Chromosomes and biological sex | AQA A-Level Psychology Revision
- Revision Notes
- Aug 5
- 16 min read
Updated: 7 hours ago
For 7182 specification, first teach in September 2025
AQA A-Level Psychology | Free Revision Notes
Estimated study time: 55 minutes
Chromosomes play a central role in biological sex development, but the relationship is more complex than simply equating XX with female and XY with male. These Chromosomes and biological sex A-Level Psychology revision notes explain typical chromosomal development, the role of the sex-determining region Y gene and patterns associated with diversity in sex development. Understanding these foundations will prepare you for the effects of testosterone, oestrogen and oxytocin and androgen insensitivity, Klinefelter’s syndrome and Turner syndrome.
Learning Objectives 🎯
By the end of this revision page, you should be able to:
Define chromosomes and sex chromosomes.
Explain the typical roles of XXand XY chromosome patterns in biological sex.
Explain how the sex-determining region YYY gene influences sexual development.
Distinguish XX, XY, X0 and XXY chromosome patterns.
Apply chromosome knowledge to androgen insensitivity syndrome, Klinefelter’s syndrome and Turner syndrome.
Analyse why chromosome pattern alone does not describe every aspect of biological sex development.
Revision Notes 📚
Chromosomes and biological sex A-Level Psychology revision overview
The AQA Gender topic requires students to understand:
The role of chromosomes in biological sex.
The interaction between chromosomes and hormones.
Diversity in sex development, including:
Androgen insensitivity syndrome.
Klinefelter’s syndrome.
Turner syndrome.
Most people have 46 chromosomes arranged into 23 pairs.
The first 22 pairs contain genetic information involved in many features of the body. The 23rd pair consists of the sex chromosomes.
The chromosomal patterns most important for this topic are:
XX
XY
X0
XXY
Androgen insensitivity syndrome usually involves an XY chromosome pattern, but the body does not respond typically to androgens. This demonstrates why biological sex development cannot always be predicted from chromosomes alone.
What are chromosomes?
Chromosomes are structures containing genetic information.
Genes are sections of genetic material that influence the development and functioning of the body. Chromosomes therefore provide biological instructions that contribute to characteristics such as:
The development of reproductive structures.
Hormone production.
Physical sexual development.
The 23rd chromosome pair is particularly important to biological sex.
These chromosomes are referred to as the sex chromosomes:
The X chromosome.
The Y chromosome.
Typical sex chromosome patterns
The two typical chromosomal patterns are:
Chromosome pattern | Typical biological development |
XX | Female biological development |
XY | Male biological development |
These are described as typical patterns because diversity in sex development means that chromosomes, hormone activity and physical development do not always follow this simple distinction.
📌 Exam tip: Write that XX and XY are the typical patterns. Avoid presenting them as the only possible patterns.
How sex chromosomes are inherited
An individual normally receives one sex chromosome from each biological parent.
The egg contains an X chromosome.
The sperm may contain:
An X chromosome.
A Y chromosome.
The combination at fertilisation typically produces one of two patterns.
An egg containing X and a sperm containing X
X+X=XX
This is usually associated with female biological development.
An egg containing X and a sperm containing Y
X+Y=XY
This is usually associated with male biological development.
The presence of the Y chromosome is especially significant because it normally contains a gene that initiates the development of testes.
The role of the Y chromosome
The YYY chromosome contains the sex-determining region Y gene, commonly abbreviated to the SRY gene.
The SRY gene initiates a developmental process that usually leads to:
The development of testes.
The production of androgens, especially testosterone.
The development of typically male sexual characteristics.
The process can be summarised as:
Y chromosome → SRY gene → testes develop → androgens are produced → typical male sexual development
This sequence explains why the Y chromosome has an important role in biological sex.
The chromosome does not directly create every male sexual characteristic. Instead, the SRY gene influences gonadal development, which affects hormone production.
The next stage of this biological process is covered in how hormones influence sexual development and behaviour.
The SRY gene
SRY stands for sex-determining region Y.
Its important function is to direct the initially undifferentiated gonads towards the development of testes.
The testes then produce androgens, including testosterone.
