Inheritance and ecology · GCSE Biology
Inherited disorders
Apply Punnett squares to GCSE Biology inherited disorders: cystic fibrosis as recessive, polydactyly as dominant, embryo screening, and the ethical arguments mark schemes expect in Combined and Triple.
Cystic fibrosis: recessive, need ff, carriers Ff look healthy. Polydactyly: dominant, one P is enough. Screening can detect alleles in an embryo; evaluation needs biology plus a social or ethical point, then a judgement.
The important bits
What you need to know
- 1
Cystic fibrosis (CF) is caused by a recessive allele. Only homozygous recessive people (ff) have the condition. Heterozygotes (Ff) are carriers with a working phenotype because one dominant allele produces enough functional protein.
- 2
CF affects cell membranes (chloride transport), leading to thick mucus in the lungs and pancreas. It is inherited, not caught like a pathogen.
- 3
Polydactyly (extra fingers or toes) is caused by a dominant allele. Genotypes PP and Pp show extra digits; only pp is the usual number. A heterozygous parent has a 50% chance of passing it on to each child (Pp × pp).
- 4
Embryo screening (PGD) or prenatal tests can detect alleles. Embryos without the disorder may be selected for implantation, or a pregnancy decision may follow — ethically contested.
- 5
Arguments for screening: reduce suffering, prepare parents, avoid a severe disease. Arguments against: destruction of embryos, “designer babies” slippery slope, disability-rights concerns, false positives/negatives, cost and access.
- 6
Huntington’s disease (often Triple) is dominant and late-onset: a person may have children before they know they have the allele. That changes the ethics of testing adults.
- 7
Carriers of recessive conditions can be found by family history and genetic tests. A carrier × carrier square is the CF classic: 25% affected, 50% carriers, 25% homozygous dominant.
- 8
These disorders are not caused by bacteria. Treatment may ease symptoms (physio for CF) but does not change the alleles in every cell — gene therapy is experimental and specification-dependent.
Quotations worth analysing
Short evidence. Real method.
“Cystic fibrosis is caused by a recessive allele; polydactyly is caused by a dominant allele.”
Learn both. Recessive needs two copies; dominant needs one. The Punnett squares then write themselves.
“Carriers are heterozygous and do not have the recessive condition.”
Ff is not “a bit ill”. One working allele is enough for a typical CF carrier phenotype.
“Embryo screening raises ethical issues.”
Give one for, one against, and a conclusion that uses the biology (alleles detected, embryos unused).
Go deeper
How do I run the cystic-fibrosis square without mixing dominance?
Let f = CF allele, F = working allele. Affected child is ff only. Two carrier parents Ff × Ff give 1 FF : 2 Ff : 1 ff, so 25% affected, 50% carriers. If only one parent is a carrier (Ff × FF), no child is ff, but 50% are carriers. If a parent has CF (ff) and the other is FF, all children are carriers and none have CF. If the other is Ff, 50% have CF. Always define the letters. People confuse CF with polydactyly and shade the heterozygote as ill. For a dominant condition, the heterozygote is ill. That single switch is the whole difference. Show working even if you can see the answer — method marks survive an arithmetic slip.
Go deeper
What does a balanced embryo-screening evaluation look like?
For: the test identifies embryos with ff (or with a dominant disease allele), so parents can implant an embryo that will not have CF, reducing a life-limiting illness and the need for intensive treatment. Against: unused embryos are destroyed, which some people see as ending a life; disability campaigners argue screening devalues people who live with the condition; tests can err; IVF is costly and stressful; selection could expand to non-medical traits. Conclusion: for a severe early-onset recessive disease, many mark schemes accept screening if parents are informed, while noting the embryo issue. Avoid “playing God” as your only line. Use alleles, embryos, and a named disorder. That is evaluation, not a sermon.
Go deeper
How is this different from vaccination or antibiotics?
Inherited disorders are in the genome of every cell from fertilisation. You cannot catch CF from a classmate, and penicillin does not rewrite the CFTR allele. Symptoms in the lungs can lead to bacterial infections that then do need antibiotics — a nested infection story — but the root cause is the allele. Gene therapy aims to add a working gene to cells; it is not a standard GCSE treatment to memorise as routine care. When a question mixes a pedigree with a cough, sort inherited versus communicable first. The pedigree is this page; the bacterium is the infection page. Examiners build those hybrids on purpose.
See the idea in action
Two CF carriers, Ff × Ff. Punnett: FF, Ff, Ff, ff. Chance of a child with cystic fibrosis is 25%; chance of a carrier is 50%. Polydactyly parent Pp × pp partner: half the children expected to have extra digits because P is dominant. For an evaluate-screening question: screening can avoid implanting ff embryos (reduces CF), but those embryos may be destroyed (ethical objection). A conclusion should weigh a severe disease against the status of the embryo, using those facts.
Exam technique
Turn knowledge into marks
State dominant or recessive before you draw. Carriers exist for recessive conditions, not for a simple dominant one in the same way. Evaluation: one medical advantage, one ethical or practical drawback, judgement. Do not treat CF as infectious.
Common mistakes
Do not give these marks away
- 01
Treating heterozygotes as having cystic fibrosis, or treating polydactyly as recessive.
- 02
Giving only an ethical slogan on embryo screening with no mention of alleles or embryos.
- 03
Saying inherited disorders are caused by viruses, or that a vaccine can prevent CF.
Two parents are carriers of cystic fibrosis (Ff). What is the chance their child has cystic fibrosis?
A0%
B25%
C50%
D100%
Show the answer
25%. Ff × Ff gives one ff in four boxes. CF is recessive, so only ff has the condition. Carriers (Ff) do not.
Quick questions
If this is the bit you searched
Is cystic fibrosis dominant or recessive?
Recessive. A person must have two copies of the CF allele (ff). Heterozygous carriers (Ff) do not have the condition.
Is polydactyly dominant or recessive?
Dominant. One copy of the polydactyly allele is enough for extra digits (PP or Pp).
What is a carrier of a genetic disorder?
Someone who is heterozygous for a recessive allele. They do not have the condition but can pass the allele to children.
What are the arguments for and against embryo screening?
For: avoid a severe disease and prepare families. Against: embryos may be destroyed, tests can be wrong, cost, and concerns about selecting people. Give both sides and a judgement.