GCSEBiologyAQARevision NotesInheritance, Variation & EvolutionReproductionSex Determination

Sex Determination (AQA GCSE Biology): Revision Note

Exam code: 8461

Written by: Lára Marie McIvor

Reviewed by: Dr Natalie Lawrence

Updated on 22 September 2026

Human chromosomes

Ordinary human body cells contain 23 pairs of chromosomes
Twenty-two pairs control characteristics only, but one of the pairs carries the genes that determine sex
In females, the sex chromosomes are the same (XX)
In males, the sex chromosomes are different (XY)

Diagram comparing male and female karyotypes and showing the inheritance of sex chromosomes. The “Male karyotype” contains chromosome pairs with maternal chromosomes shown in red and paternal chromosomes in blue. Chromosome pair 1 is labelled, and the male sex chromosomes are labelled “X chromosome” and “Y chromosome”. The “Female karyotype” contains two X chromosomes, labelled “X chromosomes” and “Female sex chromosomes”. Below, sperm and egg cells illustrate sex-chromosome inheritance. Text states, “Half of all sperm will carry an X chromosome” and “Half of all sperm will carry a Y sex chromosome”. Both illustrated egg cells contain an X chromosome, with text stating, “All egg cells will carry an X chromosome”. Further text states, “If a child inherits a Y chromosome, it will be a boy.” — **Egg cells always carry an X chromosome, while sperm carry either X or Y, giving an approximately equal chance of XX or XY offspring**

Use this image

Determining sex

The inheritance of sex can be shown using a genetic diagram, known as a Punnett square
The X and Y chromosomes taking the place of the alleles usually written in the boxes

Worked Example

Determine the probability that any offspring produced will be female.

Answer

Step 1: determine parent genotypes

Male parent = XY
Female parent = XX

Step 2: determine parent gametes

XY = X and Y
XX = X and X

Step 3: construct a Punnett square

	X	Y
X	XX	XY
X	XX	XY

Step 4: note offspring genotypes and phenotypes

2 x XX = female
2 x XY = male

Step 5: determine probability that offspring will be female

Two out of four offspring will be female

50 % probability

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent