Epistasis and Polygenic Inheritance

Sections

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EPISTASIS AND POLYGENIC INHERITANCE

Epistasis

A gene at one locus affects the expression of another gene located elsewhere

Quantitative characters

Characters that exist along a spectrum

Polygenic inheritance

Single phenotypic character that is affected by multiple genes

Epistasis

Ex. Mouse coat color

B – dominant black allele
b – recessive brown allele
C – dominant pigment deposition allele
c – recessive blocked pigment deposition allele

• Dominant B and dominant C yields black mouse
• Recessive b and dominant C yields brown mouse
• Dominant B and recessive c yields white mouse
• Recessive b and recessive c yields white mouse

Polygenic inheritance and quantitative characters

Ex. Height (exists on a spectrum)

Nature vs Nurture

Norm of reaction

Range of phenotypes that are due to environmental influences

Multifactorial characters

Many factors can influence phenotype

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• Here we will learn about epistasis and polygenic inheritance.

• First, start a table to understand some key terminology.

• Denote that epistasis refers to a how a gene at one locus affects the expression of a gene located elsewhere.

• Denote that quantitative characters are those that exist along a spectrum, such as height – we aren't just tall or short – it's on a continuum.

• Denote that polygenic inheritance describes a single phenotypic character that is affected by multiple genes.

Let's look into these concepts more closely. For epistasis, let's look to see what happens when the expression of one gene is controlled by another.

• Draw two black mice.

• Indicate that they both have the same genotype, big B, little b, big C, little C.

• Write that:
  • Big B is the dominant black allele.
  • Little b is the recessive brown allele.
  • Big C is the dominant allele that allows pigment to be deposited in the coat hair.
  • Little c is the recessive allele that blocks pigment deposition in the coat hair.

• These two alleles are independently assorted, so draw the four by four Punnett square that will show the possible offspring genotypes.

• Write in these genotype combinations.

• Draw colored squiggles in each square to represent the offspring.

Pay attention that mice that have two little c alleles are unable to deposit pigment in their coat hair, no matter the color of the pigment, so they are white!

• For those that have at least one big B allele and at least one big C allele, color them black.

• For those that have two little b alleles and at least one big C allele, color them brown.

• For those that have two little c alleles, keep them white (even if they have a big B allele) – because they cannot deposit pigment into their coat hair.

This is the point of epistasis: the allele at the "c" locus affects the expression of the gene at the "b" locus.

Now let's look a visualization of how the quantitative character of height is acquired through polygenic inheritance. We'll set this up with three hypothetical genes that play a role in human height.

• Draw two average sized people, each with a genotype: big A, little a, big B, little b, big C, little c.

How many potential genotypes can we have?

• For each gene there are 2 possible alleles and we are exploring a trait governed by three genes:
  • So 2^3 (8) independently assorted gamete possibilities from each parent so when crossed, there are a total of 64 possible genotypes in the offspring (8 times 8).
  • However, for our purposes when discussing the contribution to height, functionally all that matters is the total number of dominant alleles in the genotype (not which alleles are dominant), so we will reduce the number of genotypes to 7.

• Write seven possible genotype combinations of their offspring (these aren't all of the possible combinations, but for our purposes they are enough).

• So big A, big A, little b, little b, little c, little c gives the same amount of "height" as little a, little a, big B, little b, big C, little c.
  • As a simplification of how height is controlled in humans, consider that each dominant allele gives an equivalent amount of "height" to an offspring.

• Draw the range of height that the offspring have.

• The tallest is big A, big A, big B, big B, big C, big C.

• The shortest is little a, little a, little b, little b, little c, little c.

We apply these principles to our analysis of polygenic inheritance to simplify complex possibilities.

• Finally, a clearer way to visualize this is to draw a box with six circles in it in line with each of the genotypes of the offspring.

• Fill in one circle in each box per dominant allele in the offspring. So those with the most filled in circles end up being the tallest.

Lastly, it is important to note that genes alone do not explain all of a phenotype. The environment also plays a role. The interplay between genes and the environment is what creates the common argument of nature vs nurture.

• Denote that norm of reaction is the range of phenotypes that are due to environmental influences.

• Denote that multifactorial characters are those where many factors can influence phenotype.

If you get confused, refer to the simple analogy that the genes provide the blueprint for a house, the environment provides the builders and the materials: if there aren't enough materials, perhaps a space that was going to be two rooms now must be one room. Or if there is an abundance of materials, then an extra garage could be added.

This concludes our tutorial on epistasis and polygenic inheritance.

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