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Boy or Girl: How Genetics Determine Your Baby's Gender

The process by which your baby becomes a boy or a girl, according to what's written in his or her genetic code, is awe-inspiring. Watch it all unfold.

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-Many parents begin to wonder about the sex of their baby as soon as they discover they're having a baby. The biological process occurring before you can say it's a girl or it's a boy is a complicated one. So, here's a primer on sex determination. Humans have 1 pair of sex chromosomes. Women have 2 X chromosomes and men have an X and Y. The presence or absence of the Y chromosome determines the sex of your child. Because women have only X chromosomes, all of their eggs have only X chromosomes. Men on the other hand have an X and a Y. So, each of their sperm carries one of the 2 sex chromosomes, which sex chromosome is in the sperm that fertilizes the egg dictates the baby's sex. This means that dad, although he can't control wether an X or a Y gets to the egg first, chooses you baby's sex. Humans have roughly 25,000 genes, yet only 1 gene located on the Y chromosome, the SRY gene is required for male development. How does a single gene out of so many hold the key to whether your baby will be a boy a plain an simple explanation is that this gene activates other boy genes. Without it, girl genes are activated. The internal and external sexual organs of boys and girls developed from the same basic body plan, but you know, at birth, you can clearly distinguish the 2 sexes just by looking. What's at work here? Male and female hormones caused the embryonic gonad and 2 sets of ducts to form male or female structures. The process begins at week 7. If a Y chromosome is present, SRY is activated signaling production of the male hormone testosterone and the development of the embryonic gonads into testes. The phallus grows into a penis and the skin around it forms the scrotum. If there is no Y chromosome, you're having girl. Ovaries form and produce the female hormone estrogen. The phallus stops growing forming a clitoris and the skin around it becomes the outside of the vagina. This all starts by the end of the first trimester although you won't know if you're having a boy or a girl until your ultrasound around week 16.