Punnett Square

Genetics for dummies

Punnett Square

Punnett Square Fun


My maternal grandmother is a twin and I thought much more about the chances of giving birth to twins than to colorblind boys. Grandpa Baugh, her husband, was colorblind. That darn defective X chromosome was passed to my mother, but my brother escaped the deficiency so I didn’t think much about being a carrier until Grandpa himself made the diagnosis of our oldest son.

I have tried over the years to wrap my mind around the genetics of the disease, but most things I’ve read end up sounding something like, “Blah, blah, probabilities and statistics, blah, blah.” What has really piqued my interest at this point is the fact that not only is there a high likelihood some of my yet-to-be-born grandsons will be colorblind, but I may well have colorblind granddaughters as well. We know this because my colorblind son’s wife’s brother is colorblind, so my darling daughter-in-law could be a carrier. (Hearing the blah blah yet?)

This means not only that her sons have a 25 percent chance of being colorblind, but even her daughters have a whopping 25 percent chance. This is pretty incredible because most sources cite the occurrence of colorblindness in women at half a percent or less. Do you know any colorblind women?

In addition, if either of my sons has daughters, the girls will at the very least be carriers, so I could end up with a passel of colorblind great-grandchildren.

Since I’m way past high school biology, and didn’t pay much attention when I was there, I asked my brilliant niece Chelsea to recap these genetics. She began by explaining how human eyes detect and differentiate between color with cells called cones. The three types of cone cells identify red, blue and green light and when working correctly, help us see the entire spectrum of colors. The colorblind have a defect in at least one of the types of cone cells, altering their perception.

Here’s where the Punnett Square fun begins! Chelsea typed up the nice diagram above and gave me this explanation:

The genes responsible for cone cells are located on the X chromosome. This is the same X chromosome that helps determine gender – males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes and no Y chromosomes (XX). In order to make this pair, one chromosome is inherited from the mother and one from the father. So, everyone gets one of their mother’s X chromosomes and either an X or a Y from their father.

Since females have two X chromosomes, every cell uses genes from one of the two. So, if one of the two X chromosomes in a female has defective genes, they won’t affect all of the cells, and the abnormal gene might go unnoticed or be less severe. But, males only have one X chromosome. If that X chromosome has a mutation in a important gene, it will affect every cell that uses that gene.

Since a male only has one X chromosome, he will be colorblind if that X has defective cone genes. He gets a Y from his dad and an X from his mom (she has two; he gets one of them at random). So, a colorblind male gets a mutated X from his mom – which means that she has at least one mutated X chromosome. A female who isn’t colorblind but has a colorblind son has one normal X and one mutated X – we call her a “carrier.” A son of a non-colorblind female carrier has a 50 percent chance of being colorblind (note that this is unaffected by his father’s colorblindness!).

Although this simplistic explanation does not detail different types of color vision deficiencies, it gives us a good understanding of how things work. So if you have colorblindness in your family tree, get out a pencil and have some Punnett Square fun of your own.

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