AQA Syllabus focus:
'Diversity in sex development, including androgen insensitivity syndrome, Klinefelter’s syndrome and Turner syndrome.'
Sex development is not always strictly binary. AQA students need to understand how atypical chromosomal or hormonal processes can produce natural variation in biological sex development.
Understanding diversity in sex development
Biological sex development usually involves several linked factors, including chromosomes, gonads, hormones, and physical sex characteristics. In many people, these develop in the expected pattern of XX female or XY male, but this is not always the case. Some individuals show natural variations in one or more parts of this process. These variations are described in AQA as diversity in sex development.
Diversity in sex development: Natural variation in chromosomal, gonadal, hormonal, or anatomical sex development that does not fit a simple XX female/XY male pattern.
This matters because sex development is not controlled by chromosomes alone. A person’s phenotype, meaning their observable biological characteristics, depends on how different biological systems interact. Some variations are visible at birth, while others are only identified at puberty or during fertility investigations. The three named examples in the specification show different ways sex development can vary.
Androgen insensitivity syndrome
Androgen insensitivity syndrome (AIS) usually occurs in people with XY chromosomes. In AIS, the testes develop and produce androgens, but the body is partly or completely unable to respond to these hormones because the androgen receptors do not function typically.
This means that even though the person has XY chromosomes, physical development may not follow the usual male pattern. In complete AIS, external sex characteristics often develop in a more typically female way. In partial AIS, sex characteristics may be more mixed or ambiguous.
AIS shows an important point for psychology students: the effects of hormones depend on whether body tissues can respond to them.
Simplified model of androgen action in a target cell. The diagram shows testosterone (or DHT) binding to the androgen receptor (AR), receptor activation and nuclear entry, and subsequent binding to DNA to regulate transcription. This is a helpful visual for understanding why androgen insensitivity syndrome can occur even when androgen levels are normal: the signalling fails when AR function is impaired. Source
It is not enough for hormones to be present. Their action also depends on receptor sensitivity. In complete AIS, a person may be raised as female and may not discover the condition until puberty, when menstruation does not begin. A uterus is usually absent because testes still affect internal development during the prenatal period.
Klinefelter’s syndrome
Klinefelter’s syndrome usually involves an XXY chromosomal pattern.
Diagram showing how an XXY (Klinefelter) chromosomal pattern can arise. The figure uses color-coded X and Y chromosomes to illustrate an error in sex-chromosome separation leading to a gamete with two X chromosomes and a resulting XXY offspring. This helps link the label ‘XXY’ to a plausible biological mechanism (nondisjunction) rather than treating it as a fact to memorize. Source
The individual is typically assigned male at birth and develops male anatomy, but the additional X chromosome often affects testicular development and may lead to lower testosterone levels.
Common features can include:
tall stature
small testes
reduced facial and body hair
less muscle mass
breast tissue development
reduced fertility or infertility
Some individuals with Klinefelter’s syndrome also show language or learning difficulties, although intellectual ability varies widely. Many cases are mild and may not be diagnosed until adulthood.
For AQA purposes, the key idea is that Klinefelter’s syndrome represents male sex development with important biological variation. It does not usually produce a female phenotype, but it does show that adding an extra sex chromosome can influence the way male sex characteristics develop.
Turner syndrome
Turner syndrome usually involves a single X chromosome, written as X0, in a person who develops as female.
Because a second sex chromosome is missing or incomplete, the ovaries often do not develop typically. This can reduce estrogen production and affect puberty and fertility.
Common features can include:
short stature
underdeveloped ovaries
delayed or incomplete puberty
infertility
a webbed neck
a broader chest
Some individuals with Turner syndrome experience difficulties with spatial tasks, although verbal abilities are often less affected. As with Klinefelter’s syndrome, not everyone shows the same pattern. Severity varies, and some features are more noticeable than others.
Turner syndrome is important because it shows that female sex development can also vary as a result of chromosomal differences. It is not simply the opposite of Klinefelter’s syndrome. The mechanisms and developmental outcomes are different, even though both involve atypical sex chromosome patterns.
Comparing the three conditions
These three examples are grouped together because each one shows that biological sex development depends on several interacting factors rather than a single cause.
AIS: the chromosomal pattern is usually XY, but reduced sensitivity to androgens changes physical development.
Klinefelter’s syndrome: an extra X chromosome, usually XXY, affects male development, especially the testes and testosterone-related traits.
Turner syndrome: a missing or incomplete second sex chromosome, usually X0, affects female development, especially ovarian function and puberty.
In exam answers, avoid over-simplified claims such as saying these conditions “prove” chromosomes do not matter. A more accurate point is that chromosomes matter, but they do not act alone. Sex development can be influenced by chromosomal pattern, hormone production, and the body’s ability to respond to hormones. That is why these syndromes are important examples of diversity in sex development.
Practice Questions
Outline what is meant by androgen insensitivity syndrome. (3 marks)
1 mark for stating that the person usually has XY chromosomes.
1 mark for stating that the body is partly or completely insensitive to androgens / androgen receptors do not function typically.
1 mark for stating that sex characteristics may develop in a more typically female or mixed way despite XY chromosomes.
Explain what is meant by diversity in sex development. Refer to Klinefelter’s syndrome and Turner syndrome in your answer. (6 marks)
Award 1 mark for each correct point up to 6 marks:
Diversity in sex development means natural variation in biological sex development.
Sex development is more complex than a simple XX female / XY male model.
Klinefelter’s syndrome usually involves an XXY chromosomal pattern.
Klinefelter’s syndrome is associated with male development plus features such as lower testosterone, small testes, infertility, tall stature, or reduced body hair.
Turner syndrome usually involves a single X chromosome / X0.
Turner syndrome is associated with female development plus features such as short stature, underdeveloped ovaries, delayed puberty, or infertility.
FAQ
Diagnosis can happen at different stages:
before birth through prenatal genetic testing
in infancy if physical sex characteristics are atypical
at puberty if menstruation does not begin or puberty is delayed
in adulthood during fertility investigations
Doctors may use chromosome analysis, hormone tests, scans, and physical examination.
Mosaicism means that not all cells in the body have the same chromosomal pattern.
For example, some cells may be XXY and others XY, or some may be X0 and others XX. This can make physical features milder, less predictable, or harder to detect, which is one reason some people are diagnosed later.
Yes. Some people have relatively subtle features and are not diagnosed until adolescence or adulthood.
Late diagnosis is especially common when puberty is only mildly affected or when the first major concern is infertility. This is why prevalence estimates may be lower than the true number of cases.
Fertility is often affected because the gonads may not develop or function typically.
In Klinefelter’s syndrome, the testes may produce fewer sperm.
In Turner syndrome, the ovaries may not mature normally or may lose function early.
Fertility outcomes still vary, and specialist medical care can sometimes increase reproductive options.
These conditions involve private medical information and may also connect to identity, body image, and stigma.
Neutral language such as “variation” or “difference in sex development” is often preferred in clinical settings because it is less judgmental. However, individuals may choose different terms for themselves, so respectful communication should allow for personal preference.
