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00:00I think there are a few mustache hairs coming in.
00:08What?
00:11Hi, Tim and Moby.
00:18How come some babies are male and others are female?
00:22From Devin.
00:24Oh.
00:25Well, like most physical traits, sex is determined by your genes, or DNA.
00:31DNA stands for deoxyribonucleic acid.
00:35You can think of DNA as a blueprint for your body.
00:39It controls stuff like hair color and height, but also day-to-day functions like cell division
00:44and immune response.
00:45DNA is arranged in long, twisty strands called chromosomes.
00:50Almost every cell in your body contains 23 pairs of chromosomes.
00:55One chromosome in each pair comes from your mother, and the other comes from your father.
01:00Scientists number the different pairs to keep track of which genes are in charge of which
01:04functions.
01:05The 23rd pair of chromosomes determines your sex.
01:09There are two basic chromosome types that can make up the pair, X and Y.
01:14A Y chromosome causes the body to produce testosterone and develop male body parts.
01:20Right, so in males, the 23rd pair is made up of an X and a Y chromosome, whereas in
01:27females it's made up of two Xs.
01:30Well, being born male or female pretty much comes down to chance.
01:35See, everybody comes from the combination of a sperm cell from the father and an egg
01:40cell from the mother.
01:42Unlike all the other cells in your body, sperm and egg cells are haploid, meaning they
01:46contain 23 individual chromosomes, not 23 pairs.
01:51When a sperm cell fertilizes an egg, the single sets of chromosomes from each cell merge,
01:56forming a complete set of 23 pairs.
02:00Because females have only X chromosomes, all their egg cells contain an X for the 23rd
02:05chromosome.
02:06Males have both X and Y chromosomes.
02:09That means a sperm cell has an equal chance of containing an X or a Y.
02:14So if a Y sperm fertilizes an egg, you'll get an XY baby, or a male.
02:19If an X sperm fertilizes an egg, you'll get an XX baby, or a female.
02:25It works pretty much the same way in every organism that reproduces sexually.
02:30Uh, yeah, actually.
02:33It's pretty rare, but people can be born with uncertain sex traits.
02:37A person with male chromosomes can have some female traits, and someone with female chromosomes
02:42can have male traits.
02:44It also happens when babies inherit an unusual number of sex chromosomes, like 1 or 3.
02:51Yeah, I'd say so.
02:55Um, you're male, I think?
02:58Aren't you?