Question

If:

My grandparents on my moms side both have brown eyes.

My mom has brown eyes.

My grandparents on my dads side both have blue eyes.

My dad has blue eyes.

Then:

Why do I have hazel eyes?

Answer 1

Brown eyes are due to a dominant allele of the gene while blue eyes are due to recessive allele of a gene. The eye color is determined by amount of melanin pigment produced by the iris. Brown eyed individuals have highest melanin, with maximum melanin being present in the back layer of iris. Less concentration of melanin is present in front layer of iris. This high level of melanin will absorb most light entering the eye and hence, the eyes are brown in color. In blue eyed individuals, front layer of iris has no melanin. Thus, most light is scattered which is blue light. Hence, the eyes appear blue.

The color of eye is decided by the OCA2 gene which codes for the P protein. This P protein is required for melanin production by melanocytes. However, only 75% melanin production depends on OCA2 genes. Rest 25% is controlled by 16 other genes such as ASIP, IRF4, TPCN2, TYR, etc.. The OCA2 gene is regulated by HERC2 gene and polymorphism in HERC2 can affect melanin production in response to OCA2 gene. HERC2 has an intronic region that control OCA2 gene. HERC2 is present close to OCA2 gene on chromosome 15.

Hazel eyes are due to combination of blue and brown eye color. These individuals produce less melanin as compared to brown eyes but more melanin than brown eyed people. This allows the eyes to absorb less light as compared to brown eyed people. They also scatter the shorter wavelength of light, which is blue and green light. Hence, the eyes will be Hazel. Hazel eyes are due to reduction in melanin production by B allele due to a combination of the other genes involved in eye color.

Consider the dominant allele to be ‘B’ and the recessive allele to be ‘b’. Hence, they have bb geneotype as it is a recessive condition. Consider there to be another gene that codes for two colors-green (G) and Hazel (g). This gene is under incomplete dominance. BBGG, BBGg, BBgg, BbGG, BbGg, Bbgg will have brown eyes. The genotype bbGG will be green eyes, while bbGg will be hazel eyed (due to one G allele). The genotype bbgg will have blue eyes.

The grandparents and the father are blue eyed individual. Hence, their genotype is bbgg. However, brown eyes are determined by various genotypes. Hence, the mother and her parents can have any of the genotypes, BBGG, BBGg, BBgg, BbGG, BbGg, Bbgg. Suppose the mother’s genotype is BbGg. Consider a cross between BbGg and bbgg (one example).

		BbGg	Mother
		BG	bG	Bg	bg
bbgg	bg	BbGg Brown eyes	bbGg Hazel eyes	Bbgg Brown eyes	bbgg Blue eyes
Father	bg	BbGg Brown eyes	bbGg Hazel eyes	Bbgg Brown eyes	bbgg Blue eyes
	bg	BbGg Brown eyes	bbGg Hazel eyes	Bbgg Brown eyes	bbgg Blue eyes
	bg	BbGg Brown eyes	bbGg Hazel eyes	Bbgg Brown eyes	bbgg Blue eyes

Thus, brown eyed and blue eyes parents can give Hazel eyed offspring (25%).