Cross between blue and brown–colored eyes.
In humans, brown eyes are dominant over blue eyes. If a couple who are both heterozygous for brown eyes reproduce with one another, will the child have brown eyes or blue eyes? If we consider, B = brown eyes and b = blue eyes; the cross will be as follows.
P1: Bb × Bb
Genotypic ratio (GR): ¼ BB: 2/4 Bb: ¼ bb
Phenotypic ratio (PR): ¾ or 75% brown eyes: ¼ or 25% blue eyes
The child could have brown eyes or blue eyes. The probability that the child could be brown-eyed is 75% while the probability that it could be blue-eyed is only 25%.
Genetics of blue eyes
Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths
of light tend to be absorbed by the underlying iris pigment epithelium, and shorter
wavelengths are reflected and undergo Rayleigh scattering. The type of melanin present
is eumelanin. The inheritance pattern followed by blue eyes is considered similar
to that of a recessive trait. Eye color inheritance is considered as a polygenic trait,
meaning that it is controlled by the interactions of multiple genes.
Recent research by Hans Eiberg from Department of Cellular and Molecular Medicine
at the University of Copenhagen revealed that people with blue eyes have a single
common ancestor and that genetic mutation lead to blue eyes while initially everyone
had only brown eyes.
The study was published in Human Genetics which suggests that
a mutation in the 86th intron of the HERC2 gene reduced expression of OCA2 gene
with subsequent reduction in melanin production. The authors concluded that the
mutation may have arisen in a single individual in the Near East or around the Black
Sea region 6,000-10,000 years ago during the Neolithic revolution.
Eiberg stated, "A genetic mutation affecting the OCA2 gene in our chromosomes resulted
in the creation of a 'switch, ' which literally 'turned off' the ability to produce
brown eyes". The genetic switch is located in the gene adjacent to OCA2 and rather
than completely turning off the gene, the switch limits its action, which reduces
the production of melanin in the iris. In effect, the turned-down switch diluted
brown eyes to blue eyes.