Question

In genetic crosses ,showing recessive epistasis such as mice coat color ${{F}_{2}}$
phenotypic ratio is
A)9:3:4
B)9:6:1
C)12:3:4
D)13:3

Answer 1

Epistasis explains how phenotypes can be affected by gene interactions. Genes can either mask each other such that one is considered "dominant" or combine to create a new feature.

Complete answer:
1.It is the conditional relationship that can decide a single phenotype of certain traits between two genes.
2.Two alleles which dictate phenotypes are at each locus. They may influence one another in such a way that, irrespective of the allele of one gene, one dominant allele of the other is recessive.
3.A ratio chart or table includes an alternative expression to epistasis. There are 4 total alleles with two genes, meaning there are 16 pairs that can be produced.
4.These pairs of 16 alleles lead to 16 phenotypes. However, due to the dominant and recessive features of the dominant and recessive alleles, not all the combinations are distinct.
5.A 4 X 4 map can be used for four alleles as a visual representation of the 16 possible combinations of alleles.
6.There are six common forms of gene interactions with epistasis: dominant, inhibitory dominant, dominant duplicate, recessive duplicate, association with polymeric genes, and recessive.
7.A gene that hides the influence of another gene is called an epistatic gene in Bateson's terms. For hair colour, there are only three distinct phenotypes: agouti, black and albino.
Due to epistasis, the person with a genotype recessive for both traits, i.e. aabb, has the same albino phenotype as the individuals with aaBB and aaBb.
8.Any phenotype that may be caused by the recessive homozygous recessive bb genotype is obscured by the albino phenotype.

9.AA or Aa are Agouti rats. Mice with genotype aa are albino because, irrespective of the phenotype at the second locus, all pigment development is inhibited.
The B allele (agouti coat) dominates the b allele at its second locus (black coat). This, therefore, gives a ratio of 9:3:4 (agouti : black : albino).

| AB| Ab| aB| ab
---|---|---|---|---
AB| AABB(AGOUTI)| AABb(AGOUTI)| AaBB(AGOUTI)| AaBb(AGOUTI)
Ab| AABb(AGOUTI)| AAbb(BLACK)| AaBB(AGOUTI)| Aabb(BLACK)
aB| AaBB(AGOUTI)| AaBb(AGOUTI)| aaBB(ALBINO)| aaBb(ALBINO)
ab| AaBb(AGOUTI)| Aabb(BLACK)| aabB(ALBINO)| Aabb(ALBINO)

Hence, the correct answer is option (A)

Note: Finally, epistasis may be reciprocal in such a way that either gene expresses the same phenotype when present in the dominant (or recessive form. Epistasis can also occur when the expression of a dominant allele hides a separate gene.