Download - Epistasis TES2
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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
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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.
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Epistasis
In order to see some genes it is sometimes necessary for other genes not to be expressed.
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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.
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Tabby Cat
This cat has black stripes. We can see these stripes because the rest of its fur is a light colour.
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Black Cat
This cat could also have stripes but because the rest of its fur is black we cannot tell.
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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.
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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
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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
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Work out the phenotypes for the following:
aatbtb
AaTT
AATTB
AaTtb
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Work out the phenotypes for the following:
aatbtb = Black solid
AaTT = Vertical stripes
AATTB = Freckled
AaTtb = Vertical stripes
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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.
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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.
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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.
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Male cats are black or orange
Female cats are blotched
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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