Thursday, May 07, 2009

Reviewing 9th grade genetics

I was a biology geek anyway and enjoyed pretty much everything we did. Including dissections. (We had lunch right after that period, and while my friends would be pushing their food around their trays, not particularly hungry after what we'd just done, I'd be saying, "You gonna eat that?")

But one of my favorite units was genetics. Some of it was the sense of wonder--microscopic strands of nucleotides dance into position, resulting in a whole human being. More than anything, however, I loved the Punnett squares. All the wonder in a handy logic-puzzle format. I could calculate my chances of having a blue-eyed child with my crush at the time. (50%. Same as with the donor we've been using.) And then I could expand the square to account for more than one genetic trait.

Of course, genetics is not as cut and dried as all that. There are times when there's a step between dominant and recessive. (Exhibit A: My hazel eyes, the result of a brown-eyed and a blue-eyed parent.) And then there's sex linkage, where a gene appears only on the X-chromosome. A benign example of this is the calico and tortoiseshell patterns in cats. The gene for color is on the X-chromosome, and any tri-color pattern requires two color genes. Since males only have one X-chromosome, they will have two colors max (or be a sterile XXY male). This is also what's at work with hemophilia and color blindness.

Which brings me to the catalyst for this reminiscence.

In the past year or so, in the midst of all the tests administered to Scooter in preparation for school and general evaluation, there's been a bit of an asterisk to some of his vision testing. Various medical practitioners have pulled out the standard color-blindness testing book (the Ishihara color test, apparently) and had Scooter take a crack. Before all this, doctors either didn't try the test or didn't expect him to be able to read the numbers due to his age. But there have been honest attempts recently. The last two had the professionals shrugging and saying, "He's not totally colorblind, but he may not be seeing this correctly either. Let's try again in a bit." Scooter would read the first couple right and then got less certain. When he was asked to trace what he saw, he'd go over part of the number and then trail off. We couldn't decide if he really couldn't see or if he was just being non-cooperative.

Scooter's most recent eye exam has confirmed that he definitely cannot see all shades of color. He seems to have the most trouble with lighter shades of green and red. If you check out the linked color test, he would probably see a 71 (or a 21, as mentioned in the caption). He can see a difference in red and green, but I have no idea what they actually look like to him.

So I was interested in tracking down the genetics of this. There was no color blindness in the donor's profile--and that's more complete than my own medical history. And I wasn't aware of any color blindness in my generation or the one above me.

I remembered to mention this to my mother in one conversation. It may not have been until our next one that she was able to vaguely recall her uncle having some form of color blindness.

And so I was able to recreate this gene's path:
  • My mother's maternal uncle is color-blind. His sister, her own mother, is a carrier.
  • This gene misses her one boy-child, but is passed on at least to one of her girl-children. My mother is a carrier.
  • My mother only has girl-children and passes it along to at least one. I am a carrier.
  • I have a boy-child and am therefore responsible for his only X-chromosome. By the luck of the draw, the one he got includes the color-blindness gene.
It's a three-generation gap in expression, but it makes perfect sense, in a way I learned to figure on paper more than 20 years ago.

(And I've been told kids remember nothing from junior high.)

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