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FreeScienceLessons · AQA Biology Sex Determination and Inherited Disorders
Key facts
Sex chromosomes Humans have 23 pairs of chromosomes. The 23rd pair determines sex. Females: XX . Males: XY .Each gamete carries one sex chromosome. Egg always carries X. Sperm carries X or Y. If Y-carrying sperm fertilises the egg → XY → male. If X-carrying sperm → XX → female. The probability of each sex is 50% . Inherited disorders Cystic fibrosis — caused by a recessive allele (f). Must have ff to show the disorder.Carriers have Ff — they do not show symptoms but can pass the allele to offspring. Polydactyly (extra fingers/toes) — caused by a dominant allele (P). Only need one copy to show it.Genetic screening — testing embryos or adults for alleles associated with inherited disorders.Ethical issues: some people argue against "designer babies" or terminating pregnancies based on genetic tests. Exam questions — 3 questions · 9 marks
⚡ Extended questions included. Read the hint. Write in full sentences. AI marks against the AQA mark scheme.
1 mark Sex determination AQA 8461 style
A couple have four daughters. What is the probability that their next child will be a boy?
A 0% — they can only have daughters
B 100% — after four girls the next must be a boy
C 50%
D 25%
1 mark Cystic fibrosis inheritance AQA 8461 style
Two parents are both carriers of cystic fibrosis (Ff × Ff). What is the probability that a child has cystic fibrosis?
4 marks Punnett square — polydactyly AQA 8461 P2 style
Polydactyly is caused by a dominant allele (P). A person with polydactyly (Pp) has a child with a person without polydactyly (pp). Draw or describe a Punnett square and state the probability of their child having polydactyly. (4 marks)
Hint: Pp × pp: cross P and p from first parent with p and p from second parent. 50% Pp (polydactyly), 50% pp (unaffected).
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