Genetics and inheritance

In the 1850s and 1860s Gregor Mendel, a friar in Brno in what is now the Czech Republic, bred thousands of pea plants and tracked traits such as flower colour and seed shape. He found that traits did not blend. When he crossed purple-flowered and white-flowered plants, the offspring were all purple, but white flowers reappeared in about a quarter of the next generation.

Mendel concluded that each plant carries two hereditary factors for a trait, one from each parent, and that one factor can mask another. His work was largely ignored until 1900, when other scientists rediscovered it. The factors he described are what we now call genes.

A gene is a stretch of DNA that carries instructions, usually for making a protein. Different versions of the same gene are called alleles. Humans have around 20,000 protein-coding genes, arranged along 23 pairs of chromosomes, with one chromosome of each pair inherited from each parent.

If an allele shows its effect even when only one copy is present, it is dominant. If its effect appears only when two copies are present, it is recessive. The combination of alleles a person carries is their genotype; the observable traits that result are their phenotype.

Eggs and sperm are made by a special kind of cell division called meiosis, which halves the chromosome number. During meiosis, each pair of chromosomes is shuffled independently, and matching chromosomes swap segments in a process called crossing over.

The result is that each egg and sperm carries a unique mix of the parent's genes. Apart from identical twins, who develop from a single fertilised egg, no two siblings inherit exactly the same combination.

• Most traits, such as height and skin colour, are polygenic: influenced by many genes, each with a small effect.
• Environment interacts with genes; nutrition strongly affects how tall a person grows.
• Some genes are on the X chromosome, so conditions such as red-green colour blindness are more common in males, who have only one X.
• Some alleles are codominant, both showing their effects, as in the AB blood group.
• Epigenetic marks can switch genes on or off without changing the DNA sequence.

A mutation is a change in the DNA sequence. Many have no effect, some are harmful and a few are beneficial. Mutations are the raw material of evolution.

Some diseases are caused by a single gene. Cystic fibrosis and sickle cell disease are recessive conditions, appearing when a person inherits two affected alleles. Huntington's disease is dominant, so one copy is enough. Most common diseases, such as heart disease and type 2 diabetes, involve many genes plus lifestyle and environment.

The Human Genome Project, completed in 2003, produced the first near-complete sequence of human DNA, about 3 billion base pairs. Sequencing has since become dramatically cheaper and faster, and is used to diagnose rare diseases, guide cancer treatment and trace ancestry.

CRISPR-Cas9, developed as a gene-editing tool by Jennifer Doudna and Emmanuelle Charpentier, who shared the 2020 Nobel Prize in Chemistry, lets scientists change DNA at precise locations. In 2023 regulators approved the first CRISPR-based therapy, for sickle cell disease. Editing human embryos in ways that would be inherited remains widely prohibited and ethically contested.