I don’t know what’s gotten into me, but evidently I’m giving myself a crash course in genetics. Anyway, was reading this bit about Thomas Hunt Morgan (it’s a Flash animation) and came across this:
Huh? How did he go from observing the 3:1 ratio to knowing which chromosome the trait appears on? I’ve done a couple of Punnett Squares (X, Y, R, r, where R is the dominant red-eye trait), and can’t see how ones gets you the other. And, if the phenomenon is observed only in male fruit flies, shouldn’t that belong on a Y-chromosome instead of the X?
Well, maybe I have it figured out now…but please confirm: The X-chromosome conclusion has nothing to do with the Mendelian recessive ratio or Punnett Squares; rather, it has to do with the observation that all fruit flies with white eyes are males, thus this trait must be gender-related (otherwise it would show up on both sexes) – and from that, we can conclude that it must appear on one of the sex chromosomes. But still I have the question: Why X, not Y?
If only males have it, then it’s hormone related or something. The reason that males show more recessive traits than females is because the Y is stunted–it’s just missing some of the genes. That means that the recessive that’s carried on the X, which would be overridden by the gene on the other X, gets a chance to express itself (and we like to encourage self-expression, don’t we?) more often in males.
The gene for color-blindness in humans is the usual example. Let’s say that mom has a recessive color-blind-gene (on the X) which is recessive. The girl child gets a color-blind gene (from mom) and a non-color-blind gene (from dad) and because the non-color-blind is dominant, she can see color. The boy child gets the same set of genes from the parents, but because he got the Y from dad, there’s no non-color-blind gene to be had–he’s color blind because of the X chromosome he got from mom.
Working out the Punnett squares, using R for dominant red eyes and r for recessive white eyes (both on the X), and Y for the Y chromosome, with no eye-color locus:
The parental generation is RR female and rY male.
F1: all females are Rr (red-eyed) and all males are RY (red-eyed)
F2: 1/4 are RR (red-eyed females); 1/4 are Rr (red-eyed females); 1/4 are RY (red-eyed males); 1/4 are rY (white-eyed males).
Therefore you have a classic 3:1 Mendelian dominant-recessive ratio, with the peculiarity that all the recessives are males (50% of the males are red-eyed, 50% white-eyed). This pattern implies that the trait is carried on the X.