Sex Determination and Chromosomes for the ESAT

Sex Chromosomes and Sex Determination

In humans and most other mammals, the nucleus of a somatic cell contains 23 pairs of chromosomes. Of these, 22 pairs are known as autosomes, which are the same in both males and females. The 23rd pair consists of the sex chromosomes, which determine the biological sex of the individual.

There are two types of sex chromosomes: the $X$ chromosome and the $Y$ chromosome. The $Y$ chromosome is significantly smaller than the $X$ chromosome and consequently carries fewer genes. Crucially, the $Y$ chromosome contains the specific genes required to trigger male sexual development. Without these genes, the individual will develop as female.

The combination of these chromosomes defines the sex of the individual:

1. Females possess two $X$ chromosomes, giving them the genotype $XX$.
2. Males possess one $X$ chromosome and one $Y$ chromosome, giving them the genotype $XY$.

Sex Chromosomes in Gametes

Gametes are produced through the process of meiosis, which halves the chromosome number from the diploid state (46 chromosomes in 23 pairs) to the haploid state (23 single chromosomes). Because females are $XX$, every ovum (egg cell) produced will contain one $X$ chromosome.

Males, however, are $XY$. When sperm are produced through meiosis, the $X$ and $Y$ chromosomes separate into different cells. This means that 50 percent of the sperm produced will carry an $X$ chromosome and 50 percent will carry a $Y$ chromosome.

The Role of Fertilisation

The sex of a zygote (the fertilised egg) is determined at the moment of fertilisation by the type of sperm that successfully reaches and fuses with the egg. Since the egg always contributes an $X$ chromosome, the sperm is the deciding factor in sex determination.

1. If a sperm carrying a $Y$ chromosome fertilises the egg, the resulting zygote will have the $XY$ combination and develop into a male.
2. If a sperm carrying an $X$ chromosome fertilises the egg, the resulting zygote will have the $XX$ combination and develop into a female.

Analysing Genetic Data and Ratios

A Punnett square is the standard genetic diagram used to show the probability of offspring being male or female. By crossing the female genotype ($XX$) with the male genotype ($XY$), we can visualise the potential combinations for the offspring.

From the Punnett square, we can conclude the following:

• There is a 50 percent (or 0.5) probability that a child will be female ($XX$).
• There is a 50 percent (or 0.5) probability that a child will be male ($XY$).
• The ratio of male to female individuals in the human population should be 1 to 1.

Worked Example: Explaining Sex Development

Question: Explain why a zygote formed from the fertilisation of an ovum by a sperm cell carrying an $X$ chromosome develops into a female child.

Answer: Every somatic cell in a male contains both an $X$ and a $Y$ chromosome. The $Y$ chromosome is the carrier of key genes that initiate male sexual development. A sperm cell carrying an $X$ chromosome does not possess these specific male-initiating genes. Therefore, when it fuses with an ovum (which also always carries an $X$ chromosome), the resulting $XX$ genotype lacks the necessary triggers for male development and proceeds to develop into a female. This is why the absence of a $Y$ chromosome results in a female phenotype in humans.