Your eye color is a result of polygenic inheritance-two alleles on one gene and two alleles on another. One gene codes for either brown (B) or blue (b). The other codes for dark green (G) or light green (g). Brown is dominant over green. Someone with the homozygous genotype bbgg has very blue eyes; someone with the homozygous genotype BBGG has very brown eyes. Can a couple who both have brown eyes have a child with blue eyes? Support your answer with a Punnett square.
The correct answer and explanation is :
Yes, a couple who both have brown eyes can have a child with blue eyes if they carry the recessive alleles for blue eyes (b) and light green (g). This is possible if both parents have the genotype BbGg, meaning they each carry a recessive allele for blue (b) and light green (g), even though their phenotype is brown due to the dominance of B.
Punnett Square Analysis
Let’s analyze this with a Punnett square:
Each parent has the genotype BbGg, so we set up a dihybrid cross:
| BG | Bg | bG | bg | |
|---|---|---|---|---|
| BG | BBGG (Brown) | BBGg (Brown) | BbGG (Brown) | BbGg (Brown) |
| Bg | BBGg (Brown) | BBgg (Brown) | BbGg (Brown) | Bbgg (Brown) |
| bG | BbGG (Brown) | BbGg (Brown) | bbGG (Green) | bbGg (Green) |
| bg | BbGg (Brown) | Bbgg (Brown) | bbGg (Green) | bbgg (Blue) |
From the table, we see that one of the possible genotypes is bbgg, which results in blue eyes. The probability of this happening is 1/16 (one out of sixteen combinations).
Explanation
Eye color is controlled by polygenic inheritance, meaning multiple genes influence the final phenotype. In this case:
- The B gene determines whether the eyes are brown (dominant) or blue (recessive).
- The G gene influences green eye color but is recessive to brown.
Even though both parents have brown eyes (BbGg), they each carry the b and g alleles. If they both pass b and g to their child, the child will have the bbgg genotype, resulting in blue eyes.
Thus, it is possible for two brown-eyed parents to have a blue-eyed child if they both carry the recessive alleles for blue and light green eye color.