Brown eye color in humans is a dominant trait in gene inheritance, and a recessive gene is responsible for light eyes (gray, blue, green). But this does not mean that brown-eyed parents cannot have a blue-eyed child, because in their genome there may be recessive genes that have met with each other. In addition, genetic inheritance of eye color, like other traits, is actually a much more complex and confusing process than it seems.
Principles of Inheritance of Eye Color
Human eye color depends on the pigmentation of the iris, which contains chromatophores with melanin. If there is a lot of pigment, the eyes turn out brown or hazel, and in blue-eyed people, the production of melanin is impaired. A mutation is responsible for the light color of the eyes, which occurred not so long ago - about seven thousand years ago. Gradually, it spread, but the mutated gene is recessive, so there are much more brown-eyed people on the planet.
In a simplified form, the laws of inheritance can be described as follows: during the formation of a germ cell, a person's chromosome set is divided into two halves. Only one second of the human genome gets into the cell, including one gene responsible for eye color. When two germ cells fuse to form a fetus, genes meet each other: two genes end up in the region responsible for eye color. They will remain in the genome of the new person, but only one can manifest itself in the form of external signs - the dominant one, which suppresses the action of another, recessive gene.
If there are two dominant ones, for example, those responsible for brown eye color, then the child's eyes will be brown, if two recessive ones, then light.
Blue-eyed child with brown-eyed parents
Brown-eyed parents can have a blue-eyed child if both have recessive genes in their genome that are responsible for the light shade of the eyes. In this case, a dominant appears in part of the germ cells, which manifested itself in the form of brown eyes, and in the other part - a recessive gene. If, during conception, cells with genes for light eyes meet each other, then the child will have light eyes.
The probability of such an event is about 25%.
Much less common are situations when blue-eyed parents have brown-eyed children. From the point of view of the simplified laws of genetics described above, it is impossible to explain this: where could the dominant gene in the baby come from, if the parents did not show it, then they do not have it? And yet there are such cases, and geneticists easily explain this.
In fact, the principles of inheritance of traits are much more complicated than it seems. In humans, not one pair of genes is responsible for eye color, but a whole set in which genes inherited from many previous generations are mixed. The combinations can be very diverse, so you can never predict 100 percent what kind of eyes a child will have. Even scientists still cannot fully understand the inheritance patterns: a variety of genes in different parts of chromosomes can affect eye color.
