Understanding Inherited Disorders Revision Notes | GCSE Edexcel Higher Biology

1. What Are Inherited Disorders?

Some medical conditions run in families. A child might have the same disorder as their parent or grandparent, even though they've never been exposed to the same environment or lifestyle. These are inherited disorders — conditions written into our DNA.

Think of your DNA as an instruction manual for building and running your body. Sometimes, there are 'typos' in these instructions — faulty alleles. When these faulty alleles are passed from parent to child, they can cause inherited disorders.

Trap: Many students think all diseases are inherited. Actually, most diseases are caused by lifestyle, environment, or infections — not genetics. Only disorders caused by faulty alleles count as inherited disorders.

2. Dominant vs Recessive Disorders

Not all inherited disorders work the same way. The key difference is whether the faulty allele is dominant or recessive. This determines how likely you are to actually develop the disorder.

Picture this: dominant alleles are like loud voices in a crowd — even one copy gets heard. Recessive alleles are like whispers — you need two copies speaking together to be heard over the normal allele.

3. Polydactyly - A Dominant Disorder

Polydactyly is a perfect example of how dominant disorders work. People with this condition are born with extra fingers or toes — usually six instead of five.

Let's use P for the polydactyly allele (dominant) and p for the normal allele (recessive). Someone with polydactyly could be PP or Pp. Someone without polydactyly must be pp.

Worked example: Polydactyly Inheritance Cross

A woman with polydactyly (Pp) has children with a man who has normal digits (pp). What's the probability their children will have polydactyly?

1. Mother: Pp (has polydactyly) Father: pp (normal digits) — The mother has the disorder so must have at least one P allele
2. Mother's gametes: P or p Father's gametes: p only — Each parent contributes one allele to each offspring
3. Pp × pp Offspring: Pp, Pp, pp, pp — 50% Pp (polydactyly), 50% pp (normal)
4. Probability = 2/4 = 1/2 = 50% — Half the children will inherit polydactyly

Answer: 50% chance of polydactyly

Notice how dominant disorders can appear in every generation. If a parent has it, roughly half their children will too. This often makes dominant disorders more straightforward to track through family trees.

4. Cystic Fibrosis - A Recessive Disorder

Cystic fibrosis shows us how recessive disorders behave very differently. This serious condition affects the lungs and digestive system by causing thick, sticky mucus to build up.

Let's use F for the normal allele (dominant) and f for the cystic fibrosis allele (recessive). To have cystic fibrosis, you must be ff. People who are Ff are carriers — they don't have the disorder but can pass it on.

Worked example: Cystic Fibrosis Inheritance Cross

Two parents are both carriers for cystic fibrosis (Ff). What's the probability their child will have cystic fibrosis?

1. Mother: Ff (carrier) Father: Ff (carrier) — Both parents are carriers - they have the disorder allele but don't show symptoms
2. Mother's gametes: F or f Father's gametes: F or f — Each parent can contribute either the normal or faulty allele
3. FF, Ff, Ff, ff Ratio: 1 FF : 2 Ff : 1 ff — Only ff children will have cystic fibrosis
4. Probability = 1/4 = 0.25 = 25% — 1 in 4 children, which is the same as 0.25 or 25%.

Answer: 25% (0.25, or 1 in 4) chance of cystic fibrosis

Recessive disorders often surprise families because they can skip generations. Two healthy parents who are carriers can have a child with the disorder, even though no one in the family has shown symptoms for years.

5. Comparing Inheritance Patterns

The inheritance patterns of dominant and recessive disorders create very different family histories. Understanding these patterns helps genetic counsellors predict risks for future children.

Dominant disorders (like polydactyly): Usually appear in every generation. If a parent has it, about 50% of children inherit it. The disorder doesn't skip generations.

Recessive disorders (like cystic fibrosis): Often skip generations. Two healthy carrier parents have a 25% chance of an affected child. The disorder can appear 'out of nowhere' in families.

6. Exam Technique and Common Mistakes

Inherited disorder questions are mark-winners if you know the key patterns. Examiners love testing whether you can distinguish between dominant and recessive inheritance.

Common exam mistakes: Students often confuse which disorder is dominant or recessive. Remember the examples: polydactyly (extra digits) is dominant, cystic fibrosis (cell membrane disorder) is recessive.

For 6-mark questions on inheritance, structure your answer: define the disorder, explain the inheritance pattern, show a genetic cross with working, and state the probability with correct units (percentage or ratio).