My wife is pregnant right now with our first child, and when people see her with her big baby bump, the first question people ask, almost without fail, is, "Is it a boy or is it a girl?"
Now there are some assumptions behind that question that we take for granted because of our familiarity with our own human biology. For human babies, we take it for granted that there's a 50/50 chance of either answer, boy or girl. But why is it that way? Well, the answer depends on the sex determination system that has evolved for our species. You see, for most mammals, the sex of a baby is determined genetically with the XY chromosome system. Mammals have a pair of sex chromosomes, one passed down from Mom, and one from Dad. A pair of X's gives us a girl, and an X and a Y together gives us a boy. Since females only have X's to pass on in their egg cells, and males can give either an X or a Y in their sperm cells, the sex is determined by the father and the chance of producing a male or a female is 50/50.
This system has worked well for mammals, but throughout the tree of life, we can see other systems that have worked just as well for other animals. There are other groups of animals that also have genetic sex determination, but their systems can be pretty different from ours.
Birds and some reptiles have their sex genetically determined but instead of the sex being determined by Dad, their sex is determined by Mom. In those groups, a pair of Z sex chromosomes produces a male, so these males only have Z's to give. However, in these animals, one Z and one W chromosome together, as a pair, produces a female. In this system, the chance of a male or a female is still 50/50, it just depends on whether Mom puts a Z or a W into her egg.
Certain groups have taken genetic sex determination in completely other directions. Ants, for example, have one of the most interesting systems for determining sex, and because of it, if you are a male ant, you do not have a father. In an ant colony, there are dramatic divisions of labor. There are soldiers that defend the colony, there are workers that colect food, clean the nest, and care for the young, and there's a queen and a small group of males reproductives. Now, the queen will mate and then store sperm from the males. And this is where the system gets really interesting. If the queen use the stored sperm to fertilize an egg, then that egg will grow up to become female. However, if she lays an egg without fertilizing it, then that egg will still grow up to be an ant, but it will always be a male. So you see, it's impossible for male ants to have fathers. And male ants live their life like this, with only one copy of every gene, much like a walking sex cell. This system is called "haplodiploid" system and we see it not only in ants, but also in other highly social insects like bees and wasps.
take for granted: nhượng bộ
without fail: chắc chắn, nhất định
a pair of: đôi, cặp
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