Biological sex is shaped by genetic and hormonal processes that guide development before birth and during puberty. Understanding how chromosomes and hormones interact explains typical patterns of male and female physical development.

Chromosomes and the basis of biological sex

Biological sex begins with genetic instructions present at conception. Humans usually have 23 pairs of chromosomes, and the 23rd pair are the sex chromosomes.

This illustration contrasts the typical female (XX) and typical male (XY) sex-chromosome pair in a straightforward, labeled format. It helps cement the idea that the 23rd chromosome pair is special because it contains the sex chromosomes that usually pattern with female-typical or male-typical development. Source

Typical chromosomal patterns are XX for females and XY for males. The ovum always carries an X chromosome, but sperm carry either X or Y. This means the sperm determines chromosomal sex at fertilization.

This diagram shows how an XX mother produces eggs carrying only an X chromosome, while an XY father produces sperm carrying either X or Y. It visualizes how fertilization can result in XX or XY offspring, linking directly to how chromosomal sex is established at conception. Source

The role of chromosomes is not simply to label someone male or female. Their importance lies in the developmental instructions they contain. The Y chromosome usually carries the SRY gene, which activates the development of testes in the early embryo. If this signal is absent, ovaries typically develop instead.

Chromosomes therefore act as the starting point of a chain of events.

This pathway diagram maps the biological sequence from the Y chromosome’s SRY gene to testis development and downstream hormone signalling. It makes the ‘chromosomes → gonads → hormones → physical differentiation’ idea concrete by showing intermediate steps (e.g., Sertoli/Leydig cells and key hormones) that drive typical male-typical development. Source

They influence the development of the gonads - the testes or ovaries - and these organs then release hormones that shape later physical development. In other words, chromosomes guide biological sex indirectly as well as directly.

From genes to gonads

In early prenatal development, embryos begin with very similar structures. As development continues, chromosomal signals help direct whether the reproductive system follows a typical male or female pathway. This is why psychologists describe biological sex as a developmental process rather than a single moment.

Hormones in biological sex

Once gonads have formed, hormones help organize the body before birth and activate further changes later in life.

Hormones are important because they travel through the blood and affect specific target tissues. Their effects depend on timing, amount, and whether the body has the right receptors to respond.

Testosterone

Testosterone is produced mainly by the testes, although females also produce smaller amounts. During prenatal development, testosterone supports the formation of typical male reproductive anatomy. This helps explain why chromosomal differences lead to different bodily outcomes.

At puberty, rising testosterone levels contribute to male secondary sexual characteristics, such as a deeper voice, facial and body hair, increased muscle mass, and growth of the genitals. Testosterone is therefore central to the visible physical changes associated with male biological sex.

Estrogen (oestrogen)

Estrogen is produced mainly by the ovaries, although males also produce smaller amounts. It plays a major role in the development and regulation of the female reproductive system.

At puberty, increasing estrogen contributes to female secondary sexual characteristics, including breast development, widening of the hips, and changes in body fat distribution. It also supports maturation of reproductive organs and is closely linked to the menstrual cycle. In AQA materials, this hormone is often written as oestrogen.

Oxytocin

Oxytocin is sometimes called the "bonding hormone," but in biological sex it is especially relevant because of its reproductive functions. It is released in large amounts during childbirth, where it helps stimulate uterine contractions.

After birth, oxytocin also supports milk let-down during breastfeeding. These roles make it especially important in female reproductive processes. For this topic, it shows that hormones linked to biological sex are not limited to puberty; some are especially important in reproduction and early infant care.

Stages of sex differentiation

Prenatal development

Prenatal development is the first major stage of sex differentiation. Chromosomal patterns influence gonadal development, and gonads produce hormones that shape reproductive anatomy before birth. This means some aspects of biological sex are established long before a child is born.

Puberty

Puberty is the second major stage. Hormone production increases sharply, making sex differences more obvious. Primary sexual characteristics mature so reproduction becomes possible, while secondary sexual characteristics become more visible. Testosterone has a major role in typical male pubertal change, and estrogen has a major role in typical female pubertal change.

How chromosomes and hormones interact

Chromosomes and hormones should not be seen as separate explanations. They work together in a sequence:

• Chromosomes provide the initial genetic pattern.

• This pattern influences whether testes or ovaries develop.

• The gonads then release hormones.

• Hormones affect reproductive structures and other sex-linked physical features.

This interaction explains why biological sex develops across time. Some effects happen early in prenatal development, while others appear mainly at puberty or during reproduction in adulthood.

It is also important to remember that both males and females produce testosterone, estrogen, and oxytocin. What usually differs is the level, timing, and function of these hormones, not their simple presence or absence. For AQA Psychology, the key idea is that chromosomes set the developmental direction, and hormones carry out much of the physical differentiation associated with biological sex.