This topic summary covers Knowledge Organiser within Sex Determination for GCSE Biology. Sex chromosomes and sex determination mechanisms It is section 10 of 11 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser
Key Terms
- XX = female, XY = male
- Autosome — non-sex chromosome (pairs 1-22)
- Sex chromosome — pair 23 (X and Y)
- X chromosome — large, present in all humans
- Y chromosome — small, present only in males
- Sex-linked disorder — allele on X chromosome
- Carrier (female) — X^A X^a, unaffected but can pass allele
Must-Know Facts
- Humans have 23 pairs of chromosomes
- First 22 pairs = autosomes (same in both sexes)
- 23rd pair = sex chromosomes
- Father determines sex (X or Y sperm)
- Mother always gives X (all eggs carry X)
- 50% probability of each sex per pregnancy
- Males (XY) more likely to be affected by X-linked recessive conditions
Common Marks Lost
- Saying the mother determines sex — it is the father
- Not showing gametes before the Punnett square
- Forgetting to state probability as both fraction and percentage
- X-linked: not explaining why males are more affected
- Using wrong notation for X-linked alleles (X^A not just A)
Practice questions for Sex Determination
What determines the sex of a human baby?
Use a genetic diagram to show how sex is inherited in humans.
Quick recall flashcards
What sex chromosomes do females and males have?
Females: XX (two X chromosomes). Males: XY (one X and one Y chromosome). These are the 23rd pair of chromosomes in humans.
How do you remember XX = female and XY = male?
XX: the two X's look like chromosomes that pair up perfectly — females have a matching pair. XY: the Y looks like a little man standing with arms out — males have the odd one out. 'Why are boys different? Because of Y!'