AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Pairs control characteristics only, but one of the pairs carries the genes…
Full specification wording
Sex determination: 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
Human body cells contain 23 pairs of chromosomes.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Human body cells contain 23 pairs of chromosomes. The first 22 pairs are autosomes and carry genes that control general characteristics such as eye colour, blood group and height. The 23rd pair are the sex chromosomes and carry the genes that determine sex. In females the pair is XX; in males it is XY. The Y chromosome carries the gene that triggers male development, so the presence of a Y chromosome makes a person male. Gametes are haploid: all egg cells carry an X chromosome, while sperm cells carry either an X or a Y chromosome. At fertilisation, an X-bearing sperm produces a female zygote (XX) and a Y-bearing sperm produces a male zygote (XY). This is why sex is determined by the father.
What earns the marks
- The 22 pairs of autosomes carry genes that control characteristics other than sex, such as eye colour and blood group.
- The 23rd pair are the sex chromosomes and carry the genes that determine whether an individual is male or female.
- Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
- The Y chromosome carries the gene that causes male development, so the presence of a Y chromosome determines maleness.
- All egg cells carry an X chromosome, while sperm cells carry either an X or a Y chromosome.
- Sex is determined at fertilisation by the sperm: an X-bearing sperm gives a female (XX) and a Y-bearing sperm gives a male (XY).
Common mistakes and how to put them right
Saying the mother determines the sex of the child.
Put it right: The father determines sex because sperm carry either X or Y, while all eggs carry X.
Writing sex chromosomes without the correct pairing, such as saying males are YY.
Put it right: Males are XY and females are XX.
Thinking the 22 autosome pairs also determine sex.
Put it right: Only the 23rd pair, the sex chromosomes, carries the genes that determine sex.
Exam tips
- Use a Punnett square with X and Y gametes to show the 50:50 chance of male or female offspring.
- State clearly that all eggs carry X and sperm carry X or Y, then link this to the sex of the zygote.
- Distinguish autosomes from sex chromosomes by number: 22 pairs versus 1 pair.
Try the lesson
The start of the Sex determination: 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex. lesson, made from checked teaching notes for this specification point.
Have a go first
A human body cell contains 23 pairs of chromosomes. Which statement correctly describes the function of these chromosome pairs?
Listen
Human body cells contain 23 pairs of chromosomes. The first 22 pairs are autosomes and carry genes that control general characteristics such as eye colour, blood group and height. The 23rd pair are the sex chromosomes and carry the genes that determine sex. In females the pair is XX; in males it is XY. The Y chromosome carries the gene that triggers male development, so the presence of a Y chromosome makes a person male. Gametes are haploid: all egg cells carry an X chromosome, while sperm cells carry either an X or a Y chromosome. At fertilisation, an X-bearing sperm produces a female zygote (XX) and a Y-bearing sperm produces a male zygote (XY). This is why sex is determined by the father. A strong answer earns marks for these points. The 22 pairs of autosomes carry genes that control characteristics other than sex, such as eye colour and blood group. The 23rd pair are the sex chromosomes and carry the genes that determine whether an individual is male or female. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The Y chromosome carries the gene that causes male development, so the presence of a Y chromosome determines maleness. A common slip: Saying the mother determines the sex of the child. Instead, the father determines sex because sperm carry either X or Y, while all eggs carry X. Exam tip: Use a Punnett square with X and Y gametes to show the 50:50 chance of male or female offspring.
Key words
Say what each word means before you turn the card.
Autosomes and sex chromosomes
Try answering aloud or in your head before turning the card.
Sex determination
Try answering aloud or in your head before turning the card.
Gametes and fertilisation
Try answering aloud or in your head before turning the card.
Inheritance of sex
Try answering aloud or in your head before turning the card.
See every definition
- Autosomes and sex chromosomes
- The first 22 pairs of chromosomes are autosomes and carry genes that control general characteristics, while the 23rd pair are the sex chromosomes and carry the genes that determine sex.
- Sex determination
- Sex is determined at fertilisation by the sperm: an X-bearing sperm gives a female and a Y-bearing sperm gives a male.
- Gametes and fertilisation
- Gametes are haploid: all egg cells carry an X chromosome, while sperm cells carry either an X or a Y chromosome, and at fertilisation these combine.
- Inheritance of sex
- Females have two X chromosomes and males have one X and one Y chromosome, and the Y chromosome carries the gene that triggers male development.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Sex determination: 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge