A5. Epistasis Flashcards
What is epistasis?
Many different genes can control the same characteristic they interact to form the phenotype. This can be because the allele of one gene ______ the expression of the alleles of other genes- this is called epistasis.
Many different genes can control the same characteristic they interact to form the phenotype. This can be because the allele of one gene masks (blocks) the expression of the alleles of other genes- this is called epistasis.
Example 1-Widow’s peak
In humans a widow’s peak (see Figure 1) is controlled by one gene and baldness by others. If you have the alleles that code for baldness, it doesn’t matter whether you have the allele for a widow’s peak or not, as you have no hair. The baldness genes are epistatic to the widow’s peak gene, as the baldness genes mask the expression of the widow’s peak gene.
Example 1-Widow’s peak
In humans a widow’s peak (see Figure 1) is controlled by one gene and baldness by others. If you have the alleles that code for baldness, it doesn’t matter whether you have the allele for a widow’s peak or not, as you have no hair. The baldness genes are epistatic to the widow’s peak gene, as the baldness genes mask the expression of the widow’s peak gene.
Example 2 - Flower colour-
Flower pigment in a plant is controlled by two genes. Gene 1 codes for a yellow pigment (Y is the dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R is the dominant orange allele). If you don’t have the Y allele it won’t matter if you have the R allele or not as the flower will be colourless. Gene 1 is epistatic to gene 2 as it can mask the expression of gene 2.
Example 2 - Flower colour-
Flower pigment in a plant is controlled by two genes. Gene 1 codes for a yellow pigment (Y is the dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R is the dominant orange allele). If you don’t have the Y allele it won’t matter if you have the R allele or not as the flower will be colourless. Gene 1 is epistatic to gene 2 as it can mask the expression of gene 2.
Tip: Epistatic genes are usually at different ____.
Tip: Epistatic genes are usually at different loci (different positions on chromosomes).
Phenotypic ratios for epistatic genes
Crosses involving epistatic genes don’t result in the ____________ phenotypic ratios, eg, if you cross two heterozygous orange flowered plants (YyRr) from the example above you wouldn’t get the expected 9:3:3:1 phenotypic ratio for a normal dihybrid cross.
The phenotypic ratio you would expect to get from a dihybrid cross involving an epistatic allele depends on whether the epistatic allele is ___________or ____________
Crosses involving epistatic genes don’t result in the expected phenotypic ratios, eg, if you cross two heterozygous orange flowered plants (YyRr) from the example above you wouldn’t get the expected 9:3:3:1 phenotypic ratio for a normal dihybrid cross.
The phenotypic ratio you would expect to get from a dihybrid cross involving an epistatic allele depends on whether the epistatic allele is recessive or dominant.
Recessive epistatic alleles
If the epistatic allele is recessive then two copies of it will mask (block) the expression of the other gene. If you cross a homozygous recessive parent with a homozygous dominant parent you will produce a ::_ phenotypic ratio of dominant both: dominant epistatic, recessive other : recessive epistatic in the F² generation.
If the epistatic allele is recessive then two copies of it will mask (block) the expression of the other gene. If you cross a homozygous recessive parent with a homozygous dominant parent you will produce a 9:3:4 phenotypic ratio of dominant both: dominant epistatic, recessive other : recessive epistatic in the F² generation.
Recessive epistatic alleles - example
The flower colour example above is an example of a recessive epistatic allele. If a plant is homozygous recessive for the epistatic gene (yy) then it will be colourless, masking the expression of the orange gene. So if you cross _______________parents you should get a 9:3:4 ratio of orange : yellow :white in the F2 generation. You can check the phenotypic ratio is right using a genetic diagram, like the one in Figure 2
Figure 2: Genetic diagram of a dihybrid cross with two heterozygous parents, involving a recessive epistatic gene.
The flower colour example above is an example of a recessive epistatic allele. If a plant is homozygous recessive for the epistatic gene (yy) then it will be colourless, masking the expression of the orange gene. So if you cross homozygous parents you should get a 9:3:4 ratio of orange : yellow :white in the F2 generation. You can check the phenotypic ratio is right using a genetic diagram, like the one in Figure 2
Tip: All of the F offspring have to have the genotype YyRr because the only gametes you can get from the parents are YR and yr.
Tip: All of the F offspring have to have the genotype YyRr because the only gametes you can get from the parents are YR and yr.
Tip: This is a __________cross because you’re looking at the inheritance of two genes.
Tip: This is a dihybrid cross because you’re looking at the inheritance of two genes.
Dominant epistatic alleles
If the epistatic allele is dominant, then having at least one copy of it will mask (block) the expression of the other gene. Crossing a homozygous recessive parent with a homozygous dominant parent will produce a __:: phenotypic ratio of dominant epistatic: recessive epistatic, dominant other : recessive both in the F2 generation.
If the epistatic allele is dominant, then having at least one copy of it will mask (block) the expression of the other gene. Crossing a homozygous recessive parent with a homozygous dominant parent will produce a 12:3:1 phenotypic ratio of dominant epistatic: recessive epistatic, dominant other : recessive both in the F2 generation.
Exam Tip:
Make sure you know the difference between dominant and recessive epistatic alleles. The phenotypic ratios you’d expect to get are different for each.
Exam Tip:
Make sure you know the difference between dominant and recessive epistatic alleles. The phenotypic ratios you’d expect to get are different for each.
