17.8 - Epistasis Flashcards
What is epistasis?
- Epistasis occurs when the allele of one gene affects or masks the expression of another gene in the phenotype.
- It can alter expected Mendelian ratios in genetic crosses by modifying how certain traits are expressed.
How does epistasis affect coat colour in mice?
In mice, epistasis involves two genes.
- Gene A controls whether hairs are banded (A = banded, a = uniform black).
- Gene B controls melanin production (B = melanin, b = no melanin).
If Gene B is recessive (bb), no melanin is produced, and the coat is albino, masking the effect of Gene A.
Describe the coat colours produced by different combinations of alleles for Gene A and Gene B in mice.
- Agouti (grey-brown coat): A-B- (dominant alleles of both genes present, black-banded hairs).
- Black: aaB- (recessive alleles for gene A, dominant B, producing uniform black hairs).
- Albino: A-bb or aabb (recessive alleles for Gene B, no melanin produced, resulting in a white coat regardless of gene A).
What phenotypic ratio is produced in the F₂ generation when crossing agouti and albino mice?
The F₂ generation produces a 9:4:3 ratio:
9 agouti mice (A-B-),
4 albino mice (A-bb or aabb),
3 black mice (aaB-).
—> This deviation from Mendel’s expected 9:3:3:1 ratio occurs because the recessive bb genotype in Gene B masks the expression of Gene A.
How does the B gene in mice affect the expression of the A gene?
- Gene B controls whether melanin is produced.
- If B is homozygous recessive (bb), no melanin is produced, and the mouse’s coat is albino.
- In this case, Gene A (which controls banding of melanin) cannot be expressed because there is no melanin to distribute, regardless of its alleles.
What are the different forms of epistasis?
- Recessive Epistasis: A recessive allele of one gene (e.g., bb) masks the effect of another gene, like in mice where no melanin is produced when bb is present, resulting in an albino coat.
- Biochemical Pathway Epistasis: Genes in sequence code for enzymes in a biochemical pathway. If a gene coding for an enzyme is non-functional (recessive), the pathway stops, and no final product is made, regardless of the function of subsequent genes.
How does epistasis affect enzyme production in a biochemical pathway?
- In a pathway, two genes may code for enzymes A and B, which are needed to convert a starting molecule to a final product (e.g., red pigment).
- If either enzyme is non-functional due to recessive alleles (aa or bb), the pathway cannot proceed, and the product (red pigment) is not formed, regardless of whether the other gene functions normally.
In a biochemical pathway where genes A and B code for enzymes, what happens if either gene is homozygous recessive?
- If either gene is homozygous recessive (aa or bb), the enzyme it codes for is non-functional.
- This means the biochemical pathway is blocked, and the final product (e.g., red pigment) cannot be produced, regardless of the allele combination for the other gene.
How does epistasis modify Mendelian ratios in genetic crosses?
- Epistasis modifies the typical 9:3:3:1 ratio seen in dihybrid crosses.
- In the case of recessive epistasis, the expected phenotypic ratio changes to 9:4:3 because the recessive alleles of one gene (e.g., bb) mask the expression of another gene.
if you are given a genetic cross where you observe a 9:4:3 phenotypic ratio in the F₂ generation, what does this suggest?
- A 9:4:3 phenotypic ratio in the F₂ generation suggests that epistasis is at play.
- Specifically, it may indicate recessive epistasis, where a homozygous recessive genotype in one gene (e.g., bb) masks the expression of another gene, resulting in a modified ratio.
Using the example of epistasis in mice in this topic, consider a cross between mouse 1 that is heterozygous for gene A (colour distribution) and homozygous recessive for gene B (melanin production) with mouse 2 that is heterozygous for both genes.
State the colour of mouse 1 and the colour of mouse 2.
mouse 1 = albino(white)
mouse 2 = agouti
Using the example of epistasis in mice in this topic, consider a cross between mouse 1 that is heterozygous for gene A (colour distribution) and homozygous recessive for gene B (melanin production) with mouse 2 that is heterozygous for both genes.
Calculate, and list, the genotypes of the offspring resulting from this cross.
- AABb
- AaBb
- AAbb
- Aabb
- AaBb
- aaBb
- Aabb
- aabb
Using the example of epistasis in mice in this topic, consider a cross between mouse 1 that is heterozygous for gene A (colour distribution) and homozygous recessive for gene B (melanin production) with mouse 2 that is heterozygous for both genes.
State the ratio of different phenotypes produced by this cross.
4 albino : 3 agouti : 1 black
Some varieties of corn, Zea mays, have purple seeds due to the presence of a pigment called anthocyanin in their seed coats, In the absence of the pigment, the seeds are white. The production of anthocyanin is controlled by two genes, A and B.
One pure-breeding variety of white-seeded corn with the genotype AAbb was crossed with another pure-breeding variety of white-seeded corn with the genotype aaBB. All the offspring had purple seeds. A cross between two of the F, generation produced a ratio of nine purple-seeded plants to seven white-seeded plants.
Deduce the genotype of the F, generation.
AaBb
Some varieties of corn, Zea mays, have purple seeds due to the presence of a pigment called anthocyanin in their seed coats, In the absence of the pigment, the seeds are white. The production of anthocyanin is controlled by two genes, A and B.
One pure-breeding variety of white-seeded corn with the genotype AAbb was crossed with another pure-breeding variety of white-seeded corn with the genotype aaBB. All the offspring had purple seeds. A cross between two of the F, generation produced a ratio of nine purple-seeded plants to seven white-seeded plants.
Draw a genetic diagram showing the cross between two F, individuals.
