Mendel’s square and eye color: the method falls short.

[Chance_the_Gardener]

I remember the famous eye color square from my seventh-grade science book. Mendel’s square showed how eye color is determined, focusing on the dominant and recessive genes. Simply put (if I’ve got this right,) if a mother has two brown eye chromosomes and a father has two blue eye chromosomes, each is going to give one chromosome to their offspring, as in mother=BB, father=bb, offspring=Bb. Since brown (B) eyes are dominant and blue (b) eyes are recessive, the children who all have Bb eyes will have brown eyes, inevitably. Likewise, if brown-eyed mother=Bb and blue-eyed father=bb, then all offspring will be either Bb or bb. I found this easy to grasp, and I accepted it.

The problem is that years later, I realized that that formula seemed… sparse. I don’t know how I missed it at the time, but my own family doesn’t jibe with this at all. At least, I don’t think it does. It’s like this:

Dad: Hazel eyes
Mom: Green eyes

My brother: Brown eyes
Me: Blue eyes
Elder sister: Blue eyes
Younger sister: Hazel eyes

There we are, the oldest on top, the youngest on bottom. I know none of us are adopted, and we’re all children by the same parents. And even if we weren’t, Mendel’s square doesn’t provide for green or hazel eyes in the first place, so how to provide for those eye colors?

If Mendel’s square does make sense, then I don’t see how it can be applied to my family. It seems like there’s some genetic principle that Mendel and his pea plants didn’t cover. My faith in the humble Mendel’s square has been shaken. Don’t fret; it’s not like this is going to make me go creationist, but this does beg the question: what’s with my family’s eyes?

[Colibri]

@Chance the Gardener

I remember the famous eye color square from my seventh-grade science book.

As with almost everything, you can’t rely on what you learned in seventh grade to give you the full picture. They of course enormously simplified human eye-color genetics at that level. It is in fact polygenic, and not all of it is explained by a simple dominance-recessive model.

From here:

At one time scientists thought that a single gene pair, in a dominant/recessive inheritance pattern, controlled human eye color. The allele for brown eyes was considered dominant over the allele for blue eyes. The genetic basis for eye color is actually far more complex. At the present, three gene pairs controlling human eye color are known. Two of the gene pairs occur on chromosome pair 15 and one occurs on chromosome pair 19. The bey 2 gene, on chromosome 15, has a brown and a blue allele. A second gene, located on chromosome 19 (the gey gene) has a blue and a green allele. A third gene, bey 1, located on chromosome 15, is a central brown eye color gene.

Geneticists have designed a model using the bey 2 and gey gene pairs that explains the inheritance of blue, green and brown eyes. In this model the bey 2 gene has a brown and a blue allele. The brown allele is always dominant over the blue allele so even if a person is heterozygous (one brown and one blue allele) for the bey 2 gene on chromosome 15 the brown allele will be expressed. The gey gene also has two alleles, one green and one blue. The green allele is dominant to the blue allele on either chromosome but is recessive to the brown allele on chromosome 15. This means that there is a dominance order among the two gene pairs. If a person has a brown allele on chromosome 15 and all other alleles are blue or green the person will have brown eyes. If there is a green allele on chromosome 19 and the rest of the alleles are blue, eye color will be green. Blue eyes will occur only if all four alleles are for blue eyes. This model explains the inheritance of blue, brown and green eyes but cannot account for gray, hazel or multiple shades of brown, blue, green and gray eyes. It cannot explain how two blue-eyed parents can produce a brown-eyed child or how eye color can change over time. This suggests that there are other genes, yet to be discovered, that determine eye color or that modify the expression of the known eye color genes.

[John_Mace]

There is more than one gene that codes for eye color. What you learned in junior high is a simplified version of what is actually going on. You might find this article of interest.

[Roches]

It may also help if you consider that the statistical distribution of genes implied by Mendel’s square does not apply to small samples. (If you flip a coin 4 times, you might get tails all four times. If you flip a coin a million times, you’ll get 50% heads and 50% tails.) By Mendel’s distributions, your parents shouldn’t have had two blue-eyed children. It’s even possible that they could have had four; it’s just not very likely.

Both of your parents must be heterozygous with respect to bey 2 (Bb). Your mother must be heterozygous with respect to *gey *(bg); if she had had homozygous gey (gg), then she could not have blue-eyed children. Your father could be either heterozygous on gey (gb) or homozygous for blue eyes, depending on the role of gey in hazel eyes. He cannot be homozygous for green eyes on gey; otherwise he could not have blue-eyed children. Using this information (but assuming your father’s genotype for gey is (bX)) you could do a Mendelian square for both these genes, and try to figure out what the distribution of your sibling’s genotypes must be.

I’d really like to know more about hazel eyes, because I have them. It might be another gene, but I suspect that it also depends on the bey 2 and gey genes.