13.9 Inherited Disorders

A free GCSE Biology lesson for AQA (8461) from OBA School: polydactyly (dominant) and cystic fibrosis (recessive): symptoms, crosses and family trees. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Recall that some disorders are inherited, caused by the inheritance of certain alleles
• Describe polydactyly and explain that it is caused by a dominant allele
• Describe cystic fibrosis as a disorder of cell membranes caused by a recessive allele
• Use Punnett squares and family trees to predict the chance of a child inheriting a disorder
• Use carrier frequencies to estimate how common a disorder is

Key words

• Inherited disorder: A disorder caused by inheriting certain alleles from the parents.
• Polydactyly: Having extra fingers or toes. Caused by a dominant allele.
• Cystic fibrosis: A disorder of cell membranes that causes thick, sticky mucus. Caused by a recessive allele.
• Carrier: Someone with one copy of a recessive disorder allele (heterozygous). They are healthy but can pass the allele on.
• Gene therapy: An experimental treatment that adds working copies of an allele to a patient's cells.

Polydactyly: a dominant disorder

Some disorders are inherited. They are caused by inheriting certain alleles. Polydactyly means having extra fingers or toes. It is caused by a dominant allele (D), so a person needs only one copy to have it. Often the extra digit is small and can be removed; it rarely causes health problems.

Worked example: one parent with polydactyly (Dd × dd)

Gametes: D or d from the parent with polydactyly; d or d from the other parent.

• d; D Dd; d dd
• d; D Dd; d dd

Probability that a child has polydactyly = 2/4 = 1/2 (50%). Any child who has polydactyly must have at least one parent with it.

Cystic fibrosis: a recessive disorder

Cystic fibrosis (CF) is a disorder of cell membranes. A faulty membrane protein means cells make very thick, sticky mucus.

• Lungs and airways; Effect of thick mucus blocks small airways, so breathing is difficult and lung infections are common
• Pancreas and gut; Effect of thick mucus blocks the tubes that carry digestive enzymes, so food is not digested and absorbed well
• Reproductive system; Effect of thick mucus many people with CF have reduced fertility

CF is caused by a recessive allele (f). A person must inherit the allele from both parents (ff) to have CF. People who are Ff are carriers: they are healthy, but can pass on the allele.

Worked example: two carriers (Ff × Ff)

Offspring: FF, Ff, Ff, ff.
Probability of CF (ff) = 1/4 = 25%. Probability of a carrier (Ff) = 2/4 = 50%. Probability of FF = 25%.

Answer: 1 in 4 chance of CF for each child

Cystic fibrosis in a family tree

Worked example: working out genotypes

Amy has CF, so she is ff. Her parents Ken and Rita are unaffected, so each must be Ff.
Leo has CF (ff), so both Dan and Eve are Ff carriers.
Eve got her f from Sam or Lou, so at least one of them is a carrier.
Probability that Dan and Eve's next child has CF = 1/4.

Answer: Ken, Rita, Dan and Eve are all Ff

Exam tip: In a recessive disorder, an unaffected person with an affected parent or an affected child must be a carrier.

How common is CF? (maths skill)

In the UK about 1 person in 25 is a CF carrier. A child can only have CF if both parents are carriers.

Worked example: estimating the frequency

Probability both parents are carriers = 1/25 × 1/25 = 1/625.
Probability that a child of two carriers has CF = 1/4.
Overall probability = 1/625 × 1/4 = 1/2500.

Answer: about 1 baby in 2500 is born with CF

Treatment: there is no cure yet. Physiotherapy and exercise help clear mucus; antibiotics treat infections; enzyme tablets help digestion; newer medicines help the faulty membrane protein to work. Some people need a lung transplant. Gene therapy is being researched but is not yet a routine treatment.

Common mistakes

• Saying a recessive disorder must "skip a generation". It appears whenever a child gets two recessive alleles, which can happen in any generation.
• Calling polydactyly recessive. It is dominant: one allele is enough.
• Writing that carriers have a mild form of CF. Carriers (Ff) are healthy.
• Saying "CF affects the lungs" only. It is a disorder of cell membranes that affects the lungs, digestive system and more.

Revision questions

Try these, then check your answers below each one.

Question 1. Polydactyly is caused by: (Choose from: a dominant allele, a recessive allele, an extra chromosome, an infection during pregnancy.)

Answer: a dominant allele

Working: Polydactyly (extra fingers or toes) is caused by a dominant allele.

Question 2. Cystic fibrosis is a disorder of: (Choose from: cell membranes, red blood cells, the nervous system only, the cell wall.)

Answer: cell membranes

Working: CF is a disorder of cell membranes, caused by a recessive allele.

Question 3. True or false? A carrier of cystic fibrosis (Ff) has a mild form of cystic fibrosis.

Answer: False

Working: Carriers have one dominant F allele, so they do not have CF. They can pass the f allele on.

Related lessons

• Previous lesson: 13.8 More About Genetics
• Next lesson: 13.10 Screening for Genetic Disorders
• In this chapter: 13.1 Types of Reproduction
• In this chapter: 13.2 Cell Division in Sexual Reproduction
• In this chapter: 13.3 The Best of Both Worlds

Continue in the full course

This lesson is section 13.9 (AGCBH1309) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.