A strong examination answer should therefore link the ideas rather than write them as unrelated facts:
“The Y chromosome contains the SRY gene. This normally leads to the development of testes, which produce androgens such as testosterone. These hormones contribute to typically male sexual development.”
Simply writing “the Y chromosome makes someone male” lacks sufficient explanation.
Typical XX development
A typical XX chromosome pattern does not contain a Y chromosome or the SRY gene.
This is normally associated with:
The development of ovaries.
Oestrogen becoming an important hormone in sexual development.
Typically female sexual characteristics.
The typical sequence can be summarised as:
XX pattern → no SRY gene → ovarian development → typical female sexual development
This is a simplified account. Diversity in sex development shows that chromosomes are one part of a wider biological process.
Chromosomes and hormones work together
Chromosomes and hormones should not be treated as completely separate explanations.
Chromosomes influence:
Which gonads develop.
Which hormones are produced.
The hormonal environment affecting later development.
Hormones then act on body tissues.
The relationship can therefore be represented as:
chromosome pattern → gonadal development → hormone production → effects on body tissues
This relationship is especially important when applying chromosome knowledge to androgen insensitivity syndrome.
A person can have an XY chromosome pattern and produce androgens, but sexual development may differ if body tissues do not respond typically to those hormones.
Chromosomal pattern is not the same as gender identity
Biological sex and gender identity are related areas of study, but they are not interchangeable terms.
This lesson concerns biological development involving:
Chromosomes.
Gonads.
Hormones.
Physical sexual characteristics.
Gender identity concerns a person’s experience and understanding of their gender.
It is therefore inaccurate to assume that a chromosome pattern alone tells you:
How a person identifies.
Which gender roles they will follow.
How masculine or feminine their behaviour will be.
Which interests or personality traits they will have.
The distinction is explored further in binary, non-binary and gender-fluid identities.
Diversity in sex development
Diversity in sex development refers to variations in chromosomes, hormones or sexual anatomy.
The AQA specification names three examples:
Androgen insensitivity syndrome.
Klinefelter’s syndrome.
Turner syndrome.
These examples are important because they demonstrate that biological sex involves several connected characteristics.
A person’s development may involve:
A particular chromosome pattern.
The presence or absence of the SRY gene.
Gonadal development.
Hormone production.
The body’s response to hormones.
External and internal sexual characteristics.
These features do not always follow the most typical XX or XY pathways.
Overview of the chromosome patterns
Developmental pattern | Chromosomes | Main chromosomal feature | Typical classification |
Typical female development | XX | Two X chromosomes | Female |
Typical male development | XY | One X, one Y, usually including SRY | Male |
Turner syndrome | X0 | One X, with the second sex chromosome absent | Female |
Klinefelter’s syndrome | XXY | Two X chromosomes and one Y | Male |
Androgen insensitivity syndrome | Usually XY | Typical male chromosome pattern, but reduced or absent response to androgens | Development varies, with typically female external development in complete AIS |
The table shows why counting the number of X chromosomes is not enough to predict development.
For example:
A person with XXY chromosomes normally develops as male because a Y chromosome and SRY gene are present.
A person with XYchromosomes and complete androgen insensitivity develops typically female external characteristics because their tissues do not respond to androgens.
Turner syndrome
Turner syndrome is associated with an X0X0X0 chromosome pattern.
The notation means:
One X chromosome is present.
The second sex chromosome is absent.
Some materials write the pattern as XO, but the second symbol represents the absence of a chromosome rather than the letter O. Writing X0 makes this clearer.
A person with Turner syndrome is biologically female.
Characteristics may include:
Shorter than average stature.
Ovaries that do not develop typically.
Limited development of secondary sexual characteristics without hormonal treatment.
Infertility.
Relatively strong language or reading skills in some individuals.
Not every person will show every characteristic in exactly the same way.
Why Turner syndrome is relevant
Turner syndrome shows that two X chromosomes are not required for female biological development.
The absence of a Y chromosome and SRY gene means that typical male development is not initiated.
However, the missing second sex chromosome affects aspects of physical and reproductive development.
Turner syndrome therefore helps psychologists distinguish between:
The direction of sexual development.
The full and typical development of sexual characteristics.
