Inheritance of genes, alleles and cystic fibrosisEdexcel A-Level Biology A: Revision notes
Section 1
Key genetic terms
A gene is a length of DNA that codes for a polypeptide (or functional RNA). Different versions of the same gene are called alleles, and they occupy the same locus on a chromosome. Your genotype is the combination of alleles you have for a gene; your phenotype is the observable characteristic that results from the genotype and the environment.
An allele is dominant if it affects the phenotype even when only one copy is present (heterozygous). An allele is recessive if it only affects the phenotype when two copies are present (homozygous). A homozygote has two identical alleles; a heterozygote has two different alleles. A heterozygote for a recessive condition who shows no symptoms is a carrier.
Do not write that a gene is dominant. Alleles are dominant or recessive, not genes.
Section 2
Monohybrid inheritance
Monohybrid inheritance is the inheritance of a single gene with two alleles. Gametes are haploid, so each carries one allele. Fertilisation is random.
Worked example. Cross two heterozygotes, Cc × Cc. Gametes from each parent: C or c. Offspring: 1 CC : 2 Cc : 1 cc. If C is completely dominant, the phenotype ratio is 3 dominant : 1 recessive, and each child has a 1 in 4 chance of being cc.
Each fertilisation is an independent event: a couple who already have an affected child still have the same 1 in 4 chance for the next one. The probability of being a carrier (Cc) in this cross is 1 in 2, and of being unaffected is 3 in 4.
Write the parents' genotypes, their gametes, then the offspring genotypes, before giving any ratio or probability.
Section 3
Incomplete dominance
With incomplete dominance, neither allele is completely dominant, so the heterozygote has a phenotype intermediate between the two homozygotes. For example, crossing red-flowered snapdragons (RR) with white-flowered (WW) gives pink-flowered (RW) offspring. Crossing two pink plants gives red : pink : white in a 1 : 2 : 1 phenotype ratio, the same as the genotype ratio.
The key difference from complete dominance is that the heterozygote can be told apart from the homozygous dominant by its phenotype.
Section 4
Interpreting pedigree diagrams
A pedigree shows a family's inheritance of a characteristic: squares are males, circles are females, shaded symbols are affected, and horizontal lines join partners.
Recessive condition: two unaffected parents can have an affected child, which shows that both parents are heterozygous carriers.
Dominant condition: every affected child must have at least one affected parent, and two affected parents can have an unaffected child (both are heterozygous). Unaffected people are homozygous recessive and cannot pass the condition on.
Work out each person's genotype from their parents and children, then calculate probabilities using the Cc × Cc or Cc × cc crosses.
Two unaffected parents with an affected child show the condition is recessive. Two affected parents with an unaffected child show it is dominant.
Section 5
Cystic fibrosis: cause and inheritance
Cystic fibrosis (CF) is caused by a mutation in the CFTR gene. The most common mutation is a deletion of three bases that removes one amino acid, which changes the primary structure and so the tertiary structure of the CFTR protein.
CFTR is a chloride ion channel in the cell-surface membrane of epithelial cells. In CF the channel is faulty or missing, so chloride ions are not secreted. Less water moves into the mucus by osmosis, so the mucus is thick and sticky.
CF is inherited as an autosomal recessive condition: only people with two mutant alleles (homozygous recessive) have CF. About 1 in 25 people in the UK are carriers.
Section 6
Effects of CF on body systems
Gaseous exchange system: thick mucus is not moved by cilia, so it blocks the airways and traps bacteria. This causes repeated lung infections and inflammation, damaging lung tissue and reducing the oxygen reaching the blood.
Digestive system: mucus blocks the pancreatic duct, so pancreatic enzymes (amylase, protease, lipase) cannot reach the small intestine. Food is poorly digested and absorbed, so people with CF may be underweight. Many take enzyme capsules with meals.
Reproductive system: in males the vas deferens may be blocked or absent, so sperm cannot be released and most men are infertile. In females thick cervical mucus makes it harder for sperm to reach the egg, so fertility is reduced.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Inheritance of genes, alleles and cystic fibrosis
- A healthy couple have a baby girl who is diagnosed with cystic fibrosis (CF), which is caused by a recessive allele of the CFTR gene. Neither parent has any symptoms of CF and there is no known history of CF in either family. The couple are planning to have more children. In this question, C represents the dominant allele and c the recessive allele.Explain how two parents who do not have CF can have a child with CF.2 marks
- A rare inherited condition affects the nervous system in adults. Two affected parents, who both developed symptoms in their forties, have four children. Two of the children have the condition and two do not. The condition is controlled by a single gene with two alleles, and the gene is not on a sex chromosome. Ignore new mutations.Deduce whether the allele that causes the condition is dominant or recessive. Explain your answer.2 marks
- A girl aged six has cystic fibrosis. She has repeated chest infections and, despite a good appetite, she is underweight. Genetic testing shows that she is homozygous for a mutation in which three bases are deleted from the CFTR gene, so one amino acid is missing from the CFTR protein. The CFTR protein is a channel in the cell-surface membrane of epithelial cells that transports chloride ions out of the cells. She takes capsules of digestive enzymes with every meal.Explain how this mutation leads to the production of thick, sticky mucus.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).