Ordinary human cells contain 23 pairs of chromosomes, which is 46 chromosomes in total. 23 chromosomes come from our mother and 23 chromosomes come from our father. The 23rd pair of chromosomes are known as the sex chromosomes because they are the chromosomes that determine whether the offspring is male or female. The 23rd chromosomes can be X or Y.
- Females have two X chromosomes; they are XX
- Males have an X chromosome and a Y chromosome; they are XY
Males have one X and one Y chromosome; it is the Y chromosome that causes the embryo to develop into a male. Females do not have the Y chromosome, so the embryo develops into a female.
It is only the 23rd pair of chromosomes that determines your sex. The other 22 chromosomes determine other characteristics, such as eye colour, hair colour, whether you have cystic fibrosis etc.
We are able to draw a Punnet square for determining the sex of offspring. The male sex cells are sperm and the female sex cells are eggs. Sperm and eggs are both gametes and haploid cells; they have 23 chromosomes. A fertilised egg contains 23 pairs of chromosomes or 46 chromosomes in total. Gametes are created by meiosis. Females are XX, which means that they can only produce eggs with an X chromosome in them. Males are XY, which means that half of the sperm will have an X chromosome and the other half will have a Y chromosome. The male and female gametes are shown in the Punnett square below.
We now fill in the Punnett square by following the columns and rows. The filled in Punnett square is shown below.
There are 4 outcomes for the above Punnett square. Two of the outcomes are XX, which means that two of the offspring are females. The other two of the outcomes are XY, which means that two of the offspring are male. So, we have 2 females and 2 males. Therefore, there is a 50% chance of the offspring being female, and a 50% chance of the offspring being male. We can also say that the ratio of females to males is 50:50 or 1:1.
Also, it is always the male’s sperm that determines the sex of the offspring. This is because females can only pass on the X chromosome as they are XX, and males can pass on both the X and the Y chromosome as they are XY; it is then the Y chromosome that determines the sex of the offspring.
There is a second diagram that we can draw to show how the sex of offspring is determined. The diagram is shown below, and it works by following the lines.
The top circles in the above diagram show the chromosomes for the female (XX) and the male (XY) that are having the offspring. The next 4 circles below show the gametes that are produced. The female produces two eggs that are both X. And, the male produces two sperm where one is X and the other is Y. These gametes are produced by meiosis. The circles at the bottom show the different combinations for the 23rd pair of chromosomes for the offspring. From the 4 outcomes above, we can see that two of them are female (XX) and two of them are male (XY); there is a 50% chance of the offspring being female and a 50% chance of the offspring being female; these chances are the same as the Punnett square diagram.