GCSE Combined Science Inheritance & evolution revision

Sexual reproduction fuses a male gamete with a female gamete. The offspring inherit a mix of genes from both parents, so each one is genetically unique — variation that evolution can act on. Asexual reproduction uses just one parent and ordinary cell division, so offspring are genetically identical clones: fast and reliable, but no variety and no mixing of beneficial mutations.

Human body cells hold 46 chromosomes in 23 pairs. Meiosis halves that, so each gamete — sperm or egg — carries just 23 single chromosomes. Fertilisation restores the full 46, one set from each parent.

DNA is a polymer made of two strands coiled around each other into a double helix. A gene is a section of DNA that codes for a particular sequence of amino acids, which fold into a specific protein. The genome is the entire set of genetic material of an organism — the whole library, not one sentence.

Since the human genome was fully mapped, medicine has gained three big tools: hunting for genes linked to diseases, understanding inherited disorders (and who carries them), and tracing human migration patterns through shared DNA. Genome science lets doctors predict some disease risks and choose treatments that suit a patient’s genes.

Genes come in pairs — one allele from each parent. Alleles are different versions of the same gene. A dominant allele (written as a capital, B) shows in the phenotype even with one copy; a recessive allele (b) shows only when both copies are b. Genotype is the pair of alleles (BB, Bb or bb); phenotype is the characteristic you can see. Homozygous means both alleles match (BB or bb); heterozygous means they differ (Bb).

Inherited disorders follow the same rule. Polydactyly (extra fingers or toes) is caused by a dominant allele — a sufferer with Bb passes it to half their children with any partner. Cystic fibrosis is recessive — carriers (Cc) are healthy, but two carriers risk a 1-in-4 affected child. Family trees reveal which pattern is at play: a condition appearing in every generation suggests dominant; healthy parents with an affected child prove recessive.

Variation within a species comes from two sources. Genetic variation is inherited — eye colour, blood group, natural height. Environmental variation comes from life circumstances — a suntan, a tattoo, a stunted plant in poor soil. Many traits mix both: your genes set a height range, but diet decides where in the range you land.

New alleles arrive by mutation: random changes in DNA that occur continuously. The vast majority are neutral — they change nothing noticeable. Occasionally one is harmful, and very occasionally one produces a new phenotype that happens to suit the environment. That rare case is evolution’s raw material.

Darwin’s theory runs as a loop. First, variation: mutations and sexual reproduction make individuals different. Second, selection pressure: predators, disease, competition or climate mean resources are limited. Third, survival and reproduction: individuals with the best-suited phenotypes are more likely to survive and breed. Fourth, inheritance: their offspring inherit the helpful alleles, so those alleles become more common — the population evolves. Given enough time, this produces new species.

Selective breeding is ancient and needs no laboratory, but it is slow and shrinks the gene pool — close relatives get crossed, letting harmful recessive alleles surface. Genetic engineering is precise and fast: crops can resist insects or herbicides and yield more food, and bacteria mass-produce medicines. Concerns include escaped genes affecting wild populations, unknown long-term effects of eating GM food, and control of seed supplies by large companies.

The Linnaean system sorts organisms into ever-smaller groups — kingdom, class, genus, species — judged by shared structures and characteristics. Every species gets a binomial name: two Latin words, genus then species, as in *Homo sapiens*. Closely related species share recent common ancestors, so they sit close together in the scheme.

Evolutionary trees turn that idea into a picture. Branch points are common ancestors; the closer two species’ branches join, the more recently they shared one. Trees are built from fossils, DNA and protein comparisons — the more similar the DNA, the closer the relationship. Reading a tree is a favourite easy-mark question: trace the branches back to find the shared ancestor.