At least two pairs of genes control eye color. Both pairs influence the production of the pigment, melanin, but act independently. One pair of alleles is B (Brown color; dominant) and b (blue color); the other pair is A (pigment production; dominant) and a (no pigment production; albino). The gene pair aa is epistatic to (masks) B and b and produces the nonpigmented eyes of the albino. What is the type of gene interaction that exists between the two gene pairs? Give the genotypes and phenotypes of the possible offspring of the mating bbAa x Bbaa and indicate the parental phenotypes. (any method)
The Correct Answer and Explanation is:
Type of Gene Interaction:
The interaction between the two gene pairs (B/b and A/a) represents recessive epistasis. In this case, the aa genotype is epistatic, meaning it masks the effect of the B/b gene pair regardless of whether B or b alleles are present. Therefore, individuals with the aa genotype will exhibit the albino phenotype regardless of the B gene’s contribution.
Parental Genotypes and Phenotypes:
- Parent 1: bbAa → blue eyes
- Parent 2: Bbaa → albino
Explanation:
- bb gives blue eyes.
- Aa allows pigment production.
- B gives brown eyes, but in this parent it’s masked by aa, which prevents pigment (albino).
Possible Gametes:
- Parent 1 (bbAa): bA, ba
- Parent 2 (Bbaa): Ba, ba
Offspring Genotypes and Phenotypes:
| Offspring Genotype | B allele | A allele | Pigment? | Phenotype |
|---|---|---|---|---|
| BbAa | B | A | Yes | Brown eyes |
| Bbaa | B | a | No | Albino |
| bbAa | b | A | Yes | Blue eyes |
| bbaa | b | a | No | Albino |
Phenotypic Ratio:
This genetic scenario illustrates recessive epistasis, where one gene (in this case, the A/a gene controlling pigment production) masks the expression of another gene (the B/b gene for brown or blue eye color). The A allele allows for pigment production, which is necessary for either brown or blue eye color to be expressed. However, the aa genotype prevents pigment production entirely, leading to an albino phenotype regardless of the alleles present at the B locus.
When a bbAa (blue-eyed, as bb is recessive and Aa allows pigment) individual mates with a Bbaa (albino, due to the epistatic aa genotype), the offspring can have different combinations of the B and A alleles.
The four potential genotypic outcomes from this cross are:
- BbAa (brown eyes): Both dominant alleles present; pigment is produced and B dominates.
- bbAa (blue eyes): Pigment is produced, but bb results in blue color.
- Bbaa and bbaa (both albino): Despite B or b presence, the aa genotype stops pigment production.
This example demonstrates how gene interactions can complicate classic Mendelian inheritance. Even though one might expect B to always result in brown eyes due to dominance, its effect is suppressed when paired with aa. This kind of interaction is critical in understanding genetic diseases, pigmentation, and complex traits influenced by multiple genes.
