Mad Prime: Tag development

The scientific kitty

Madeleine Ball — Sun, 22 Apr 2007 18:46:00 +0100

As Schrödinger so famously demonstrated, whenever one is illustrating fundamental scientific principles, the optimum choice for such an illustration is a cat. In that spirit, I'd like to present one of my favorite examples of how fundamental biology phenomena are visible in our everyday life.

Some background: Gene copy number is important. Variations in gene copy number are, perhaps, a subtle sort of problem -- having 50% more or less copies of a gene available for expression is conceivably a minor thing in the biochemical world, where feedback loops regulate gene expression to increase or decrease as necessary. (That's why so many disorders are recessive; as long as one functional copy of a gene exists, things seem to work fine.) Nevertheless, the duplication or deletion of entire chromosomes has a severe effect. Within the autosomes (non-sex chromosomes), no cases of chromosome loss are viable. The only extra chromosome that is mild enough to be viable in humans is trisomy 21, which causes Down syndrome. Chromosome 21 is the smallest autosome.

Because of this, when it comes to the sex chromosomes, mammals are faced with a copy number problem. Males (XY) have only one copy of the X chromosome, while females (XX) have two. The ways biology addresses this issue is called "dosage compensation". In mammals, dosage compensation is achieved by randomly inactivating all but one X chromosome in all cells. Thus, regardless of an animal being male or female, only one X chromosome is active in any given cell.

This X-inactivation occurs early in embryonic development. Once a cell has decided to inactivate a given X chromosome, that decision is inherited by all its daughter cells. As a result, female mammals exist as a "mosaic" of X-inactivations -- in their bodies, whole patches of tissue have one or the other X inactivated.

An interesting consequence of X-inactivation is that, unlike genes on other chromosomes, only one allele of a gene on the X chromosome is expressed in any given cell. This phenomenon is easily visible in tortoiseshell cats -- tortoiseshell coloration arises from X-inactivation, so these cats are almost always female. The coat color gene, which has alleles for orange or black coats, exists on the X chromosome. Because of X-inactivation, only one of the two genes is active in various patches of skin, giving rise to a pattern of orange and black patches. Since the process of X-inactivation is random, this pattern of patches is random.

I love this example of X-inactivation so much, I added a picture of a kitty to the wikipedia page on X-inactivation. It's always cool to have a everyday visualization of what would otherwise be an abstract genetic and developmental phenomenon.

Bios and Zoe

Madeleine Ball — Wed, 15 Mar 2006 03:44:00 +0000

I was listening to the Long Now lectures again, this one by Michael West on the subject of human life extension. I can't say much of it stuck with me, but there was one topic that really caught my attention. And that was this: Ancient Greek had two words for life -- "bios" for the life of an individual, finite and mortal, and "zoe" for the infinite and general phenomenon of life.

He applies this language to the contrast between the somatic tissue of our bodies and the germ line tissue of our gametes. The gametes are immortal, an unbroken line that extends back to the first life from which we all descended. They've never died. But every time they move through a new generation a set of cells is created to house and protect this royal lineage -- our bodies. Thus, the body is the "bios", the somatic mortal tissue of finite span. And that cycle of embryonic stem, germ stem, and gamete cells is the "zoe", the immortal life that is unbroken.

After hearing that, of course, I thought Zoe was pretty much the best name ever to give one's daughter. There she is, made from your immortal fragment, the part that can live on.

To my dismay, Chris pointed out that there's already someone named Zoe Ball, a somewhat famous person. I was crushed. (I even whined about changing our last name.) Anyway, I'm passing along the name to you guys, in case you get any daughters and don't know what to call them.

Dragon's Teeth

Madeleine Ball — Tue, 14 Mar 2006 23:09:00 +0000

I recently bought some dried dragon fruit at Trader Joe's, because I'm a total xenophile. (At least, with respect to food -- chicken feet? Hey, I've never tried that! Hm... chewy....) The dried fruit turned out to be not particularly sweet or tasty, really, but interesting in how terribly full of seeds it was. So I looked up some images of dragon fruit on the internet.

Well, dragon fruit is the fruit of a cactus. One page I found talked about growing up plants from the seeds of fruit acquired at the market.

The cool thing about this is this image in particular:

I'd never thought about it - are cacti monocots or dicots? (Monocots have seeds that form a single leaf, dicots have seeds that have two halves and form a pair of leaves - more info here.) Cacti don't have leaves! But they do, when they first grow, in their vegetable version of an embryo. Cotyledon leaves are like leaves, but not true leaves, and that's what that picture is. They don't look like cacti at all! It reminded me of the ontogeny/phylogeny thing -- embryonic humans look like fish, and embryonic cacti look like leaved plants.

PS - the dragon's teeth name given to the jpg is a cute reference to this myth.

PPS - I admit, cacti can have leafy things, technically speaking, sticking out near their needles in a structure called an "areole".