Is it possible to calculate the eye colour before birth?
The eye colour is genetically determined and depends on the eye colours of both parents. However, the final eye colour of the newborn cannot be calculated exactly, only probabilities can be given. The genes determine how much melanin is produced.
Each gene is present in two copies (alleles) (diploid). One allele comes from the mother, the second from the father. The trait for brown eyes is inherited dominantly, while the trait for blue eyes is inherited recessively.
Dominant means that one copy of the trait is sufficient for the child to have brown eyes. For blue eyes, which are inherited recessively, both copies must carry the blue trait. If there is only one recessive copy, the feature is “overwritten” by another, dominant copy.
It is now known that several genes are responsible for the inheritance of eye color, so that the inheritance is a little more complex. For example, other genes are responsible for the respective colour intensity and shades of the individual eye colour. Not only brown eyes are inherited dominantly.
Green eyes are also passed on dominantly, but are less dominant than brown eyes. Blue eyes are passed on recessively. Grey eyes have even less assertiveness than blue eyes, they are also a recessive trait.
Parents’ eyes brown – the child’s blue?
Why children whose parents both have a brown eye colour can also develop a blue eye colour is explained by heredity. Brown eye color is inherited as the dominant trait, while blue eye color is inherited recessively. So in the parents it is sufficient if one of the two copies of a gene has the brown trait, because the brown trait always displaces another eye color trait, because it has the strongest dominance.
The second copy of the trait can therefore carry the blue trait, but this is not expressed. In inheritance, only one copy of a gene, i.e. one eye color trait, is passed on from mother and father. If the child then has blue eyes, two copies with the blue characteristic must be present in the child because the gene for blue eyes is recessive.
Probabilities for the different eye colours
The final eye colour cannot be predicted one hundred percent. But the eye colour of mother and father can be used to determine different probabilities. If both parents have brown eyes, more than 50% of the child will also have brown eyes later.
However, about 20% of the eyes can also be green and just under 10% blue. Since the brown eye trait is inherited dominantly, the parents with brown eyes may also have the trait for green or blue eyes, which is not pronounced, but can be inherited. If one parent has brown eyes and the other blue eyes, the probability is 50% for brown or blue eyes.
The same applies if one parent has brown eyes and the other parent has green eyes. The best way to predict the probability for eye colour is if both parents have blue eyes. In this case, the eye colour is almost one hundred percent likely to be blue.
This is because blue eyes require both parents to have two blue features, so the child will automatically have two blue features. If both parents have green eyes, then the child has 75% green eyes and 25% blue eyes. Brown eyes are out of question, because then one of the parents must have brown eyes.
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