Epistasis TES2
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Transcript of 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