Sex Chromosomes and Genetic Crosses Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Students learn about human sex chromosomes (XX for females, XY for males), the 23 pairs of chromosomes in human cells including the sex-determining pair...

• Explain how sex chromosomes determine gender in humans
• Calculate the probability of offspring being male or female using genetic crosses
• Apply Punnett squares to predict sex inheritance patterns
• Compare the chromosome composition of males and females

Key ideas to keep in view

Use this lesson to stay inside B5.1 Inheritance while connecting the detail back to the wider B5: Genes, inheritance and selection topic.

• B5: Genes, inheritance and selection
• B5.1 Inheritance
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from Sex Chromosomes and Genetic Crosses before testing yourself from memory.

• 1. Why Are There Equal Numbers of Boys and Girls?
• 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.
• The 23rd pair of chromosomes that carry genes determining biological sex. Females have two X chromosomes (XX), males have one X and one Y chromosome (XY).
• 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.
• 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.

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.