AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Sex determination: 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
Of the 23 pairs of chromosomes in a human body cell, 22 pairs are autosomes.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Of the 23 pairs of chromosomes in a human body cell, 22 pairs are autosomes. These autosomes carry genes that control general characteristics such as eye colour, blood group and height. The remaining pair is the sex chromosomes. In females the pair is XX, and in males it is XY. The sex-determining region of the Y chromosome carries the gene that triggers male development, so the presence or absence of a Y chromosome determines sex. Gametes carry one sex chromosome: all eggs carry X, while sperm carry either X or Y. At fertilisation, an X-bearing sperm produces XX offspring, and a Y-bearing sperm produces XY offspring.
What earns the marks
- Identify the 22 pairs of autosomes as carrying genes that control characteristics other than sex.
- Identify the 23rd pair as the sex chromosomes, XX in females and XY in males.
- Explain that the Y chromosome carries the gene that determines male sex, so its presence results in a male.
- Describe gametes as carrying one sex chromosome, with eggs always X and sperm either X or Y.
- Use a Punnett square or genetic cross to show the 50:50 chance of XX or XY offspring at fertilisation.
Common mistakes and how to put them right
Stating that the 22 pairs control only sex.
Put it right by saying the 22 pairs are autosomes controlling other characteristics, while the 23rd pair determines sex.
Believing that females are XY and males are XX.
Put it right by recalling females are XX and males are XY.
Thinking that the egg determines sex.
Put it right by explaining that sperm carry either X or Y, so the sperm determines the sex of the offspring.
Exam tips
- Label a genetic cross clearly with parental genotypes, gametes and offspring genotypes to show sex determination.
- Use the terms autosome and sex chromosome accurately when describing the 23 pairs.
- When asked about probability, state that there is a 50% chance of each sex, based on equal numbers of X-bearing and Y-bearing sperm.
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
In a human body cell, how many pairs of chromosomes carry the genes that determine sex?
Listen
Of the 23 pairs of chromosomes in a human body cell, 22 pairs are autosomes. These autosomes carry genes that control general characteristics such as eye colour, blood group and height. The remaining pair is the sex chromosomes. In females the pair is XX, and in males it is XY. The sex-determining region of the Y chromosome carries the gene that triggers male development, so the presence or absence of a Y chromosome determines sex. Gametes carry one sex chromosome: all eggs carry X, while sperm carry either X or Y. At fertilisation, an X-bearing sperm produces XX offspring, and a Y-bearing sperm produces XY offspring. A strong answer earns marks for these points. Identify the 22 pairs of autosomes as carrying genes that control characteristics other than sex. Identify the 23rd pair as the sex chromosomes, XX in females and XY in males. Explain that the Y chromosome carries the gene that determines male sex, so its presence results in a male. Describe gametes as carrying one sex chromosome, with eggs always X and sperm either X or Y. A common slip: Stating that the 22 pairs control only sex. Instead, saying the 22 pairs are autosomes controlling other characteristics, while the 23rd pair determines sex. Exam tip: Label a genetic cross clearly with parental genotypes, gametes and offspring genotypes to show sex determination.
See it
Key words
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Autosomes
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Sex chromosomes
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Y chromosome
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Gametes
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See every definition
- Autosomes
- The 22 pairs of chromosomes that carry genes controlling characteristics other than sex.
- Sex chromosomes
- The remaining pair of chromosomes, XX in females and XY in males.
- Y chromosome
- Carries the sex-determining region with the gene that triggers male development, so its presence or absence determines sex.
- Gametes
- Carry one sex chromosome, with all eggs carrying X and sperm carrying either X or Y.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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