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XX and XY are the sex chromosomes: females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Free GCSE Biology notes and 12 FAQs.
Sex Chromosomes and Genetic Crosses is part of Reproduction in GCSE Edexcel Higher Biology.
3 min
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GCSE Edexcel Higher
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XX and XY are the sex chromosomes: females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Free GCSE Biology notes and 12 FAQs.
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Use this lesson to stay inside Reproduction while connecting the detail back to the wider Topic 3: Genetics topic.
Read the core explanations from Sex Chromosomes and Genetic Crosses before testing yourself from memory.
Walk into any school and you'll notice something remarkable - roughly half the students are male, half are female. This isn't random chance. It's the result of a precise biological mechanism involving one special pair of chromosomes.
Every human body cell contains 23 pairs of chromosomes - that's 46 chromosomes total. Twenty-two of these pairs control characteristics like eye colour, height, and blood type. But the 23rd pair is different. This pair determines whether you develop as male or female.
Here's the key difference: females have two X chromosomes (written as XX), while males have one X and one Y chromosome (written as XY). The Y chromosome is much smaller than the X and carries the genes that trigger male development.
Trap: Many students think males are XY because they're 'stronger' or 'different'. Actually, the Y chromosome is smaller and carries fewer genes than the X. It's the presence of specific genes on the Y chromosome that triggers male development.
During reproduction, each parent contributes one chromosome from each pair to their offspring. For sex chromosomes, this creates a fascinating pattern. Let's trace what happens:
The mother (XX) can only contribute an X chromosome - she has no Y to give. The father (XY) can contribute either an X or a Y chromosome. This means the father's contribution determines the child's sex.
Worked example: Sex Inheritance Genetic Cross
Show how sex is inherited when a female (XX) and male (XY) have children.
Answer: 50% chance of female (XX), 50% chance of male (XY)
This 50:50 ratio explains why populations have roughly equal numbers of males and females. It's not that families always have equal numbers of boys and girls - it's that across the whole population, the probabilities balance out.
Sex inheritance follows simple mathematical patterns that you need to master for exams. The key is understanding direct proportion - if the probability of male offspring is 1:1, then in a family of 4 children, you'd expect 2 boys and 2 girls on average.
However, this is probability, not certainty. A family could have 4 boys or 4 girls - each child is an independent event with a 50% chance of being male or female.
Worked example: Calculating Sex Ratios in Populations
In a town of 10,000 people, predict how many would be male and female based on sex chromosome inheritance.
Answer: 5,000 males and 5,000 females expected
Sex chromosome questions are mark-winners if you know the technique. Examiners test your ability to set up genetic crosses, calculate probabilities, and explain the biological mechanism.
Exam depth extension: In sex chromosomes and genetic crosses, connect why are there equal numbers of boys and girls? to the sex chromosome system using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.
Common exam mistakes to avoid: Writing 'XY' for females (it's XX), forgetting that the father determines sex, and thinking that families 'balance out' - each child has a 50% chance regardless of siblings.
For 6-mark questions on sex inheritance, structure your answer: state parent genotypes, show gamete formation, draw the Punnett square, calculate probabilities, and explain why the ratio is 1:1.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The sex chromosomes are the 23rd pair of chromosomes, which carry genes determining biological sex. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY); the Y chromosome is much smaller and carries the genes that trigger male development.
XX and XY are the sex chromosomes: females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). A common mistake is writing XY for females.
The father determines the sex of a baby. The mother (XX) can only contribute an X chromosome, but the father (XY) can contribute either an X or a Y chromosome, so his gamete determines whether the child is XX (female) or XY (male).
The probability of having a boy or a girl is 50% each, a 1:1 ratio. A genetic cross of XX × XY gives 2 out of 4 XX (female) and 2 out of 4 XY (male) offspring, and each child is an independent event.
To draw a Punnett square for sex inheritance, put the female gametes (X and X) across the top and the male gametes (X and Y) down the side. The offspring are XX, XX, XY and XY, so half the offspring are female and half are male.
The Y chromosome is much smaller than the X chromosome and carries fewer genes. It is the presence of specific genes on the Y chromosome that triggers male development.
A mother cannot pass on a Y chromosome because she is XX and has no Y to give, so all her eggs carry an X. The father (XY) can pass on either an X or a Y, so his contribution determines the child's sex.
The 23rd pair of chromosomes determines sex. These sex chromosomes are XX in females and XY in males, while the other twenty-two pairs control characteristics like eye colour, height and blood type.
There are roughly equal numbers of males and females because a cross between XX and XY gives 50% XX and 50% XY offspring. Across a whole population this 1:1 ratio balances out, even though each family can differ.
Having boys does not change the chance of having a girl: each child is an independent event with a 50% chance of being male or female. A family could have 4 boys or 4 girls, because this is probability, not certainty.
Sperm cells carry either an X or a Y chromosome, while egg cells carry only an X chromosome. This is because males are XY and females are XX, and each gamete gets one chromosome from each pair.
The number of males in a population is predicted using the 1:1 sex ratio, so the male proportion is 1/2 = 0.5. In a town of 10,000 people, expected males = 10,000 × 0.5 = 5,000, and expected females are also 5,000.