Applying Turner syndrome
Consider the following scenario:
Genetic testing shows that a person has one X chromosome and no second sex chromosome. The person has developed as female but experiences limited ovarian development.
This can be explained as follows:
The chromosome pattern is X0.
This is associated with Turner syndrome.
There is no Y chromosome or SRY gene.
Male sexual development is therefore not initiated.
The missing second sex chromosome affects ovarian and secondary sexual development.
Klinefelter’s syndrome
Klinefelter’s syndrome is associated with an XXY chromosome pattern.
The person has:
Two X chromosomes.
One Y chromosome.
A person with Klinefelter’s syndrome is biologically male.
This is because the Y chromosome usually contains the SRY gene, initiating the development of testes.
Possible characteristics include:
Greater than average height.
Small testes.
Reduced facial or body hair.
Lower testosterone production.
Infertility.
Difficulties with reading or writing in some individuals.
Greater emotional sensitivity in some individuals.
These are possible features rather than a checklist applying identically to every person.
Why Klinefelter’s syndrome is relevant
Klinefelter’s syndrome shows that the presence of two X chromosomes does not automatically produce female biological development.
The YYY chromosome and SRY gene normally initiate male development.
This provides an important exam distinction:
XXY is associated with male biological development, not female development.
The additional X chromosome affects physical and reproductive development, but it does not cancel the influence of the YYY chromosome.
Applying Klinefelter’s syndrome
Consider this scenario:
A person has two X chromosomes and one Y chromosome. Tests show reduced testosterone production and small testes.
A developed application would explain that:
The chromosome pattern is XXY
This is associated with Klinefelter’s syndrome.
The YYY chromosome normally contains the SRY gene.
The SRY gene initiates testicular development.
The person therefore develops as male, although the additional X chromosome affects later sexual development.
Androgen insensitivity syndrome
Androgen insensitivity syndrome, abbreviated to AIS, usually occurs in a person with an XY chromosome pattern.
The person normally has:
A Y chromosome.
The SRY gene.
Testes that produce androgens.
However, body tissues are partly or completely insensitive to those androgens.
The hormones may be present, but the body cannot respond to them in the typical way.
Complete androgen insensitivity syndrome
In complete androgen insensitivity syndrome, the body does not respond to androgens.
Although the person has an XY chromosome pattern:
Typically male external sexual characteristics do not develop.
External development is typically female.
The person is generally assigned female at birth.
AIS therefore demonstrates that chromosomes influence biological sex through a chain of processes.
The presence of XY chromosomes does not by itself guarantee typical male external development. Hormones must also be produced and body tissues must be able to respond to them.
Applying androgen insensitivity syndrome
Consider this scenario:
Genetic testing shows an XY chromosome pattern. The SRY gene has initiated the development of testes, but the person has typically female external sexual characteristics because body tissues do not respond to androgens.
This can be explained as follows:
The chromosome pattern is XY.
The presence of SRY led to testicular development.
Androgens were produced.
Body tissues were insensitive to the androgens.
Typical male external development therefore did not occur.
The pattern is consistent with androgen insensitivity syndrome.
Why AIS matters for understanding chromosomes
AIS is particularly useful for separating three processes:
Chromosomal sex
The person has XYXYXY chromosomes.
Hormone production
Testes produce androgens.
Hormone response
The body does not respond typically to the androgens.
The example shows that hormone action depends on both the presence of a hormone and the sensitivity of body tissues to it.
This interaction is examined more fully in androgen insensitivity, Klinefelter’s syndrome and Turner syndrome.
Comparing Turner, Klinefelter’s and AIS
Feature | Turner syndrome | Klinefelter’s syndrome | Androgen insensitivity syndrome |
Chromosome pattern | X0 | XXY | Usually XY |
Y chromosome present? | No | Yes | Yes |
SRY usually present? | No | Yes | Yes |
Typical developmental direction | Female | Male | Female external development in complete AIS |
Main reason development differs | Missing second sex chromosome | Additional X chromosome | Body tissues do not respond typically to androgens |
Key exam point | One X, not no chromosomes | Two X chromosomes do not make the person female | XY does not always lead to typical male external development |
What these patterns show about biological sex
The chromosome patterns demonstrate several important principles.
Chromosomes play a major role
Changes in the sex chromosome pattern are associated with predictable changes in biological development.
For example:
X0 is associated with Turner syndrome.
XXY is associated with Klinefelter’s syndrome.
XY normally initiates male development through SRY.
This supports the view that chromosomes are a significant biological influence.
The Y chromosome has a particularly important role
Klinefelter’s syndrome demonstrates that a person with two X chromosomes may still develop as male when a Y chromosome is present.
This is because the SRY gene normally initiates testicular development.
Chromosomes do not act alone
AIS shows that chromosome pattern is not the final stage of biological sex development.
Typical development also depends on:
Gonadal development.
Hormone production.
Hormone receptors.
The response of body tissues.
A chromosome-only explanation is therefore incomplete.
Biological sex is more complex than a strict binary chromosome rule
Most people follow typical XX or XY pathways.
However, DSD patterns show that:
Additional chromosomes can occur.
A chromosome may be absent.
Hormonal responses may differ.
Chromosomal and external sexual characteristics may not align in the typical way.
This does not mean that chromosomes are unimportant. It means their influence operates within a broader biological system.
Chromosomes as a biological explanation
Chromosomes also appear later in the Gender topic as part of evaluating biological accounts of gender development.
A chromosome-based explanation has several strengths.
It identifies a clear biological mechanism
The sequence from the Y chromosome to the SRY gene, testes and androgen production provides a clear account of typical male biological development.
This is stronger than merely observing that males and females often differ biologically because it identifies a mechanism through which development occurs.
DSD patterns provide useful evidence
Turner syndrome, Klinefelter’s syndrome and AIS show that variations in chromosomes or hormone response are associated with variations in development.
These patterns help psychologists examine the separate contributions of:
Chromosomes.
Hormones.
Hormone sensitivity.
It can become biologically reductionist
Explaining biological sex solely through XX and XY patterns reduces a complex developmental process to one variable.
This is incomplete because AIS demonstrates that hormone response also matters.
The broader issue of explaining complex phenomena through lower-level biological components is covered in biological and environmental reductionism.
It can become biologically deterministic
A strongly deterministic account may imply that chromosomes entirely and inevitably determine later gendered behaviour.
Chromosomes have a major role in biological sex development, but they do not by themselves determine:
Gender identity.
Gender roles.
Interests.
Personality.
Behaviour.
The difference between biological influence and complete determination can be explored through biological, environmental and psychic determinism.
Applying chromosome knowledge in examinations
Application questions may present:
A chromosome pattern.
A description of sexual development.
Information about hormone production.
Information about hormone sensitivity.
A comparison between two people.
Use the following strategy.
Step 1: Identify the chromosome pattern
Look for:
Two X chromosomes: XX.
One X and one Y: XY
One X and no second sex chromosome: X0
Two X chromosomes and one YYY: XXY
Step 2: Identify whether a Y chromosome is present
If a Y chromosome is present, the SRY gene will normally initiate the development of testes.
Step 3: Consider hormone action
Ask:
Are androgens produced?
Can body tissues respond to them?
This is essential when AIS is described.
Step 4: Name the relevant developmental pattern
X0: Turner syndrome.
XXY: Klinefelter’s syndrome.
XY with reduced or absent androgen response: AIS.
Step 5: Explain, do not merely label
A weak answer states:
“This is AIS.”
A developed answer states:
“The person has an XY chromosome pattern and the SRY gene has initiated testicular development. However, their tissues are insensitive to androgens, so typical male external sexual development does not occur. This is consistent with androgen insensitivity syndrome.”
Using respectful and accurate terminology
Diversity in sex development concerns real people, not simply unusual examination examples.
Use person-first and precise language, such as:
“A person with Turner syndrome.”
“A person with Klinefelter’s syndrome.”
“A person with androgen insensitivity syndrome.”
Avoid:
Defining a person only by a condition.
Describing every variation as an “abnormality”.
Assuming the same characteristics apply to every individual.
Inferring gender identity from a chromosome pattern.
Research in this area can be socially sensitive because inaccurate conclusions may affect how individuals or groups are perceived. This connects with the consequences of socially sensitive research.
