I remember most of 10th grade biology as an exercise in mega-memorization. I stuffed as many scientific terms in my brain as it could hold before the tests, and afterward they started leaking out again. (When my son took 10th grade biology – in 9th grade – I discovered how few of those Latin- and Greek-derived terms I could still dredge out of the depths of memory.) But one unit was more about logic – and counting – than memorization, so I particularly enjoyed studying genetics.
Besides, I had always wanted to understand better how I came to be lefthanded (my father may have been also but was forced to learn to write righthanded) and have red hair (some relative I had never met - I think she had died before I was born - was supposed to be the source of it). I had been told since I was young that it had something to do with dominant and recessive genes, but it was nice to finally understand what that meant.
I noticed (thanks to my daily check at infoplease.com) that “Today’s Birthday” (July 22) is that of Gregor Mendel, whom I fondly remember – along with his peas - for that enjoyable unit in biology class. In case you have forgotten your 10th (or 9th or whatever) grade biology and would like a review, or never learned it and would like an easy primer, I culled the web (well, I did a goodsearch and went through a couple dozen promising hits, though most were too technical) for a good “genetics made easy.”
I was beginning to think I had been wrong to expect to find such a thing, when I found it. It’s even called “Genetics Made Easy“ (just think - I could have tried typing that in the address line and saved myself all that searching). The site is designed for prospective parents who are concerned about whether they will have a “normal” baby, which is why the examples are of human reproduction rather than peas or fruit flies. (It was even developed by a Spanish doctor for Spanish-speaking people, and then translated to English, which only makes it of greater interest to me, since I studied and lived in Spain.)
Included in the explanation are why and how “mistakes” sometimes happen in passing along the genetic instructions. What I don’t think it probably covers is the very unusual (one-in-a-million) case of twins born with different skin color, such as those just born in Germany.
July 23, 2008 at 7:46 am |
Genetics was my favorite part of Biology class, too.
I also had seen pics of twin girls, around 2 yrs. old, one black, one white. They were dressed alike, & each had their hair in little ponytails. So adorable!
July 23, 2008 at 11:58 am |
I once had a coworker who was dark-skinned, but told me that she was actually mixed race, and all of her siblings were lighter than she was. So if she and a sibling had been twins, they would also have qualified.
July 23, 2008 at 4:44 pm |
It seems easy enough to me to understand how siblings could have different skin color, just like different hair color, eye color, etc., because they receive different sets of genes from their parents. But with twins, where – as I understand it – both come from the same fertilized egg and share the same DNA, I don’t understand how the different skin color happens. According to the article, “experts say that there is a one-in-a-million chance for a birth of this kind but it is possible if genes are mixed in certain way.” But it doesn’t explain how they could be mixed that way or provide links to any explanations.
July 23, 2008 at 9:21 pm |
In my reading somewhere I came across something about DNA not being the entire story. There may be other factors that determine the exact ways that certain genes are expressed or not expressed.
July 23, 2008 at 9:26 pm |
Oh, and if the two boys are fraternal twins, then they are just like any two siblings in a family. They just happen to have been born at the same time.
July 23, 2008 at 10:28 pm |
If they’re fraternal twins, then it makes sense. I just didn’t see why it would be much of a news story if that were the case. (And I couldn’t find any news article identifying which type of twins they were.)
I did find something tonight explaining the situation – and identifying such cases as fraternal twins:
from http://www.impactlab.com/2006/02/22/identical-twins-one-black-one-white/
Skin color is believed to be determined by up to seven different genes working together.
If a woman is of mixed race, her eggs will usually contain a mixture of genes coding for both black and white skin.
Similarly, a man of mixed race will have a variety of
different genes in his sperm. When these eggs and sperm come together,
they will create a baby of mixed race.
But, very occasionally, the egg or sperm might contain
genes coding for one skin color. If both the egg and sperm contain all
white genes, the baby will be white. And if both contain just the
versions necessary for dark skin, the baby will be dark.
For a mixed-race couple, the odds of either of these
scenarios is around 100 to one. But both scenarios can occur at the
same time if the woman conceives non-identical twins, another 100 to
one chance.
This involves two eggs being fertilized by two sperm at the same time, which also has odds of around 100 to one.
The odds of this happening are 100 x 100 x 100 — a million to one.
I also read another blog where someone said those calculations are wrong, that it’s one in several thousand but not close to a million.
July 24, 2008 at 11:35 am |
I wonder if the discrepancy in odds depends what you are taking for granted. If you are “already” talking about twins, then the odds of two different colors would be 1 in 100 x 100, or 10,000. But if you talking about any birth at all, then only 1% of those are fraternal twins, so 1% of different colored siblings (like my coworker and her siblings) are born at the same time.