📘 GCSE Biology · Notes & worksheet
Inheritance and Evolution.
Notes
Key terms
- DNA: double helix, holds the genetic code. A gene is a section of DNA coding for one protein. A chromosome is a long coiled DNA molecule. The genome is all the genetic material of an organism.
- Allele: a version of a gene. Dominant alleles show in the phenotype if present once. Recessive alleles show only when two copies are present.
- Homozygous: two identical alleles. Heterozygous: two different alleles.
- Genotype: the alleles (for example Tt). Phenotype: the observable characteristic.
Genetic crosses (Punnett squares)
Cross Tt × Tt gives TT, Tt, Tt, tt: genotype ratio 1 : 2 : 1, phenotype ratio 3 dominant : 1 recessive.
- Probability is independent each time: each child has a 1/4 chance of being tt, regardless of earlier children.
- Probability of two events: multiply (¼ × ¼ = 1/16).
Sex determination
Females XX, males XY. Each child has a 50% chance of being male or female.
Genetic disorders
- Cystic fibrosis: recessive allele. Two carriers (Cc × Cc) have a 25% chance of an affected child.
- Polydactyly: dominant allele.
Variation and mutation
Variation comes from genes, environment, or both. Mutations change DNA sequences. Most have no effect, some change the protein, and a few are beneficial and are passed on.
Evolution by natural selection
- Variation exists within a population because of mutations.
- Some individuals are better adapted and survive and breed more successfully.
- They pass on the advantageous alleles.
- Over many generations the proportion of that allele increases.
Antibiotic resistance: a random mutation makes a bacterium resistant. Antibiotics kill non-resistant bacteria, so the resistant ones survive and reproduce. Resistant strains such as MRSA spread.
Selective breeding and genetic engineering
- Selective breeding: choosing parents with desired traits. Can reduce genetic variation and cause inbreeding.
- Genetic engineering: moving a gene from one organism into another (for example insulin production in bacteria, or golden rice).
Evidence for evolution
Fossils, antibiotic resistance, and DNA comparison. Speciation occurs when populations become so different they cannot interbreed.
Common mistakes
- Writing that organisms "adapt on purpose". Natural selection acts on existing variation.
- Giving only the genotype ratio when asked for the probability that a child is affected.
- Forgetting to state alleles with letters and a key.
Exam tips
- Always define your letters ("T = tall, t = short").
- Describe natural selection in four clear steps.
Worksheet
Prints the questions only, without answers. Choose “Save as PDF” in the print window to download.
Total: 28 marks.
- Define the terms allele, genotype and phenotype. (3)
- In pea plants, T (tall) is dominant to t (short). Two heterozygous plants are crossed.
(a) Draw a Punnett square for the cross. (2)
(b) State the genotype ratio and the phenotype ratio. (2)
(c) Find the probability that two offspring are both short. (2) - Cystic fibrosis is caused by a recessive allele (c). Two parents do not have the disorder, but their first child does.
(a) State the genotypes of both parents and explain how you know. (3)
(b) Find the probability that their next child has cystic fibrosis. (2) - A couple already have three sons. What is the probability that their fourth child is a son? Explain your answer. (2)
- Describe how natural selection could cause a population of rabbits to become faster over many generations. (4)
- Explain how antibiotic-resistant bacteria such as MRSA develop and spread. (4)
- Compare selective breeding and genetic engineering. (2)
- Give one benefit and one risk of genetically modifying crop plants. (2)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- Allele: a version of a gene (B1). Genotype: the combination of alleles an organism has (B1). Phenotype: the observable characteristics resulting from the genotype and environment (B1).
- (a) Tt × Tt: gametes T, t and T, t gives TT, Tt, Tt, tt. (B1 correct gametes, B1 correct square)
(b) Genotype ratio 1 TT : 2 Tt : 1 tt. Phenotype ratio 3 tall : 1 short. (B1, B1)
(c) P(short) = ¼, so P(both short) = ¼ × ¼ = 1/16. (M1, A1) - (a) Both parents are Cc (carriers). (B1) The child is cc, so received a recessive c from each parent (B1). The parents do not show the disorder, so each also carries the dominant C allele (B1).
(b) ¼ or 25%, or 0.25. (M1 Punnett or reasoning, A1). Each pregnancy is independent of earlier children. - ½ or 50% (B1). Each child's sex is independent of the sex of earlier children, because the father's sperm carries X or Y with equal probability. (B1)
- Variation exists in speed because of different alleles (B1). Faster rabbits are more likely to escape predators and survive (B1). They breed and pass on the alleles for speed (B1). Over many generations the frequency of those alleles increases and the average speed of the population rises. (B1)
- A random mutation in a bacterium makes it resistant to the antibiotic (B1). When antibiotics are used, non-resistant bacteria are killed but the resistant one survives (B1). It reproduces and passes on the resistant allele (B1). The population becomes mainly resistant and spreads between people, especially if antibiotics are overused. (B1)
- Selective breeding chooses parents with desirable characteristics over many generations (B1). Genetic engineering moves specific genes directly from one organism into another in one step. (B1)
- Benefit: crops can have higher yield, pest resistance, or added vitamins. Risk: possible effects on other organisms or ecosystems, gene transfer to wild plants, or ethical and cost concerns. (B1 each)