Genetic InheritanceAQA GCSE Biology: Revision notes

DNA (deoxyribonucleic acid) is a polymer made up of two strands that coil together to form a double helix. DNA is contained in structures called chromosomes, found in the nucleus of cells. DNA is built from four different nucleotides, each containing a common sugar and phosphate group and one of four different bases: A, C, G and T. A sequence of three bases (a 'triplet') codes for one particular amino acid.

• A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which make up a specific protein
• The genome of an organism is its entire genetic material — all of its DNA
• The whole human genome has now been studied (mapped). This has great importance for medicine, including searching for genes linked to disease, understanding inherited disorders, and tracing human migration patterns

The order of bases along a gene controls the order in which amino acids are assembled during protein synthesis, which in turn determines the protein produced. Genetic variants can occur in both coding DNA (which codes directly for proteins) and non-coding DNA, and both can influence an organism's phenotype (its observable characteristics) — non-coding DNA variants can affect how genes are switched on or off. A change in the structure of DNA (a mutation) may therefore result in a different protein being made, which can change the phenotype.

Understanding of genetics developed gradually over more than a century:

• Mid-19th century — Gregor Mendel carried out breeding experiments on pea plants, observing that inheritance of each characteristic is determined by 'units' passed unchanged from parents to offspring
• Late 19th century — scientists observed chromosome behaviour during cell division
• Early 20th century — chromosomes and Mendel's 'units' (now called genes) were found to behave in the same way, linking the two discoveries
• Mid-20th century — the structure of DNA was determined and the mechanism of gene function was worked out

Mendel's discovery of the units of inheritance was not recognised as significant until after his death. This is because at the time, scientists did not yet know about chromosomes or DNA, so there was no physical structure that could explain how Mendel's 'units' were passed on or how they worked — his mathematical patterns could not yet be linked to any known biological mechanism.