The Code of Life: DNA and Genetics

Look at your hands for a moment. You might notice you have your father's long fingers, or your mother's dimples when you smile. Have you ever wondered exactly what passes from parent to child to make this happen? It is not magic — it is a molecule so thin that ten billion of them laid side by side would only be as wide as your thumb. That molecule is called DNA, and it contains the instructions that build every living thing on Earth.

Every cell in your body — except red blood cells — contains a complete set of instructions for building you. These instructions are written in a molecule called DNA (deoxyribonucleic acid). DNA is shaped like a twisted ladder, a shape scientists call a double helix. The 'rungs' of the ladder are made from pairs of chemical bases: adenine always pairs with thymine, and cytosine always pairs with guanine. This pairing rule is called complementary base pairing. Your DNA is packed tightly into thread-like structures called chromosomes, found inside the nucleus of each cell. Humans have 46 chromosomes, arranged in 23 pairs. One chromosome in each pair came from your biological mother; the other came from your biological father. A gene is a specific section of DNA that carries instructions for one particular characteristic, such as eye colour, blood type, or whether earlobes are attached or free. Humans have roughly 20,000–25,000 genes spread across their chromosomes. Each gene can exist in different versions called alleles. For example, the gene that influences earlobe shape has an allele for free earlobes and an allele for attached earlobes. Because you have two copies of most chromosomes, you have two alleles for most genes — one inherited from each parent. Some alleles are dominant: even one copy is enough to produce the trait. Others are recessive: you need two copies of the recessive allele for that trait to show. Geneticists use capital letters for dominant alleles (e.g. F for free earlobes) and lower-case letters for recessive alleles (e.g. f for attached earlobes). A person with FF or Ff will have free earlobes; only someone with ff will have attached earlobes. A Punnett square is a simple grid scientists use to predict the possible combinations of alleles an offspring could inherit. The parent's alleles are placed along the top and side of the grid, and the boxes inside show every possible outcome.