The Role of Chromosomes & Hormones (AQA A Level Psychology): Revision Note

The role of chromosomes in biological sex

• Chromosomes are made of nucleic acids (mainly DNA in humans) and protein; they are found in the nucleus of cells

• Humans have 23 pairs of chromosomes in each cell; the 23rd pair determines the baby’s sex

  • The chromosome genotype for a male is XY (one of the pair looks like an X and one looks shorter, like a Y)

  • Females have two of the larger X chromosomes, so their genotype is XX

• Chromosomes determine a baby’s sex at fertilisation, as all eggs carry an X chromosome

  • If the egg is fertilised by a sperm carrying a Y-chromosome, the child will be male: XY genotype

  • If the sperm carries an X chromosome, then the child will be female: XX genotype

• The Y chromosome carries the SRY gene, which, during prenatal development, causes testes to develop in an XY embryo; this then produces androgens

  • Androgens cause the embryo to become male, and without them, the embryo develops into a female

The role of hormones in biological sex

• Hormones are chemical messengers that are released into the bloodstream from glands

• Most sex development is a result of the influence of hormones prenatally in the uterus, which act upon brain development of the foetus and cause the development of reproductive organs (testes or ovaries)

• During puberty, hormonal activity triggers the development of secondary sexual characteristics, e.g., pubic hair

• Males and females produce the same hormones but in different concentrations, such as:

  • Testosterone, which stimulates the development of male sexual characteristics (such as facial and chest hair) and is produced in the testes

  • Oestrogen, which stimulates the development of female sexual characteristics (such as breasts and widening hips) and is produced in the ovaries

  • Oxytocin is produced in the hypothalamus. Women typically release it in larger amounts than men; it stimulates labour and breast-feeding in mothers, as well as bonding behaviour in both sexes

Research which investigates chromosomes and hormones

• Imperato-McGinley et al. (1974) studied children that were genetic males (XY) but appeared female at birth and were raised as girls. They became masculinised at puberty when testosterone surged, with most adopting a male identity, showing the strong influence of androgens on biological sex development

• Phoenix et al. (1959) studied female guinea pigs exposed to testosterone in the womb, who developed masculinised genitals and showed male-typical mating behaviour in adulthood. This shows the role of prenatal androgens in shaping biological sex development and demonstrates the organisational effect of hormones on the developing brain.

Evaluation of the role of chromosomes and hormones in biological sex

Strengths

• Research into diversity in sex development (DSD) supports the role of chromosomes and hormones in biological sex, e.g., individuals with complete androgen insensitivity syndrome (CAIS) have XY chromosomes but develop female external anatomy due to androgen receptor dysfunction

  • This shows that hormones (not just chromosomes) are essential to biological sex development

• Reiner & Gearhart (2004) studied 16 genetic males (XY) born without a functional penis: 14 were raised as female, but 8 of these participants reassigned themselves as male by the age of 16

  • This demonstrates that biological factors can override socialisation in sex development

Limitations

• Cause and effect cannot be firmly established, as it is unethical to experimentally manipulate prenatal hormones or chromosomes in humans. Most evidence comes from naturally occurring DSD cases, where multiple factors (genes, hormones and socialisation) interact

  • This means that the relationship between biology and sex development is complex

• The role of hormones in brain sexual dimorphism may be overstated as reviews by Eliot et al. (2021) have found that publication bias has inflated apparent sex differences. Most brain features show extensive overlap between males and females rather than clear dimorphism

  • This weakens the claim that prenatal hormones produce distinctly 'male' or 'female' brains and suggests hormonal effects on brain structure are smaller than first thought

Issues & Debates

• This topic supports the nature side of the debate, as it emphasises how biological factors (chromosomes and hormones) determine sex and sexual development

  • However, critics argue that environmental factors (e.g. upbringing or social expectations) also shape sex-related behaviours and identity, challenging a purely biological view

• Focusing solely on SRY gene activation or hormone levels reduces complex sex development and behaviour to simple biological causes (reductionism)

  • A holistic view would consider psychological, cognitive, and social influences alongside biology