Question

If an agouti (CcAa) is crossed with albino mice (ccAA), then many albino mice are produced in resulting progeny?
A) 4
B) 9
C) 2
D) 3

Answer 1

Coat colour in mice is a phenomenon which is regulated by the presence of recessive epistasis where the recessive allele of one gene dominates the other gene and alternates the genotype and the phenotype.

Complete answer: The coat colour of mice is an example of recessive epistasis. Recessive epistasis occurs when the recessive allele of one gene dominates the allele of the second gene. In mice, the wild type coat colour i.e., agouti is dominated over the albino coat colour. When there is a cross between the agouti and albino mice, i.e. the cross of progenies ‘CcAa’ and ‘ccAA’, it gives the result of progeny with CcAA, CcAa, ccAA, ccAa. In this cross, the phenotypic ratio would be $9:3:4$. Here, 9 belongs to the agouti colour, 3 belongs to the solid colour and 4 have the albino colour. Therefore, a cross between CcAa X ccAA would generate four types of offspring with the genotype CcAA, CcAa, ccAa, and ccAa. Here, two are agouti, and the other two are albino. The mice with the genotype cc will have the albino coat colour, so here in this question, there will be 2 progenies produced or $50\%$ progenies.
Parents: CcAa X ccAA
Gametes: CA, Ca, cA, ca
F1 generation:

| CA| Ca| cA| ca
---|---|---|---|---
cA| CcAA (Agouti)| CcAa (Agouti)| ccAA (Albino)| CCAa (Albino)
cA| CcAA (Agouti)| CcAa (Agouti)| ccAA (Albino)| ccAa (Albino)

Hence, the correct answer is option C.

Note: Dominant epistasis is when only one allele of the gene that shows epistasis can mask alleles of the other gene which produces a segregation ratio such as 12:3:1 and can be viewed as a modification of the 9:3:3:1 ratio.