The real problem in teaching genetics is blowtorching all the fuzz off concerning what the cursed things do and don’t do, which is an ongoing topic for this whole blog. But say you’ve cleared that out a bit and some students are willing to learn what you’re planning to say … and then another conceptual speedbump appears, no warning sign ‘r nothin’. It concerns the number 2, or more precisely, three independent 2’s.
This post really relies on Q & A to be effective. Ask what you need to!
Also, for profs & other teachers who might be reading, if you know of a good introductory text, as in a textbook used by actual students, which nails these points to the wall, please let me know.
The first “2”: no X’s
You’ve seen’em. Everyone knows chromosomes look like little X’s, which is to say two lumpy string-things joined in the middle.
This is however merely an artifact of one logistic constraint on looking at chromosomes. This “2” really doesn’t mean anything when talking about genes doing things, and in so talking, clear that image out and think of one lumpy sausage-y thing per chromosome.
The trouble? A crucial point of genetic physiology, especially for creatures such as ourselves, involves something called pairing, as in paired chromosomes, often called homologous. And looking at those two joined things in the pictures, the student perfectly reasonably and in my experience universally assumes that these must be the two parts of the pair.
But they’re not.
The second “2”: pairing
This point’s the oldest; scientifically speaking, it arrived waaaay before DNA, before double helices, before genetic engineering, and any of that fancy stuff. At this late date, it’s best taught with a big overview: that prokaryotes (“bacteria” to you, mostly) have one chromosome, and each gene is (not “has,” is) a location on it. So, per location, there’s one “instruction” or more accurately, one unit of content. But we are eukaryotes and don’t do it that way at all.
(Terms fun: the name for this condition of having two chromosomal locations for one gene, i.e., two genetic instructions for each gene, is called diploidy.)
The model was nailed down by the Hardy-Weinberg principle in 1926, which gets a whole post of its own one day, and for present purposes I’m only saying that allelic diversity on paired chromosomes became the root mechanic of Darwinian selection. Roughly a century following its intellectual mission statement by William Lawrence, modern biology had found its physiological and theoretical solid ground.
But it only makes sense if you understand that the paired chromosomes are never stuck together and there’s no indication that they ever touch.
The third “2”: double helix
In the classroom, it’s about this point when someone nods confidently because they are thinking, “Yes, I know this,” as they think about the complementary DNA strands within the double helix.
“He’s talking about two of something and I’ve seen pictures of two of something, so there you go.” Which is why the conversation needs a full stop. This is yet another “2” all by itself.
Headmelting department: the trouble is, these complementary strands are called “paired” too. You simply have to learn the hard way that this pairing is inside one chromosome, so if you’re talking about two paired chromosomes, then each one has its own paired-DNA-strands double helix in it.
I won’t go so far to say that understanding the double helix is a footnote. Its properties rate their own post someday. And as far as technological significance goes, well, yeah. But for purposes of understanding genes in action, this “2” can be shelved as well for a while.
Remember the four questions: what are they mechanically, what do they do, who has what, and how has any of this changed. The first question – now well-illuminated – isn’t a magic wand regarding the other three, and to get into them, a course must get past this2 x 2 x 2 speedbump. It’s that second “2” that matters.