Inheritance, variation and evolution — GCSE Biology Revision
Everything you need to revise inheritance, variation and evolution for GCSE Biology: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Genetic Material and Inheritance
Inheritance is the passing of genetic information from one generation to the next. Chromosomes, found in the nucleus of eukaryotic cells, carry genetic material. Each chromosome is a long molecule of DNA, coiled up. Genes are small sections of DNA on a chromosome that code for a specific sequence of amino acids, which in turn form a specific protein. These proteins determine an organism's characteristics. Humans have 23 pairs of chromosomes, 22 autosomal pairs and one pair of sex chromosomes (XX for female, XY for male).
Alleles and Phenotype
Alleles are different forms of the same gene. For example, a gene for eye colour might have an allele for blue eyes and an allele for brown eyes. An individual inherits one allele from each parent for every gene. If the alleles are the same, the individual is homozygous; if they are different, they are heterozygous. Dominant alleles always express their characteristic if present, while recessive alleles are only expressed if two copies are present. The combination of alleles an organism has is its genotype, and the observable characteristics are its phenotype.
Variation and Evolution by Natural Selection
Variation within a species arises from both genetic differences (mutations, sexual reproduction) and environmental factors. Genetic variation allows for evolution. Natural selection is the process where individuals better adapted to their environment are more likely to survive and reproduce, passing on their advantageous alleles. Over time, this leads to an increase in the frequency of these beneficial alleles in the population, resulting in a gradual change in the characteristics of the species – this is evolution. Mutations introduce new alleles into the population, providing the raw material for natural selection.
Selective Breeding and Genetic Engineering
Selective breeding (artificial selection) involves humans choosing organisms with desirable characteristics to breed together over several generations. This increases the frequency of those traits in the population, for example, producing high-yielding crops or docile farm animals. Genetic engineering, on the other hand, involves directly altering an organism's DNA. A gene from one organism can be inserted into another, creating a transgenic organism. This technique has applications in medicine (e.g., insulin production) and agriculture (e.g., pest-resistant crops), but also raises ethical considerations.
Key facts
- DNA is a polymer made of two strands forming a double helix.|A gene is a section of DNA that codes for a specific protein.|Alleles are different versions of a gene.|Evolution is the change in inherited characteristics of biological populations over successive generations.|Natural selection drives evolution, favouring advantageous traits.|Selective breeding and genetic engineering are human interventions influencing inheritance.
Key terms
- Allele::A different form of a gene.|Genotype::The genetic makeup of an individual; the combination of alleles possessed.|Phenotype::The observable characteristics of an organism, determined by genotype and environment.|Homozygous::Having two identical alleles for a particular gene.|Heterozygous::Having two different alleles for a particular gene.|Mutation::A random change in the DNA sequence.
Common mistakes
- Confusing 'gene' with 'allele' – a gene is the trait, an allele is the version of that trait.|Assuming acquired characteristics can be inherited – only genetic changes are passed on.|Believing individuals evolve during their lifetime – populations evolve over generations.|Not understanding that recessive traits only appear if two recessive alleles are present.
Exam tips
- Practice drawing and interpreting Punnett squares for monohybrid crosses.|Clearly distinguish between genotype and phenotype in your answers.|Remember to link genetic variation to natural selection in evolution questions.|Be able to explain the advantages and disadvantages of selective breeding and genetic engineering.
Quick quiz
1. Which of the following describes an organism's observable characteristics?
- Genotype
- Allele
- Phenotype
- Chromosome
Show answer
Phenotype — The phenotype refers to the physical or observable characteristics, resulting from the genotype and environmental influences.
2. What is the term for different forms of the same gene?
- Chromosomes
- Alleles
- Genotypes
- Phenotypes
Show answer
Alleles — Alleles are the alternative versions or forms of a gene, such as the allele for blue eyes or brown eyes.
3. An individual with two identical alleles for a particular gene is described as:
- Heterozygous
- Dominant
- Homozygous
- Recessive
Show answer
Homozygous — Homozygous means having two identical alleles for a gene (e.g., AA or aa), while heterozygous means having two different alleles (e.g., Aa).
4. Which process involves humans selecting organisms with desirable traits to breed?
- Natural selection
- Genetic mutation
- Genetic engineering
- Selective breeding
Show answer
Selective breeding — Selective breeding, also known as artificial selection, is when humans intentionally breed organisms for specific desired traits.
5. What is the primary source of genetic variation that natural selection acts upon?
- Environmental changes
- Acquired characteristics
- Mutations
- Phenotypic expression
Show answer
Mutations — Mutations are random changes in DNA that create new alleles, introducing new genetic variation into a population, which natural selection can then act on.
Exam-style questions
Explain how natural selection leads to evolution. [4 marks]
Show mark scheme
- Variation exists within a population (due to mutation/sexual reproduction).
- Individuals with advantageous alleles/characteristics are better adapted to the environment.
- These individuals are more likely to survive and reproduce, passing on their advantageous alleles.
- Over time, the frequency of these advantageous alleles increases in the population, leading to a change in the species' characteristics (evolution).
Cystic fibrosis is a genetic disorder caused by a recessive allele (f). A couple are both carriers for cystic fibrosis (heterozygous, Ff). Complete a Punnett square to show the probability of their child having cystic fibrosis. [3 marks]
Show mark scheme
- Correctly drawn Punnett square showing parental gametes (F, f) across top and side.
- Correctly filled Punnett square showing genotypes FF, Ff, Ff, ff.
- Correctly states 25% (or 1 in 4) probability of the child having cystic fibrosis (genotype ff).
Describe two differences between selective breeding and genetic engineering. [4 marks]
Show mark scheme
- Selective breeding involves breeding within the same species; genetic engineering can transfer genes between different species.
- Selective breeding selects for existing desirable traits; genetic engineering involves directly altering/inserting genes into an organism's DNA.
- Selective breeding is a slower process (multiple generations); genetic engineering can be much faster.
- Selective breeding is less precise; genetic engineering is more precise in changing specific genes.
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