The child inherits eye color from their parents in a recessive-dominant manner. For the distribution of pigment in the anterior layer of the iris, two pairs of genes are responsible to a greater extent, a varied combination of alleles of which determines the color of the eyes in a child.
Eyes have three main colors - brown, blue and green, their inheritance determines two pairs of genes. The shades of these colors are determined by the individual characteristics of the organism to distribute melanin in chromatophores, which are located in the iris. Other genes, which are responsible for hair color and skin tone, also affect the shade of eye color. For fair-haired people with fair skin, blue eyes are typical, and representatives of the Negroid race have dark brown eyes.
The gene, which is responsible only for eye color, is located on chromosome 15 and is called HERC2, the second gene, EYCL 1, is located on chromosome 19. The first gene carries information about hazel and blue colors, the second - about green and blue.
The brown color is dominant in the HERC2 allele, green in the EYCL 1 allele, and blue eyes are inherited in the presence of a recessive trait in two genes. In genetics, it is customary to designate a dominant with a capital letter of the Latin alphabet, a recessive trait is a lowercase letter. If capital and lowercase letters are found in the gene, the organism is heterozygous for this trait and has a dominant color, and a child can inherit a hidden recessive trait. A "suppressed" trait will appear in a baby when an absolute recessive allele is inherited from two parents. That is, brown-eyed heterozygous parents may well have a blue-eyed child or with green eyes.
Using Latin letters, the brown eye color, which is determined by the HERC2 gene, can be designated AA or Aa, the set aa corresponds to blue eyes. When a trait is inherited, one letter is transmitted to the child from each parent. Thus, if the father has a homozygous sign of brown eyes, and the mother is blue-eyed, then the calculations look like this: AA + aa = Aa, Aa, Aa, Aa, i.e. the child can only get the set Aa, which is manifested by the dominant, i.e. the eyes will be brown. But if the father is heterozygous and has a set of Aa, and the mother is blue-eyed, the formula looks like: Aa + aa = Aa, Aa, aa, aa, i.e. there is a 50% chance that a blue-eyed mother's baby will have the same eyes. In blue-eyed parents, the formula for eye inheritance looks like: aa + aa = aa, aa, aa, aa, in this case the baby inherits only the recessive allele aa, i.e. the color of his eyes will be blue.
In the EYCL 1 allele, the eye color trait is inherited in the same way as in the HERC2 gene, but only the letter A denotes green. It is arranged by nature so that the existing dominant trait of brown eyes in the HERC2 gene "wins" the present green trait in the EYCL 1 gene.
Thus, a child always inherits brown eye color if one of the parents has a homozygous dominant AA set in the HERC2 gene. If a parent with brown eyes passes on the recessive gene a to the child, i.e. a sign of blue eyes, the color of the eyes determines the presence of a green dominant trait in the EYCL 1 gene. In cases when a parent with green eyes does not transmit the dominant trait A, but "presents" the recessive allele a, the child is born with blue eyes.
Since eye color is determined by two genes, its shades are obtained from the presence of non-manifested signs. If the child has a genetic set of AA in the HERC2 allele, then the eyes will be dark brown. The presence of type Aa brown eyes in the HERC2 gene, and the recessive aa trait in the EYCL 1 gene, results in light brown eyes. The homozygous AA green eye trait at the EYCL 1 locus determines a more intense color than the heterozygous Aa set.