 6. Determination Did you know that nearly all tortoise shell cats are female? We'll find out why later. In this video we are going to discover how sex is determined. Human body cells have 46 chromosomes arranged in 23 pairs. There are 22 pairs of autosomes and one pair of sex chromosomes or allosomes. For a male, the sex chromosomes are usually XY. Whilst for a female, they are XX. To make a baby, a sperm has to join with an ovum or an egg cell. We looked at sperm and egg cells in detail in this video. You should know that they only have half of the DNA of a body cell. They are haploid. Instead of 46 chromosomes, so 23 pairs, they just have 23 chromosomes in total. 22 autosomes and one sex chromosome or allosome. An ovum will always have an X chromosome, but the sperm will have an X or a Y. If we looked at this in a Punnett square, the gametes from the female would always be X. So XX and the gametes from the male would be either X or Y. Can you complete the Punnett square to show the probability of having a male or a female child? Pause the video, complete the Punnett square and click play when you're ready to check. Did you get it right? So the X from the ovum and the X from the sperm would produce a female baby. And the X from the ovum and the Y from the sperm would give us a male child. So really, it is the man who determines the gender of the child. He brings the Y, which is the only chance of a male child. Unfortunately for Ann Boylan, a wife of King Henry VIII of England in the 16th century, she was executed for not providing a male heir to the throne when really his sperm were to blame. From the Punnett square, we would expect 50% of the offspring to be male and 50% to be female. But the actual outcomes may be different. One couple could, for example, end up with four goals. Discounting social factors such as sex-selective abortions, at birth the natural male-to-female sex ratio for humans is about 105 males to 100 females. It is thought that this is nature's way of overcoming the fact that males have a higher risk of dying younger than females. The overall world male-to-female sex ratio drops down from 101 males to every 100 females, so it is closer to the expected 50% of each. Now let's get back to our cat. Most tortoiseshell cats are female because this fur color is a characteristic found on the X chromosome. But males have one X chromosome, so why can't males be tortoiseshell? Two X chromosomes are needed to show the three different colors. So as females have XX6 chromosomes, they can be tortoiseshell, but XY males can't. Now I say, most rather than all, because you can occasionally get a male tortoiseshell cat, just like you can occasionally get a human man with XX chromosomes. These rare individuals have an extra strand of DNA, and so are XXY, rather than just XY. So the male has the double X's needed to be tortoiseshell. There are other rare chromosomal disorders that mean you can have XX human males and XY females, or ternosodendrome, when one sex chromosome is present, a solitary X. XYY and XXY are two more syndromes, and they are many others. From this video, you should know that females usually have XX6 chromosomes, and males usually have XY. The father provides the Y chromosome, and so the chance of having a male child falls in his shoulders. If you liked the video, give it a thumbs up, and don't forget to subscribe, comment below if you have any questions. Why not check out our Fuse school app as well? Until next time!