The foundations of genetics
Genetics is the science of heredity: why offspring resemble their parents, and why they differ. Its central discovery is that inheritance comes in discrete units, written in a chemical code that almost every living thing reads the same way.
A gene is a contribution, not a verdict. Most traits depend on many genes and on the environment, and which genes are switched on varies by tissue and over time. Heritability measures variation in a population, never the fate of an individual.
Hardy and Weinberg showed in 1908 that allele frequencies stay constant unless something acts on them. Selection, drift, mutation and migration are those somethings — and in the 1920s and 1930s this joined Mendel’s genetics to Darwin’s evolution.
DNA is copied with very high but not perfect fidelity. Changes in sequence are the raw material of both inherited disease and evolution. Two unrelated people differ by a single letter about once in every thousand positions — and by roughly 0.4 per cent of their DNA once insertions, deletions and larger rearrangements are counted.
DNA is transcribed into RNA, and RNA is translated into protein, three bases to one amino acid. Francis Crick set out the scheme in 1958; the code itself was worked out between 1961 and 1966. It is nearly identical in every organism.
Avery, MacLeod and McCarty showed in 1944 that DNA, not protein, carries hereditary information. The double helix of 1953 explained how: two complementary strands, each a template for the other.
Sutton and Boveri proposed in 1902–03 that chromosomes carry the hereditary factors. Thomas Hunt Morgan’s fruit flies confirmed it from 1910, and genes that travel together gave the first genetic maps.
It does not. Gregor Mendel showed in 1866 that traits pass as discrete units, two per individual and one from each parent, which separate and recombine unchanged. The word “gene” for these units dates from 1909.
Read from the bottom up. The order is also roughly the order of discovery: units first, then where they sit, then what they are made of, then how they are read.