Inheritance

Heredity is the passing on of characteristics from parents to offspring, through their genes. You have 46 chromosomes in each body cell, in 23 pairs. One of each pair came from your mother and one from your father.

Gametes are sex cells: egg cells and sperm cells. Each carries only 23 chromosomes, half the usual number. At fertilisation a sperm cell and an egg cell join, 23 + 23, and the new cell has the full 46. That cell divides again and again to build a baby. Every body cell with a nucleus carries the same full set of instructions; only the gametes carry half.

Inherited (in your genes)	Environmental (from your life)	Both together
Natural eye colour, natural hair colour, blood group, dimples, earlobe shape	Scars, a suntan, the language you speak, skills such as riding a bike or playing the piano	Height, weight, skin colour, how fast you can run

Why you look like your family, but not exactly. Each egg and each sperm carries a different selection of a parent’s chromosomes, so every child gets a different mix of the same two sets of genes. That is why brothers and sisters resemble each other without being the same. Identical twins come from one fertilised egg that splits in two, so they share all their genes. Even so, the environment shapes them: their fingerprints differ, and so can their height and weight. Non-identical twins come from two eggs, and are as alike as any brother and sister.

Stretch Some instructions are hidden by others, so a characteristic can skip a generation: two brown-haired parents can have a red-haired child if both carry the red-hair instruction without showing it. Changes that happen to your body during your life, such as big muscles from training, are not written into your genes and are not passed on. Gregor Mendel worked out the rules of inheritance by counting pea plants in the 1860s, before anyone knew what genes were made of.