First up, responding to the most recent comment. Off-hand I don't think the metatype is tied to gender, that might be a personal preference, but I don't it founded in anything that is written up in the game text. At least nowhere that I recall.
And doing a cursory look into the genetics of it. First up, there is no such thing as a "male X" or a "female X." The reason for male-pattern baldness for example is because the gene that causes it is on the X chromosome, but is recessive. In males, this means that because they don't have a second X chromosome, a single occurrence of the gene is enough to cause the disorder. So, by that principle it actually means that genetics tied to the X chromosome are more likely to be passed from mother to son, not mother to daughter. But, I think it is much more likely that the genetics for metatype are probably carried on one of the other 22 chromosomes.
Now, I think Wyrm's numbers might be closer to correct, I definitely don't think the values are 40/40/20 in terms of percentage chances. I think the proposed values are much closer to single-digit chance of a child born to elf parents (for example) being born human rather than elf. And it might be much lower than that even, (~0.2% chance rather than 1%-5%). Genetic reversion (as it is referred to) is a rare occurrence. Enough so that it is mentioned in the Giant metavariant as an oddity, because there is an increased chance of genetic reversion. Apparently one-fourth of female giant offspring result in a sapiens expression, that is to say, a quarter of the daughters of giant parents are born human rather than troll.
Statistically speaking (assuming even chances of a boy/girl), that means approximately 12.5% of giant offspring result in a human rather than another giant. This is considered "unique" in its frequency, which is why I think it is probably 10-20 times more frequent than normal (if not more) making it a statistically significant event. It's even mentioned as causing newsworthy events with people trying to research the sapiens reversion.