Genetics in the news: Aging, silent mutations, "Virgin birth" by Komodo Dragons

It's been a busy week for genetics news.

Genetics of Aging
First, there was the discovery of a single gene that influences (and may control) the aging process. (If you'd like to know more, see the post on my Genetics for Dummies Amazon blog here.)

Eye Color is Controlled by "Junk" DNA
Next, an Australian team published their findings on the genetics of eye color in the 20 Dec 2006 issue of the American Journal of Human Genetics. Researchers learned that only a couple of letters of the genetic code control eye color. The most amazing part, though, is that these genetic changes (called single nucleotide polymorphisms or SNPs; pronounced snips) are not in a gene. To appreciate what this discovery means, you need to understand how genes are put together.

Genes consist of regions of DNA that code for the character in question (these coding parts are called exons for 'expressed'). The coding parts of genes are interrupted by chunks of DNA that used to be thought of as junk. That is, these regions that interrupt exons (called introns for 'interruption') were said to be non-coding and therefore "useless." Now, scientists are finding that introns actually do things. In the case of eye color, the intron says 'turn on color using this gene (known as OCA2) and the color to use is blue or green.' To get a more detailed explanation, see the BBC News article here which includes a handy illustration.

Changing the Dogma: Silent Mutations Matter
The third discovery also involves SNPs and was published in Science. It turns out that so-called silent mutations may not be quite as quiet as originally thought. The dogma is that the genetic code is redundant. Redundancy means that small changes in particular codons (the three letter "words" of the genetic code) were believed to have no affect in the sense that the same amino acid was produced using slightly different spellings. For example, UUU (three uracils in a row) and UUC (uracil, uracil, cytosine) both code for phenylalanine. From this new discovery, it appears that those slight changes may produce the same amino acid but the protein produced may be shaped differently. There are two articles that go into more detail : this Science Now article, and a second piece published in The Scientist.

Reproduction without Sex by Komodo Dragons
Finally, just in time for Christmas, is the much trumpeted "virgin" birth by a female Komodo Dragon named Flora who resides at the Chester Zoo. Flora produced a total of 25 eggs, eight of which are viable and expected to hatch any day now. The phenomenon that allows Flora to pull this feat is called parthenogenesis. Parthenogenesis is the production of offspring by asexual means. Insects such as bees and wasps reproduce asexually quite frequently but vertebrates do not. That said, parthenogenesis isn't all that rare (some 70 species, mostly snakes and fish are known to reproduce this way).

One feature of interest in the case of Flora is that all her offspring will be males. Like birds, female Komodo Dragons have sex chromosomes, ZW (the analogous condition in humans: males are XY). The sex that bears those differing sex chromosomes is the one that determines the sex of the offspring. Flora's offspring are all ZZ (and thus, male) because of the way that her eggs become viable to produce offspring without fertilization. Here's how that worked: Flora produced oogonia that skipped the second round of meiosis. This left her with eggs that were diploid (that is, with chromosomes in pairs), completely homozygous (identical gene pairs), yet not clones of Flora (who has heterozygous loci), and male (that is, either ZZ or WW).

I suspect that only the ZZ eggs survive (accounting for the low viability rate) but I'm only guessing. I have not yet found a reference that explains how the eggs of parthenogenic females begin development and cell division.