Nature is very capricious at times, and genetics too, so much so that a few days ago I met a couple who had brown eyes, both the father and the mother whose baby had blue eyes. Well, I said capricious for not calling it another way, because they told me that they were beginning to be a little tired of looks and comments, with more or less grace, related to electricians, plumbers and the butane one.
It is true that it is highly unlikely that two parents with brown eyes will have a child with blue eyes, but it is not impossible. In the same way, they may also have children with green eyes and in fact they are more likely to come out green than blue. So if you have brown eyes and your partner does too, don’t convince yourself still believing that none of your children will have green or blue eyes , because it is possible and now I am going to explain why.
The basic rules of genetic inheritance
Sorry, I have to do a little reminder of Mendel’s Laws to be able to explain the subject of the eyes, so put your mind back to your time of GBS , dig into your memories and show me that going to school served you for something else than to heat the chair (it helped me for that and to have a new case every September … little more).
Well, we begin by explaining that Gregor Mendel discovered the basic rules that govern genetic inheritance with various experiments with plants, the most famous being that of peas. Mendel grew green peas and yellow peas. He crossed the two varieties obtaining the following result: 100% of the peas were yellow. Then he took these yellow peas and crossed them between them, obtaining a very curious result: 75% of the peas were yellow and the remaining 25% were green .
Not content to see that all this was very strange, he fertilized the new generation of peas, seeing that the same thing happened again with the yellow ones (75% yellow and 25% green) and that with the green ones all came out green, 100%. Thus he realized that there were dominant hereditary characteristics and recessive hereditary characteristics .
We repeat, with an image
Now we explain it again with the image, to see it more graphically. The roughness of the pea is also specified in the drawing, but since we don’t need it, we will only have the first two letters of each type of pea. Yellow is a dominant characteristic, so it is called A (uppercase) and green, which is recessive, is called a (lowercase). Above all you have a yellow pea (AA) and next to it a green one (aa). I repeat, look only at the first two letters.
By crossing these peas Mendel got yellow peas, all of them. The reason is that they were all Aa . When two species are joined, one gives a gene and the other of another gene, as you can see, whatever you do the combination, Aa always comes out (the first A from yellow with the first a from green, the first A with the second a , the second A with the first a and the second A with the second a ). Therefore, being all Aa , dominating yellow over green, all are yellow .
Now, when crossing all the Aa, the unexpected happened, because 25% of green peas came out . In the painting I think it is quite well understood. Whenever one of the peas crossed yielded a capital pea was yellow, but in a quarter of cases a year gave way to lowercase and the other Aa too, resulting in aa pea, ie green. If those greens then crossed each other, they would undoubtedly be always green because none of them bore an A.
Explaining the color of the eyes
The issue of eye color is a bit more complex than peas, because we have two chromosomes to blame for eye color . One of them is chromosome 15 , where there is a gene that influences color and whose possible alleles are brown and blue. The other is chromosome 19 , which also has a gene that influences color and whose possible alleles are green and blue (that is, we all carry genes for green and / or blue, because on chromosome 19 there are no fit for brown).
As with the colors of peas, there are dominant and recessive colors. Brown is dominant and as long as it is there it will “win” over the rest, blue is recessive and green is halfway there, as it is dominated by brown, but it dominates blue. To make it easy to call the brown M, V to the green and to the blue.
There are many possible combinations, because as we have said there are two chromosomes that give the color of the eyes. A person whose eyes are blue has the easiest combination, aa on chromosome 15 and aa on chromosome 19. Only if it was Ma-aa , they would already have brown eyes. If it were Ma-Va , it would also have them brown and, if it had aa-Va , it would have them green (then there are many more combinations, of course, because 15 can be MM, Ma, aM or aa and 19 can be VV, Va, aV or aa ). In addition, then you have to take into account that more things influence , because there are people who have brown eyes but tending to greenish, there are those who have dark brown, there are people with one eye of each color, etc. But hey, to get an idea of the generalities, today’s explanations are more than enough.
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The key is in the recessive “a” of both genes
To explain how it can be that a couple with brown eyes have a child with blue eyes, we simply have to know that the key is for both parents to have an a on each chromosome. If the father is, for example, Ma-Va and the mother is also Ma-Va (they both have brown eyes) they will have a 75% chance of having children with brown eyes ( MM, Ma or aM on chromosome 15), 18.75% of having green eyes ( aa on chromosome 15, and VV, Va or aV on 19), and 6.25% of having blue eyes ( aa-aa ).
A case with more probability of having children with blue eyes would be if one of the two was Ma-aa and the other Ma-Va and the case of parents with brown eyes would be more likely if a father and a mother were together with the combination Ma-yy and Ma-yy . In the latter case, the probability of having brown eyes in the children would be 75% and that of having blue eyes the remaining 25%.
So if you ever meet a brown-eyed couple with a blue-eyed baby, know that it is possible . It is true that it is also possible that the child belongs to another, but right off the bat it is better to trust the fidelity of the couples, right?
In Babies and more | Baby’s eye color: when is it defined
