The genetics of eye color is contingent on two genes:
Each human has two genes for eye color - one Brown/Blue and one Green/Hazel. Brown is dominant
over all other alleles. Green and hazel have incomplete dominance. The heterozygous allele
combination Gg produces hazel eyes. The chart below shows the phenotypes for all of the gene
combinations.
Brown
Green
BBGG
BBGg
BBgg
BbGG
BbGg
Bbgg
Hazel
bbGG
Blue
bbGg
bbgg
So how do we calculate the probability of inheritance of eye color?
Mom has allele combination BbGG and she has brown eyes.
Dad has allele combination bbGg had he has hazel eyes.
Look at the Punnett Squares that result.
Brown/Blue
B
b
b
Bb
bb
b
Bb
bb
Green/Hazel
G
G
G
GG
GG
g
Gg
Gg
So the possibilities of the offspring’s genotypes are: BbGG, BbGg, bbGG, or bbGg.
BbGG and BbGg will result in brown eyes.
bbGG will result in green eyes.
bbGg will result in hazel eyes.
The probability of brown eyed children is 50% (or 2 out of 4 possibilities). The probably of green eyes
and of hazel eyes are both 25% (1 out of 4 possibilities). The probability of a blue eyed offspring is 0%.