DNA, Genetics and Inheritance
DNA, deoxyribonucleic acid, is the polymer that carries genetic information in almost every cell, made of two strands coiled together in a double helix, with each strand a chain of alternating sugar and phosphate groups attached to one of four bases: A, T, C and G. A gene is a small section of DNA on a chromosome that codes, through the sequence of its bases, for a specific sequence of amino acids that make one particular protein.
Method
- Learn the core vocabulary precisely: a chromosome is a long, coiled molecule of DNA found in the nucleus (humans have 23 pairs); a gene is a section of a chromosome that codes for one protein; an allele is an alternative version of a gene; a genome is an organism's entire genetic material.
- Learn the difference between homozygous, two identical alleles for a gene such as BB or bb, and heterozygous, two different alleles such as Bb, and between dominant, an allele expressed even when only one copy is present, and recessive, an allele expressed only when two copies are present.
- For every genetic-diagram question, follow the same four steps: write the genotypes of both parents, list the gametes each parent can produce, combine the gametes in a Punnett square, then read off the resulting genotype and phenotype ratios.
- Learn sex determination as a special case of a genetic diagram: human egg cells all carry an X chromosome, sperm cells carry either an X or a Y chromosome, so a cross of XX (mother) with XY (father) always gives a 1 to 1 ratio of XX (female) to XY (male) offspring.
- Learn cystic fibrosis and polydactyly as the two named example disorders: cystic fibrosis is caused by a recessive allele, so a sufferer must inherit the recessive allele from both parents, meaning two unaffected carriers, each Ff, can have an affected child, ff; polydactyly is caused by a dominant allele, so only one copy is needed for a person to be affected.
- For Human Genome Project questions, link the science to its uses: understanding the whole human genome helps scientists search for genes linked to inherited disorders, understand and treat those disorders, and trace patterns of human migration.
- For embryo screening questions, give a scientific point about what the screening detects and why, plus a balanced ethical point, for example weighing the benefit of avoiding a serious inherited disorder against concerns about the moral status of a discarded embryo.
Worked example
Cystic fibrosis is caused by a recessive allele, f. Two parents who are both unaffected carriers, genotype Ff, have a child. Using a genetic diagram, calculate the probability, as a percentage, that their child has cystic fibrosis.
- Write the genotype of each parent: Ff and Ff, and list the possible gametes from each: F or f.
- Combine the gametes in a Punnett square to find the offspring genotypes: FF, Ff, Ff, ff.
- Read off the phenotypes: FF and Ff are both unaffected (Ff is a carrier), and only ff has cystic fibrosis.
- Convert to a probability: 1 out of the 4 possible offspring is ff, so the probability is 1 in 4.
- Final answer: 1/4 x 100 = 25% probability that the child has cystic fibrosis.
Practice questions
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Q1State how many chromosomes are found in a normal human body cell, and how they are arranged.Show answer
Answer: 46 chromosomes, arranged as 23 pairs.
Q2Define homozygous.Show answer
Answer: Having two identical alleles for a particular gene, for example BB or bb.
Q3In pea plants, purple flower colour (allele H) is dominant over white flower colour (allele h). State the phenotype of a plant with genotype Hh.Show answer
Answer: Purple, since H is dominant and only one copy is needed to show the purple phenotype.
Q4A man has genotype XY. State his sex, and state which sex chromosomes are found in the sperm cells he produces.Show answer
Answer: Male; his sperm cells carry either an X chromosome or a Y chromosome, in roughly equal proportions.
Q5State whether cystic fibrosis is caused by a dominant or a recessive allele.Show answer
Answer: A recessive allele.
Q6A man who is a known carrier of cystic fibrosis (Ff) has children with a woman who has been confirmed to carry no cystic fibrosis allele at all (FF). Explain why none of their children can have cystic fibrosis.Show answer
Answer: The FF parent can only pass on the F allele, so every child must inherit at least one F allele; since a child would need to inherit the recessive f allele from both parents to have genotype ff and be affected, this is impossible, and every child will be FF or Ff (unaffected).
Q7Explain one benefit of the Human Genome Project.Show answer
Answer: Any valid point, for example it allows scientists to identify genes linked to particular inherited disorders, helping with diagnosis or treatment, or it allows scientists to trace patterns of human migration.
Q8State one ethical concern raised by embryo screening.Show answer
Answer: Any valid concern, for example that it involves discarding embryos that are not implanted, raising concerns about the moral status of the embryo, or that the technology could eventually be used to select for non-medical characteristics.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A pedigree diagram shows that two parents who do not have cystic fibrosis have a child who does have cystic fibrosis. Using genetic terms, explain how this is possible.
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A couple already have three daughters and are expecting a fourth child. (a) Using a genetic diagram, state the probability that their fourth child will be a boy. (b) Explain why having three daughters already does not change this probability.
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A couple who are both known carriers of a serious recessive genetic disorder are using IVF and are offered embryo screening before an embryo is implanted. Evaluate the use of embryo screening in this situation, considering both the potential benefits and the ethical objections.
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