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B12.6 Inherited Disorders

GCSE Combined Science · AQA · Chapter B12: Reproduction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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 and direct proportion to estimate numbers

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 (CF): 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.
  • Communic- able disease: A disease caused by a pathogen that can spread between people. Inherited disorders are not communicable.

Polydactyly: a dominant disorder

Some disorders are inherited: they are caused by inheriting certain alleles, not by a pathogen. 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. The extra digit is often small and can be removed; it rarely causes health problems.

Two of the four boxes are Dd, so the probability that a child has polydactyly = 2/4 = 1/2 (50%). A child with 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 the thick mucus blocks small airways and reduces gas exchange; breathing is hard and lung infections are common
  • Pancreas and gut; Effect of the thick mucus blocks the tubes that carry digestive enzymes, so food is not digested and absorbed well
  • Reproductive system; Effect of the 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. Treatments include physiotherapy to loosen mucus, antibiotics for infections and enzyme tablets; there is no cure yet.

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

Cystic fibrosis in a family tree

Worked example: working out genotypes

Kai has CF, so he is ff. His parents Raj and Hana are unaffected, so each must be Ff.
Nia has CF (ff), so both of her parents, Lena and Finn, are Ff carriers.
Finn got his f allele from Owen or Bea, so at least one of them is a carrier.
Probability that Lena and Finn's next child has CF = 1/4.

Answer: Raj, Hana, Lena and Finn 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 numbers

How many carriers would you expect in a town of 45 000 people? 45 000 ÷ 25 = 1 800.
Probability that two random parents are both carriers = 1/25 × 1/25 = 1/625.
Probability that their child has CF = 1/625 × 1/4 = 1/2500.

Answer: 1 800 carriers; about 1 baby in 2 500 has CF

Common mistakes

  • Saying a recessive disorder must "skip a generation". It appears whenever a child gets two recessive alleles.
  • 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 only affects the lungs. It is a disorder of cell membranes that affects the lungs, digestive system and more.
  • Calling an inherited disorder infectious. You cannot catch it from another person.

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, a pathogen, having an extra chromosome.)

Answer: a dominant allele

Working: Polydactyly (extra fingers or toes) is caused by a dominant allele, so one copy is enough.

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

Answer: cell membranes

Working: Cystic fibrosis is a disorder of cell membranes. It causes thick, sticky mucus in the lungs and digestive system.

Question 3. True or false? A person who is a carrier of cystic fibrosis (Ff) has a mild form of the disorder.

Answer: False

Working: Carriers have one normal dominant allele, so they do not have cystic fibrosis at all. They can pass the f allele to their children.

Related lessons

Continue in the full course

This lesson is section B12.6 (AGCSC1206) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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