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.
Topic position
Section 10 of 11
Practice
26 questions
Recall
20 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)