype of offspring from parental genotypes

Explain what X inactivation is

Recap

So far we have looked at dominant and recessive alleles. However, it is very rare for a characteristic to be controlled by a single gene.

Today we will look at how genes interact with one another.

Epistasis

In order to see some genes it is sometimes necessary for other genes not to be expressed.

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.

How is this controlled?

In tabby cats there are two different genes involved.

The first gene control whether the cat has is the Agouti gene- this determines grey bands or solid black. There is the dominant A and the recessive a. A means stripes and a means solid coat colour.

AA or Aa = Grey bands

Aa = Solid black coat

The 2nd allele involved is the allele for coat pattern. There are three possible alleles:

T= Vertical stripes

TB= Freckled

tb= Blotched

TT, or Ttb = Vertical stripes

TTB, TBTB or TBtb = Freckled

tbtb = Blotched

Work out the phenotypes for the following:

aatbtb

AaTT

AATTB

AaTtb

Work out the phenotypes for the following:

aatbtb = Black solid

AaTT = Vertical stripes

AATTB = Freckled

AaTtb = Vertical stripes

Draw a Punnett for the following cross

AAtbtb + aaTT

First work out what the gametes will be- then decide on the genotypes and phenotype of the offspring.

X inactivation As females have

two copies of the x chromosome early in the development of the of the embryo one x chromosome in each cell is supercoiled to prevent transcription. This supercoiled chromosome forms a visible lump know as the Barr body.

The process seems to be random so groups of cells in females will have different different X chromosomes inactivated.

This process can lead to interesting coat colouration. If the X chromosome from the father has the gene for a orange coat and the X chromosome from the mother has a gene for a black coat a cat can have a blotched appearance.

Male cats are black or orange

Female cats are blotched

Have we learned?

By the end you should be able to:

Define epistasis

Describe the way in which genes interact

Determine the phenotype of offspring from parental genotypes

Explain what X inactivation is