Some diseases are passed from parent to child through genes rather than caught from pathogens. Dominant conditions show in every carrier; recessive ones need two faulty alleles. Cystic fibrosis, sickle-cell disease, and polydactyly are the key examples at GCSE, each following predictable inheritance patterns.
What makes a disease "inherited"?
An inherited disease (also called a genetic disorder) is caused by a faulty allele — an altered version of a gene — that is passed from parent to offspring through the gametes (sperm and egg). Unlike infectious diseases, inherited conditions cannot be caught from another person and are present from conception.
All inherited diseases at GCSE are caused by a single gene with two alleles — one dominant (written as a capital letter) and one recessive (written in lower case). The combination of alleles an organism carries is its genotype; what you can observe is its phenotype.
Key terms:
- Homozygous dominant (AA) — two dominant alleles; shows the dominant phenotype
- Homozygous recessive (aa) — two recessive alleles; shows the recessive phenotype
- Heterozygous (Aa) — one of each; shows the dominant phenotype but carries the recessive allele
What is a carrier?
A carrier is a person who is heterozygous (Aa) for a recessive condition. They carry one faulty recessive allele but do not develop the disease because the dominant allele masks it. Carriers can pass the faulty allele on to their children without knowing they carry it.
This is critical for recessive inherited diseases: two carriers (Aa × Aa) have a 25% chance of producing an affected child (aa), a 50% chance of producing a carrier (Aa), and a 25% chance of producing an unaffected non-carrier (AA).
What is cystic fibrosis and how is it inherited?
Cystic fibrosis (CF) is caused by a recessive allele — we use F for the dominant (unaffected) allele and f for the recessive (faulty) allele. Only individuals with genotype ff develop cystic fibrosis.
CF causes thick, sticky mucus to accumulate in the lungs and digestive system, making breathing difficult and blocking enzymes needed for digestion.
Punnett square: two carrier parents (Ff × Ff)
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
Predicted outcomes: 25% FF (unaffected, non-carrier), 50% Ff (carrier, unaffected), 25% ff (cystic fibrosis).
The probability that any one child has cystic fibrosis is 1 in 4 (25%).
What is polydactyly and how is it inherited?
Polydactyly is caused by a dominant allele — even one copy of the faulty allele causes the condition. We use D for the dominant (causes extra digits) allele and d for the recessive (normal) allele. An affected person is Dd (heterozygous) or DD (homozygous dominant, rare).
Polydactyly causes a person to be born with extra fingers or toes — usually one extra digit on one or both hands. It is a relatively harmless condition and does not affect life expectancy.
Punnett square: one affected parent (Dd) × one unaffected parent (dd)
| D | d | |
|---|---|---|
| d | Dd | dd |
| d | Dd | dd |
Predicted outcomes: 50% Dd (polydactyly), 50% dd (unaffected).
The probability that any one child inherits polydactyly is 1 in 2 (50%).
What is sickle-cell disease and how does codominance work?
Sickle-cell disease involves two codominant alleles, making it slightly different from a simple dominant/recessive pattern. We use H^A for the normal haemoglobin allele and H^S for the sickle-cell allele. Both alleles contribute to the phenotype in a heterozygous individual.
| Genotype | Phenotype |
|---|---|
| H^A H^A | Normal — no sickle cells |
| H^A H^S | Sickle-cell trait — carrier; mostly unaffected but slightly protected against malaria |
| H^S H^S | Sickle-cell disease — red blood cells become sickle-shaped under low-oxygen conditions, causing crises, anaemia, and pain |
The H^S allele distorts red blood cells into a crescent (sickle) shape. These cells clump together and block capillaries, depriving tissues of oxygen. People with H^A H^S (the sickle-cell trait) are naturally more resistant to malaria because the malaria parasite (Plasmodium) cannot survive well in cells that partially sickle — this explains why the H^S allele is common in populations from regions where malaria is endemic.
How can genetic counselling help families?
Genetic counselling involves advising couples about their risk of having a child with an inherited condition. If both parents are known carriers (Ff) of cystic fibrosis, for example, a genetic counsellor will explain the 25% risk and the options available:
- Prenatal testing — chorionic villus sampling (CVS) or amniocentesis can detect the foetus's genotype during pregnancy.
- Pre-implantation genetic diagnosis (PGD) — used with IVF to test embryos before implanting only unaffected ones.
- Newborn screening — the NHS heel-prick test checks for CF and other conditions shortly after birth, enabling early treatment.
Genetic counselling does not make decisions for families; it provides accurate risk information so informed choices can be made.
Frequently asked questions
Why do two unaffected parents sometimes have an affected child?
This happens when both parents are carriers (Aa) of a recessive condition. Each parent carries one recessive allele (a) but does not develop the disease because the dominant allele (A) masks it. When two carriers reproduce, there is a 1 in 4 (25%) probability that a child will inherit the recessive allele from both parents (aa) and therefore develop the condition. Neither parent was ill, so the condition appeared to come from nowhere — but it was always present in the family's genetics.
What is the difference between dominant and recessive inheritance?
In dominant inheritance, only one faulty allele is needed for the disease to appear — every person who carries the allele shows the phenotype. Polydactyly is an example: a parent with genotype Dd passes D to half their children, all of whom develop the condition. In recessive inheritance, both alleles must be faulty for the disease to appear — carriers (Aa) are unaffected. Cystic fibrosis follows recessive inheritance: only the ff genotype causes the disease.
Can you work out the probability of an inherited disease from a family tree?
Yes. In a family tree (pedigree diagram), shaded symbols indicate affected individuals and unshaded ones indicate unaffected. By looking at whether affected individuals have unaffected parents, you can determine whether the condition is dominant (affected child must have at least one affected parent) or recessive (two unaffected parents can have an affected child if both are carriers). From that, you can assign likely genotypes to individuals and calculate the probability for future children using a Punnett square.
Why is sickle-cell disease more common in some populations than others?
The H^S allele is most common in people with ancestry from sub-Saharan Africa, the Mediterranean, the Middle East, and South Asia — all regions where malaria historically caused very high death rates. Individuals with sickle-cell trait (H^A H^S) are partially protected against Plasmodium falciparum malaria, so in malaria-endemic regions they survived and reproduced at higher rates than those with H^A H^A (normal haemoglobin), passing the H^S allele on more frequently. This is natural selection acting on human genetics. In regions without malaria, the H^S allele confers no survival advantage, so it remains rare.
For Socratic GCSE biology with Professor Darwin — following a faulty allele from chromosome to patient — visit aitutors.me.