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Sex Determination | Genetics | Biology | FuseSchool

Human body cells have 46 chromosomes arranged in 23 pairs. There are 22 pairs of autosomes and one pair of sex chromosomes (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 egg cell). 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 X, X, and the gametes from the male would be either X or Y.

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 Anne Boleyn, a wife of King Henry 8th 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 exampl, end up with 4 girls. 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 to 101 males to every 100 females, which is closer to the expected 50% of each.

Most tortoiseshell cats are female because this fur colour is a characteristic found on the X chromosome. But males have one X chromosome, so why can’t males be tortoiseshell?

For a tortoiseshell cat, two X chromosomes are needed to show the three different colours. So as females have XX sex-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 Turner syndrome when only one sex chromosome is present, a solitary X. XYY and XXYY are two more syndromes, and there are many others.

From this video you should now know that females usually have XX sex-chromosomes, and males usually have XY. The father provides the Y-chromosome, and so the chance of having a male child falls on his shoulders.


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