Two Inherited Disorders for AQA
Two Inherited Disorders for AQA
CYSTIC FIBROSIS (Recessive)
Caused by: Recessive allele (ff) — both parents must be carriers
Effects:
- Thick, sticky mucus in lungs and digestive system
- Difficulty breathing, frequent infections
- Digestive problems (enzymes can't reach food)
Inheritance: If both parents are carriers (Ff), 25% chance of affected child
POLYDACTYLY (Dominant)
Caused by: Dominant allele (P) — only one copy needed
Effects:
- Extra fingers or toes
- Can be surgically corrected
- Not usually harmful
Inheritance: If one parent has Pp, 50% chance of affected child
Biology glossary
- What is polydactyly and how is it inherited?
- Polydactyly is a dominant disorder — having just ONE copy of the dominant allele (Pp or PP) causes extra fingers or toes. Only one affected parent is needed to pass it on. It is usually not harmful and can be corrected surgically.
- What is cystic fibrosis and how is it inherited?
- Cystic fibrosis is a recessive disorder caused by having two copies of the faulty allele (ff). It causes thick, sticky mucus to build up in the lungs and digestive system, leading to breathing difficulties and frequent infections.
Caused by: Recessive allele (ff) — both parents must be carriers
Earn the mark scheme marks
Memory Aids
CF = ff (both copies faulty): "CF needs two Faulty alleles — ff. You can remember this because the disorder is Fully expressed only when there is no Functional copy." Think: CF is like a car with two flat tyres — you need both tyres flat before the car stops.
Polydactyly = Powerful P (dominant): "Polydactyly only needs one Powerful P to appear." Think of the P standing for both Polydactyly and Powerful — one dominant allele is powerful enough to cause extra digits.
Carrier cross result — always 25%: When two carriers (Ff x Ff) have children: 25% FF (unaffected, not a carrier), 50% Ff (carrier), 25% ff (affected). Remember: "25-50-25 for two carriers".
Dominant disorder: one parent affected means 50% risk: If one parent has polydactyly (Pp) and the other is unaffected (pp), half the children are expected to inherit P. This is the 1:1 ratio cross (Pp x pp).
Quick Check: A couple are both carriers of cystic fibrosis. Use a Punnett square to calculate the probability that their first child will have cystic fibrosis, and explain why neither parent has the condition.
Both parents are carriers: Ff x Ff. Punnett square gives FF, Ff, Ff, ff. Probability of ff (cystic fibrosis) = 1/4 = 25%. Neither parent has the condition because they are heterozygous (Ff) — they have one dominant allele (F) which produces enough functional CFTR protein for normal health. CF only develops when an individual has two recessive alleles (ff) and no functional allele to produce working CFTR. Carriers do not know they carry the allele unless they are genetically tested.
Quick Check: A parent has polydactyly with genotype Pp. Explain why there is a 50% chance, not a 100% chance, that their child will also have polydactyly.
The affected parent (Pp) produces two types of gametes during meiosis: gametes carrying P and gametes carrying p, in equal proportions. If the other parent is unaffected (pp), they can only contribute p gametes. The possible offspring are Pp (polydactyly) and pp (unaffected) in a 1:1 ratio. Each child has an independent 50% probability of inheriting the P allele. Since polydactyly is dominant, only the Pp offspring will show the condition; the pp offspring will be completely unaffected.
Quick Check: A student says that genetic screening of embryos "should be compulsory to eliminate genetic diseases." Evaluate this claim using scientific and ethical reasoning.
Scientifically, genetic screening such as PGD can identify embryos carrying alleles for conditions like CF, allowing parents to select unaffected embryos for implantation. This could reduce the incidence of inherited disorders. However, making it compulsory raises serious ethical issues: it removes individual reproductive autonomy; people with genetic conditions (e.g., polydactyly) may not consider their condition a disease requiring elimination; it could lead to discrimination; and selecting embryos based on genetic traits raises questions about the moral status of embryos that are not implanted. Most ethicists argue that screening should remain a personal, informed choice rather than a state-mandated requirement.
Now try it yourself
Quiz · Question 1 of 23
What is an inherited disorder?
Tap an answer to check it
This topic in real past papers
Every real exam question we've found on inherited disorders, with a full worked answer.