Inheritance, Variation & Evolution: Key AQA Biology Paper 2 Notes

DNA Structure and Inheritance Basics

Think of DNA as your body's instruction manual - it's the genetic material that makes you uniquely you. This polymer forms a double helix structure and lives in tightly coiled bundles called chromosomes in your cell nucleus. You've got 23 pairs (46 total), including the sex chromosomes that determine if you're male (XY) or female (XX).

Each gene is a small section of DNA that codes for a specific protein. Since proteins determine what type of cell you become, your DNA essentially decides whether a cell becomes a red blood cell, nerve cell, or any other type. Your complete genome - all your genetic material together - helps scientists identify disease-causing genes and even trace human migration patterns.

When it comes to inheritance, you get two versions (called alleles) of every gene - one from each parent. If these alleles are identical, you're homozygous; if different, you're heterozygous. In heterozygous cases, the dominant allele always shows up in your characteristics.

Key Distinction: Your genotype is the collection of alleles you have, whilst your phenotype is the actual characteristics you display.

Reproduction and Meiosis

Sexual reproduction involves combining genetic material from two parents through fertilisation, creating loads of genetic variation. Asexual reproduction uses just one parent with no genetic mixing, producing identical offspring through processes like mitosis (plants/fungi) or binary fission (bacteria).

Meiosis is the special cell division that creates sex cells (gametes) with only half your genetic material. Your chromosomes first replicate, then line up randomly at the cell centre before being pulled apart. This randomness ensures genetic diversity - each gamete gets a unique mix of maternal and paternal chromosomes.

The process happens twice, leaving you with four genetically unique cells that develop into sperm or egg cells. When these fuse during fertilisation, they restore the full chromosome number and create a genetically unique individual.

Genetic diagrams help predict inheritance patterns using Punnett squares. These simple grids show all possible offspring combinations when you know the parents' genotypes.

Remember: Meiosis creates diversity, whilst mitosis creates identical copies!

Genetic Disorders and Evolution

Some disorders get passed down through families via inherited alleles. Polydactyly (extra fingers/toes) comes from a dominant allele, whilst cystic fibrosis needs two recessive alleles and affects cell membranes, causing sticky mucus in lungs and pancreas. Around 1 in 10 people carry the cystic fibrosis allele without showing symptoms.

Scientists can screen embryos during IVF to check for genetic disorders. This reduces suffering and healthcare costs but raises ethical questions about discrimination and "designer babies."

Variation exists because everyone has a unique genome producing different proteins. Your phenotype results from both genes and environment working together. Mutations - changes in DNA code - usually don't affect proteins, but beneficial ones help organisms survive and reproduce better.

Natural selection means the "survival of the fittest" - useful traits get passed on more often. This process drives evolution, where characteristics change in populations over many generations. All current species evolved from simpler life forms that first appeared 3 billion years ago.

Evolution Fact: Every species alive today descended from ancient, simpler organisms!

Selective Breeding and Genetic Engineering

Selective breeding involves choosing the best plants or animals to breed together, creating better offspring with desired traits. However, this reduces the gene pool (collection of different alleles in a population), leading to inbreeding problems, disease susceptibility, and less genetic variation.

Genetic engineering takes genes from one organism and transfers them to another, even across different species. Plants now resist diseases and produce more food, whilst bacteria create useful substances like insulin for diabetics. Gene therapy aims to replace faulty genes with healthy versions to treat inherited diseases.

The process involves isolating the desired gene using enzymes, inserting it into a vector (virus or bacterial plasmid), then introducing this vector into the target organism. The cells take up the vector and start producing the new protein.

Genetic modification offers huge benefits: better crop yields, disease resistance, and cheaper food production. However, concerns include unknown health risks, potential environmental damage if modified organisms spread into wild populations, and ecosystem disruption.

Engineering vs Breeding: Genetic engineering works across species boundaries, whilst selective breeding stays within the same species!

Fossils, Evolution Evidence, and Classification

Fossils provide crucial evidence for evolution by showing us extinct organisms and how life has changed over time. Most species that ever lived are now extinct, leaving behind traces through gradual replacement (shells/bones replaced by minerals), casts (impressions in hardened materials), or preservation in amber/tar pits.

Many early life forms were too soft-bodied to fossilise well, and geological activity has destroyed much evidence. Extinction happens when environmental changes, habitat destruction, climate shifts, new predators, diseases, or competing species wipe out populations.

Antibiotic resistance demonstrates evolution in action. Random mutations help some bacteria survive antibiotic treatment, and these resistant strains multiply. Superbugs like MRSA resist multiple antibiotics, partly due to overuse in farming.

Classification organises life using the Linnean system: Kingdom, Phylum, Class, Order, Family, Genus, Species. Scientists use Latin names for universal understanding. Above kingdoms sit three domains: Eukarya (organisms with nuclei), Bacteria (single-celled prokaryotes), and Archaea (different prokaryotes living in extreme conditions).

Classification Tip: Remember "King Philip Came Over For Good Soup" to memorise the classification hierarchy!