4.3.3 - Chromosomes, hormones and diversity in sex development

Biological sex as a pattern

Biological sex concerns the bodily features involved in sex development and reproduction, especially chromosomes, gonads, hormones, internal reproductive organs, external genitalia and secondary sex characteristics.

Biological sex is often classified as male or female at birth, usually from external genitalia. For most people, the different features line up in a typical way. For example, a 46,XX karyotype usually develops with ovaries and higher oestrogen at puberty, while a 46,XY karyotype usually develops with testes and higher testosterone at puberty. However, sex development is not controlled by one switch. A person's chromosomal, hormonal, gonadal and anatomical features can vary in ways that do not fit the simplest XX/XY model.

Diversity in sex development (DSD) includes variations in chromosomes, hormone production or hormone response. It is broader than a difference in chromosome number. The three required examples illustrate different mechanisms:

Use careful language. It is accurate to describe AIS, Klinefelter syndrome and Turner syndrome as medical or genetic conditions, but avoid wording that suggests a person is "wrong", "less male", "less female" or reducible to a chromosome pattern. AQA wants biological understanding, not judgement.

Chromosomes and typical sex development

Humans usually have 46 chromosomes arranged in 23 pairs. The first 22 pairs are autosomes. The 23rd pair are the sex chromosomes, usually XX or XY.

Egg cells usually carry an X chromosome. Sperm cells usually carry either an X or a Y chromosome. This means the sperm usually determines the chromosomal pattern of the embryo:

The Y chromosome is important because it usually carries the SRY gene.

In a typical 46,XY pathway, SRY activity leads to the development of testes. The testes then produce hormones, including testosterone, that guide male-typical development before birth and again at puberty. Testosterone is also converted in some tissues into dihydrotestosterone (DHT), which is especially important for male-typical external genital development. You do not need DHT as a separate AQA hormone, but it helps explain why "testosterone" is often discussed as part of a wider androgen system.

In a typical 46,XX pathway, ovarian development involves an active network of genes. Female-typical prenatal reproductive anatomy does not simply arise because an ovary releases oestrogen: the absence of testicular signals is also important. Testes normally release anti-Müllerian hormone, which causes structures that would form the uterus and uterine tubes to regress. Androgen action supports other male-typical internal and external structures. These details explain why a person can have testes and no uterus yet have female-typical external genitalia in complete AIS. Later, ovarian oestrogen supports pubertal changes and reproductive functioning. The additional hormone name is explanatory background, not an extra named hormone to memorise for this bullet.

The exam point is causal but cautious: chromosomes help start sex development, but hormones and hormone sensitivity shape what happens next. AIS makes this especially clear. A person may have XY chromosomes and produce testosterone, but if their cells cannot respond to androgens, sex development will not follow the usual XY pathway.

Testosterone, oestrogen and oxytocin

Hormones are chemical messengers released into the bloodstream. They affect target organs and tissues that have the right receptors. In biological sex, hormones matter at different stages: before birth, at puberty, during menstrual cycles, during pregnancy and birth, and across adult reproductive functioning.

Testosterone and oestrogen are often called sex hormones because they are directly involved in sexual development and reproductive functioning. They are present in all people, but typical levels and timing differ. For example, testosterone rises substantially in many males at puberty, while oestrogen rises substantially in many females at puberty. These changes contribute to secondary sex characteristics.

Oxytocin has a different role. It is not a chromosome-linked sex-determining hormone. Its clearest physical functions are in childbirth and breastfeeding: it helps the uterus contract during labour and helps move milk from breast tissue during lactation. It is also involved in sexual arousal, ejaculation, parent-infant bonding and social recognition. A good AQA answer treats oxytocin as part of reproductive biology and social bonding, while avoiding the oversimplified claim that it "causes women to bond" or that men lack it.