Inheritance, cystic fibrosis and genetic screening — Edexcel International AS Level Biology
Genetic terms, monohybrid inheritance, pedigrees and sex linkage, the effects of cystic fibrosis, genetic screening and the ethical issues it raises.
Revision notes
6 short notes, in the order of the specification. Each one in short:
A gene is a length of DNA coding for a polypeptide, and an allele is one version of a gene. The genotype is the alleles an organism has, and the phenotype is its observable characteristics. A dominant allele shows in the heterozygote, a recessive allele only in the homozygote, and codominant alleles both show.
Monohybrid inheritance is the inheritance of one gene with two alleles. A genetic cross between two heterozygotes gives a 3 : 1 ratio of dominant to recessive phenotypes. A pedigree diagram shows the phenotypes of family members over generations, and can be used to deduce whether an allele is dominant or recessive.
A sex-linked gene is carried on a sex chromosome. Red-green colour blindness is caused by a recessive allele on the X chromosome. Males (XY) have only one X chromosome, so one copy of the allele causes the condition, whereas females (XX) need two copies, so the condition is more common in males.
Cystic fibrosis is caused by a recessive mutation in the gene for the CFTR protein, a chloride ion channel in cell-surface membranes. The faulty protein reduces the movement of chloride ions and water out of cells, so mucus is thick and sticky. This blocks and damages the gaseous exchange, digestive and reproductive systems.
Genetic screening tests people or embryos for particular alleles. It can identify carriers of a recessive allele, test embryos made by IVF before implantation (PGD), or test a fetus in the uterus by amniocentesis or chorionic villus sampling (CVS). Results help people make decisions, but screening has risks and implications.
Genetic screening raises ethical and social questions, which can be discussed from different viewpoints. Religious views differ on the status of the embryo and on termination. Moral arguments weigh the right to know and to choose against the value of every life. Social issues include cost, discrimination and pressure on parents.
Exam questions
7 exam-style questions (26 marks), each with its mark scheme.
Answer the questionsFlashcards
20 cards: flip them, mark what you knew, and practise the rest.
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The whole of membranes, proteins, dna and gene expression on one page, so you can see where this subtopic fits.
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Quick check questions
What is the difference between genotype and phenotype?
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Genotype is the alleles an organism has; phenotype is its observable characteristics.
What is codominance?
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Both alleles are expressed in the phenotype of a heterozygote.
Why is red-green colour blindness more common in males?
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The allele is recessive and on the X chromosome; males have only one X, so one copy is expressed.
Name the protein that is faulty in cystic fibrosis.
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The CFTR protein, a chloride ion channel in cell-surface membranes.
Name two methods of prenatal testing.
Show answer
Amniocentesis and chorionic villus sampling (CVS).
Frequently asked questions
What is the difference between an allele and a gene?
A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide. An allele is one of the different forms of that gene. A person has two alleles of each gene, one from each parent, and these may be the same or different.
How do you tell from a pedigree diagram whether an allele is dominant or recessive?
If two unaffected parents have an affected child, the allele for the condition is recessive and both parents are carriers. If two affected parents have an unaffected child, the allele is dominant and both parents are heterozygous. Always name the individuals and give their genotypes.
Why does cystic fibrosis cause problems in the lungs, digestive system and reproductive system?
The faulty CFTR chloride channel means less water moves out of cells by osmosis, so mucus is thick and sticky. It blocks the airways, causing infections, blocks the pancreatic duct so enzymes cannot reach the intestine, and blocks the tubes that carry sperm.
What is the difference between amniocentesis and chorionic villus sampling?
Amniocentesis tests fetal cells in the amniotic fluid, from about 15 weeks. Chorionic villus sampling takes cells from the placenta and can be done earlier, from about 11 weeks. Both use ultrasound to guide a needle, and both carry a small risk of miscarriage.
What are the ethical issues of genetic screening?
Issues include the status of embryos in PGD, termination after a positive prenatal test, the right of parents to choose, and pressure to screen. Social issues include discrimination by insurers or employers, privacy of genetic information, cost and access, and attitudes towards people with disorders.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026