Since our own 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 baby 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 allligators and most of 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 sometimes in the middle of the overall development period, when a critical time is reached. And during this time, the sex is completely determined by temperature in the nest. In pained turtles, for example, warm temperature above the critical temperature will produce females whithin the eggs, and cool temperature will produce a male. I'm not really 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 will it be a boy or will it be a girl isn't settled until even later in life. You see, clownfish all start out their life as males, however, as they mature, they become female. They also spend their life in a small group with a strict dominance hierachy, where only the most dominant male and female reproduce. And 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 the other males will move up one rank in the hierachy.
In another very different ocean animal, the Green Spoon worm, the sex of the babies is determined by completely different aspect of the environment. For this species, it is simply a matter of where a larval happens to randomly fall on the sea floor. If a larval lands on the open sea floor, then it will become a female. But if it lands on top of a female, then it will becomes a male. So for some species, the question of boy or girl is answered by genetics. For others, it's anwered 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. There are a nearly all-female species, and althought 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 really all depend on the system of sex determination. For human, that system is a genetic XY system. And for me and my wife, we found out, it's gonna be a baby boy.
embryo (n): phôi, phôi thai
critical (adj): phê bình, phê phán, nguy cấp, nguy kịch
mnemonic (adj): thuộc về trí nhớ
tropical (adj): nhiệt đới
settle (v) bố trí, xếp đặt
mature (v) trưởng thành
strict (adj): chính xác, nghiêm ngặt
dominance (n): ưu thế, thế trội hơn, địa vị, địa vị thống trị
hierachy (n): hệ thống phân cấp, cấp bậc
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