My elder sister is pregnant right now with her 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 human biology. For human babies, we take it for granted that there's a 50/50 chance of either a boy or a girl answering. 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. Other groups of animals 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.
Some soldiers defend the colony, some workers collect food, clean the nest, and care for the young, and there's a queen and a small group of male reproductives. Now, the queen will mate and then store sperm from the males. And this is where the system gets interesting. If the queen uses 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, male ants can't 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 a haplodiploid system, and we see it not only in ants but also in other highly social insects like bees and wasps.
Since our sex is determined by genes, and we do know of these other animals that have their sex determined by genes, it's easy to assume that for all animals the sex of their babies still must be determined by genetics. However, for some animals, the question of whether it will be a boy or a girl has nothing to do with genes at all, and it can depend on something like the weather. These are animals like alligators and most turtles. In these animals, the sex of an embryo in a developing egg is determined by the temperature.
In these species, the sex of the baby is not yet determined when the egg is laid, and it remains undetermined until sometime in the middle of the overall development period when a critical time is reached. During this time, the sex is completely determined by the temperature in the nest. In painted turtles, for example, warm temperatures above the critical temperature will produce females within the eggs, and cool temperatures will produce males. I'm not sure who came up with this mnemonic, but you can remember that when it comes to painted turtles, they are all hot chicks and cool dudes.
For some tropical fish, the question of whether will it be a boy or will it be a girl isn't settled until even later in life. You see, clownfish all start their lives as males, However, as they mature, they become female. They also spend their lives in small groups with a strict dominance hierarchy where only the most dominant male and female reproduce. Amazingly, if the dominant female in the group dies, the largest and most dominant male will then quickly become female and take her place, and all of the other males will move up one rank in the hierarchy.
In another very different ocean animal, the green spoonworm, the sex of the babies is determined by a completely different aspect of the environment. For this species, it is simply a matter of where a larva happens to randomly fall on the sea floor. If a larva lands on the open sea floor, then it will become a female. But if it lands on top of a female, then it will become a male. So for some species, the question of boy or girl is answered by genetics. For others, it's answered by the environment.
And for others still, they don't even bother with the question at all. Take whiptail lizards, for example. For those desert lizards, the answer is easy. It's a girl. It's always a girl. They are a nearly all-female species, and although they still lay eggs, these eggs hatch out female clones of themselves. So will it be a girl or will it be a boy Throughout the entire animal kingdom, it does all depend on the system of sex determination. For humans, that system is a genetic XY system. And for me and my wife, we found out it's going to be a baby boy.