sex determination | Genetics | Biology

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Human body cells have 46 chromosomes 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 while for a female they are XX.

In order to make a baby, a sperm has to attach to an ovum (or egg cell). You should know that they only have half of a body cell's DNA; They are 'haploid'. Instead of 46 chromosomes, 23 pairs, they have a total of 23 chromosomes. 22 autosomes and one sex chromosome, or allosome. An ovum will always have an X chromosome but a sperm will have an X or a Y. If we look at this in the Punnett class, the gametes from the female are always X, so X, X, and the gametes from the male are either X or Y.

So really, it is the man who determines the sex of the child. He brings Y, which is the only possibility of a male child. Unfortunately for Anne Boleyn, wife of King Henry 8th of England in the 16th century, she was executed for not providing a male heir to the throne when in fact her sperm was to blame!

From the Punnett class, we would expect 50% of the offspring to be male and 50% female. But actual results may be different. For example, a couple may end up with 4 girls. Exemption of social factors such as sex-selective abortion at birth, the natural male-to-female sex ratio for humans ranges from about 105 males to 100 females. It is thought that this is nature's way of overcoming the fact that men have a higher risk of dying younger than women. The overall world male-to-female sex ratio drops to 101 males for every 100 females, closer to the expected 50% of each.

Most tortoiseshell cats are female because this fur coloration is a trait found on the X chromosome. But males have one X chromosome, so why can't males be tortoiseshells?

For the tortoiseshell cat, two X chromosomes are needed to show the three different colors. So as females have XX sex-chromosomes, they can be tortoiseshell. But XY men can't. Now I say 'most' instead of all because you can sometimes get a male tortoiseshell cat, just as you can sometimes get a human male with XX chromosomes. These rare individuals have an extra strand of DNA, and are therefore XXY (instead of just XY). Therefore the male needs a double X to become a tortoise.

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 women usually have XX sex-chromosome, and men usually have XY. The father provides the Y-chromosome, and so the prospect of having a male child falls on his shoulders.

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