XY-Men and XX-Women

I saw “X2” over the weekend -- twice -- and the rumors are true: it’s fantastic, even better than the original (which I liked quite a bit). So many bloggers have already commented on this subject that I can’t begin to link them all; check out Jacob Levy’s post for a sample. (When I read Jacob's post yesterday, it had lots of links to others’ posts, but they seem to be gone now.)

However, I am constitutionally incapable of watching a movie, even a great one, without picking a few nits. (Don’t worry -- no major spoilers here.) At one point in the movie, a character states that the “mutant gene” is carried on the Y-chromosome, and hence is passed only from fathers to children. I’ve been trying to find a way that this could make sense from a genetic standpoint, and I’ve failed.

First, since only men have Y-chromosomes, we would have to conclude that *only men can be mutants*. X-linked traits like hemophilia and colorblindness can be possessed by either sex, because both men and women have X-chromosomes. But Y-linked traits can only be possessed by men. But we know that there are numerous female mutants in the movie, so it can’t be true that the mutant gene is Y-linked. (Incidentally, my college biology professor told me that biologists have only discovered one Y-linked trait: hairy ears. But that was back in 1995, so maybe they’ve discovered more Y-linked traits by now.)

Second, something is fishy about the idea that changes in *one* gene could result in all the many different mutant powers we observe in the movie, from teleportation to weather control to fast healing to body morphing. Some are mental, others physical; it’s pretty odd that they would all be controlled by the same gene. Gene interactions are certainly possible, though, so perhaps it works like this: the “mutant gene” interacts with the other genes in one’s body to produce a variety of different phenotypes (expressed traits). This still doesn’t overcome the men-only problem, though.

Here’s a more complicated explanation: there is a single gene, found on the Y-chromosome, that causes mutations to occur in genes on *other chromosomes*. This mutation-inducing effect only occurs during the process of meiosis in the male’s germ cells. As a result, a child can only be born with mutations (of the super-power variety) if her father possessed the mutant gene. This explanation works, except for one problem: once a female child is born with a mutation in her non-Y chromosomes, she can pass it on to her children (unless she’s sterile). So even if super-power mutations had their origin in the mutation-inducing effects of a Y-linked trait, the mutations themselves would not be Y-linked and could be passed on by either men or women.

One more nitpick about the movie: I think they changed the nature of Storm’s powers. In the original movie (and, as I recall, the comic book), it is strongly implied that Storm can only control the weather when she has a direct line of sight to the sky. (Warning: minor spoiler from the first movie coming.) When Sabretooth pins her against the wall of the train station and starts choking her, she is unable to hit him with a lightning bolt until Cyclops accidentally blows a hole in the roof. Yet in the current movie, she causes major weather effects inside a room that is entirely closed off from the outside world.