Hints from the Examiner Reports 💡
Examiner hint: For a short “describe” question, develop the biological sequence. A strong answer should move beyond stating that females are XX and males are XY. Link the YYY chromosome to the SRY gene, testicular development and androgen production.
Examiner hint: Use accurate specialist terminology. AQA’s 2021 mark scheme rewarded clear and detailed knowledge including the 23rd chromosome pair, the SRY gene, hormone production and X0X0X0 or XXY patterns.
Examiner hint: Do not confuse the number of chromosomes with the number of pairs. Humans normally have 23 pairs, not 23 chromosomes.
Examiner hint: Shape every point to the question. In the 2022 extended response, students generally demonstrated strong knowledge but often produced weaker discussion because their evidence about testosterone or the Y chromosome focused on aggression without linking back effectively to sex or gender.
Examiner hint: A “describe” question requires accurate knowledge, not evaluation. A “discuss” question requires both description and developed analysis. Pay close attention to the command word.
Examiner hint: Avoid writing an essay entirely about chromosomes when the question asks about chromosomes and hormones. Address the complete wording of the question.
Examiner hint: Do not merely list Turner and Klinefelter’s syndromes. State the relevant chromosome pattern and explain how it affects biological development.
Examiner hint: With an AIS scenario, separate chromosome pattern, hormone production and hormone response. This prevents the inaccurate claim that an XY pattern always produces typical male external development.
Examiner hint: Keep biological sex and gender identity distinct. Do not infer a person’s identity, roles or behaviour solely from their sex chromosomes.
Common Mistakes ⚠️
Mistake: Writing that humans have 23 chromosomes
Why this is incorrect:
Humans normally have 46 chromosomes organised into 23 pairs.
How to improve:
Refer to the X and Y chromosomes as the 23rd pair.
Mistake: Saying the mother determines whether the pattern is XX or XY
Why this is incorrect:
The egg normally supplies an X chromosome. The sperm supplies either an X or a Y.
How to improve:
Use:
X+X=XX
or:
X+Y=XY
Mistake: Saying that the Y chromosome directly produces testosterone
Why this is incorrect:
The SRY gene on the Y chromosome normally initiates the development of testes. The testes then produce androgens.
How to improve:
Explain the complete sequence from chromosome to gonad to hormone.
Mistake: Treating X0 as the letters “X” and “O”
Why this is incorrect:
The zero indicates that the second sex chromosome is absent.
How to improve:
Write X0X0X0 and explain that only one X chromosome is present.
Mistake: Saying a person with Turner syndrome has no X chromosome
Why this is incorrect:
A person with Turner syndrome has one X chromosome rather than the usual two sex chromosomes.
How to improve:
Remember:
Turner = one X, X0
Mistake: Saying XXY must produce female development because there are two X chromosomes
Why this is incorrect:
The Y chromosome normally carries the SRY gene, so XXY is associated with male development.
How to improve:
Look first for the presence of a Y chromosome and SRY.
Mistake: Saying all XY individuals show typical male external development
Why this is incorrect:
In AIS, the body does not respond typically to androgens.
How to improve:
Consider chromosomes, hormone production and hormone sensitivity separately.
Mistake: Treating chromosomes and hormones as the same thing
Why this is incorrect:
Chromosomes contain genetic information. Hormones are chemical messengers produced by glands or gonads.
How to improve:
Explain how chromosomes influence which hormones are produced.
Mistake: Assuming chromosome pattern determines gender identity
Why this is incorrect:
Chromosomes contribute to biological sex development but do not by themselves establish a person’s gender identity.
How to improve:
Keep biological sex, gender identity and gender-related behaviour conceptually distinct.
Mistake: Presenting possible syndrome characteristics as universal
Why this is incorrect:
Individuals vary, and not everyone experiences every associated characteristic.
How to improve:
Use phrases such as “may include” or “is commonly associated with”.
