Inheritance, Variation and Evolution — AQA Combined Science: Trilogy

This unit covers how characteristics are passed on, how variation arises, and how species change over time through evolution.

Reproduction

• Sexual reproduction involves the fusion of gametes (sperm and egg in animals; pollen and egg cells in plants). Offspring inherit a mixture of genes from two parents, so they show variation.
• Asexual reproduction involves only one parent and no gametes. Offspring are genetically identical clones. It uses mitosis.

Some organisms (e.g. malarial parasites, many plants, fungi) reproduce both ways depending on conditions.

Meiosis

Gametes are produced by meiosis, which occurs in the reproductive organs.

• It produces four genetically different gametes from one cell.
• Each gamete has half the normal chromosome number (it is haploid).
• At fertilisation, two gametes fuse to restore the full chromosome number, and the new cell divides by mitosis to form an embryo.

Meiosis produces variation because the genes are shuffled.

DNA and the genome

• DNA is the molecule that carries the genetic code. It is a polymer made of two strands forming a double helix.
• DNA is contained in structures called chromosomes.
• A gene is a small section of DNA that codes for a particular sequence of amino acids, making a specific protein.
• The genome is the entire genetic material of an organism. Understanding the human genome helps us search for genes linked to diseases, understand inherited disorders and trace human migration.

Genetic inheritance

Key terms:

• Allele — a different version of a gene.
• Dominant allele — only one copy needed to show (written as a capital letter).
• Recessive allele — needs two copies to show (lower-case letter).
• Homozygous — two identical alleles (e.g. BB or bb).
• Heterozygous — two different alleles (e.g. Bb).
• Genotype — the alleles present.
• Phenotype — the characteristic shown.

Genetic crosses

Use a Punnett square to predict the outcomes of a cross and express the result as a ratio or probability. For example, crossing two heterozygous parents (Bb × Bb) gives a 3:1 ratio of dominant to recessive phenotypes.

Sex determination

Humans have 23 pairs of chromosomes. One pair determines sex:

• Females are XX, males are XY. A Punnett square of X and Y shows there is a 50% chance of each sex.

Inherited disorders

• Polydactyly (extra fingers/toes) — caused by a dominant allele.
• Cystic fibrosis (affecting cell membranes) — caused by a recessive allele, so a child must inherit it from both parents.

Embryos can be screened for some disorders, raising ethical questions you should be able to discuss.

Variation

Variation within a species is caused by:

• Genes (genetic variation).
• The environment (e.g. a scar, a plant grown in poor light).
• A combination of both.

Mutations are random changes to DNA that occur continuously. Most have no effect on the phenotype; some have a small effect; very rarely one significantly changes a characteristic and may, in a changed environment, give a survival advantage.

Evolution

The theory of evolution by natural selection (Charles Darwin) states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Natural selection works like this:

1. There is variation within a species.
2. Individuals with characteristics best suited to the environment are more likely to survive and breed.
3. They pass on the beneficial alleles to their offspring.
4. Over many generations, the species changes — it evolves.

If two populations become so different they can no longer interbreed to produce fertile offspring, a new species has formed (speciation).

Selective breeding and genetic engineering

• Selective breeding (artificial selection): humans choose organisms with desired characteristics to breed together over generations (e.g. cows that produce more milk, disease-resistant crops). A risk is reduced variation, leading to inbreeding problems.
• Genetic engineering: transferring a gene from one organism into another to give a desired characteristic (e.g. crops resistant to herbicides, bacteria producing human insulin). Benefits include higher yields and medical uses; concerns include effects on wild populations and ethics.

Evidence for evolution

• Fossils — the remains of organisms preserved in rocks, showing how organisms have changed over time. The fossil record is incomplete because many early organisms were soft-bodied.
• Antibiotic resistance in bacteria — a modern, observable example of natural selection: resistant bacteria survive antibiotics and reproduce.
• Extinction — a species dies out when it cannot adapt to environmental change, new predators, disease or competition.

Classification

Living things are classified into groups. The traditional Linnaean system groups organisms into kingdom, phylum, class, order, family, genus and species. The binomial system names species by genus and species (e.g. Homo sapiens). As understanding of biochemistry and DNA improved, the three-domain system (Archaea, Bacteria, Eukaryota) was proposed by Carl Woese.

Exam tips

• Practise Punnett squares and converting outcomes to ratios and percentages.
• Learn the four steps of natural selection and be ready to apply them to any example.
• Distinguish meiosis (gametes, genetically different, halves chromosomes) from mitosis (identical, growth/repair).
• Be able to give the three pieces of evidence for evolution.