Exam-Style Questions ✍️
Questions
1. Which chromosome pattern is typically associated with male biological development?[1 mark]
A. XX
B. XY
C. X0
2. Outline the role of the SRY gene in biological sex development.[3 marks]
3. Explain how typical XX and XY chromosome patterns influence biological sex development.[4 marks]
4. A genetic test shows that a person has one X chromosome and no second sex chromosome.
Identify the chromosome pattern and explain the likely direction of biological sex development.[3 marks]
5. A person has two X chromosomes and one Y chromosome.
Explain why the person usually develops as male despite having two X chromosomes.[4 marks]
6. Alex has an XY chromosome pattern and testes that produce androgens. However, Alex’s body tissues do not respond to the androgens, and Alex has typically female external sexual characteristics.
Use your knowledge of chromosomes and diversity in sex development to explain Alex’s development.[4 marks]
7. Complete the following table.[4 marks]
Description | Chromosome pattern or DSD |
Typical female chromosome pattern | __________ |
One X chromosome with the second sex chromosome absent | __________ |
Two X chromosomes and one Y chromosome | __________ |
XY pattern with reduced or absent response to androgens | __________ |
8. Explain why chromosome pattern alone is not always sufficient to predict external sexual development.[4 marks]
9. Discuss the role of chromosomes in biological sex. Refer to diversity in sex development in your answer.[8 marks]
Answers and Mark Scheme
Question 1
One mark for:
B, XYXYXY.
Question 2
Award up to three marks:
The SRY gene is normally located on the YYY chromosome.
It initiates the development of testes.
The testes produce androgens, including testosterone, which contribute to typically male sexual development.
Question 3
Award up to four marks:
XXXXXX is the chromosome pattern typically associated with female biological development.
It does not normally include a YYY chromosome or SRY gene.
XYXYXY is the chromosome pattern typically associated with male biological development.
The YYY chromosome normally contains SRY, which initiates testicular development and androgen production.
Question 4
Award up to three marks:
The pattern is X0X0X0.
It is associated with Turner syndrome.
There is no YYY chromosome or SRY gene, so the person normally develops as female, although the missing second chromosome affects ovarian and secondary sexual development.
Question 5
Award up to four marks:
The pattern is XXYXXYXXY.
It is associated with Klinefelter’s syndrome.
The YYY chromosome normally contains the SRY gene.
SRY initiates testicular development, so the person develops as male, although the additional XXX affects later physical and reproductive development.
Question 6
Award up to four marks:
Alex’s XYXYXY pattern normally includes the SRY gene.
SRY has initiated testicular development and androgen production.
Alex’s tissues are insensitive to the androgens.
Typical male external development does not occur, which is consistent with androgen insensitivity syndrome.
Question 7
Award one mark for each correct response:
Description | Answer |
Typical female chromosome pattern | XX |
One X chromosome with the second sex chromosome absent | X0, Turner syndrome |
Two X chromosomes and one Y chromosome | XXY, Klinefelter’s syndrome |
XY pattern with reduced or absent response to androgens | Androgen insensitivity syndrome |
Question 8
Award up to four marks:
Chromosomes influence gonadal and hormonal development.
Hormones must act on body tissues to affect sexual development.
In AIS, an XYXYXY individual produces androgens but the body does not respond typically to them.
Therefore, chromosomes, hormone production and hormone sensitivity must be considered together.
Question 9
A strong response should include:
Knowledge and understanding
Chromosomes as carriers of genetic information.
The 23rd chromosome pair.
Typical XXXXXX and XYXYXY patterns.
The YYY chromosome and SRY gene.
Development of testes.
Production of androgens.
The relationship between chromosomes and hormones.
X0X0X0 and Turner syndrome.
XXYXXYXXY and Klinefelter’s syndrome.
XYXYXY chromosomes and androgen insensitivity syndrome.
Discussion
The SRY mechanism provides a clear biological explanation.
DSD patterns demonstrate that chromosomes have significant developmental effects.
Klinefelter’s syndrome shows the importance of the YYY chromosome even when two XXX chromosomes are present.
AIS demonstrates that chromosomes alone do not determine external sexual development.
A chromosome-only account may be biologically reductionist.
Chromosome patterns should not be used to make automatic claims about gender identity or behaviour.
Research and language concerning DSD must be handled sensitively.
Higher-level answers will accurately link chromosomes, gonadal development and hormone action rather than presenting them as isolated facts.
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