All organisms inherit a specific pattern of growth, which, for any sexually reproduced organism, comes from the organism's parents. The pattern of growth from one parent combines with that of the other, determining how the organism will develop. The information that governs the pattern of growth is found in the genetic material that exists in each living cell. In most cells, the genetic material is located inside the cell's nucleus, although in some single-celled organisms without nuclei, it lies in the cytoplasm Viruses, which do not have cells, are made up almost entirely of genetic material.

Within each nucleus, the genetic material is arranged into tiny bodies called chromosomes. Each chromosome contains several types of proteins and deoxyribonucleic acid (DNA). DNA consists of two strands, each composed of a long chain of subunits called nucleotides, that form a double helix (the shape of two stretched, intertwined springs). The number of chromosomes in a cell and the size of the DNA (as determined by the number of nucleotides it contains) vary from species to species.

There are four kinds of nucleotides in DNA. Inheritable traits are determined by genes, which are DNA segments that contain specific sequences of the four kinds of nucleotides. The entire sequence of nucleotides in DNA is the genetic code. This code supplies the instructions by which an organism's cells make proteins, the chemical compounds that are necessary for all biological functions.

To make proteins from the genetic code's instructions, the code must be transferred from DNA to another type of nucleic acid, ribonucleic acid (RNA). RNA transports the genetic message from the DNA to the protein-making parts of the cell.

Sometimes a gene is missing or has an error in the coded instructions. Such abnormalities of the genetic material cause genetic diseases and birth defects.

All the genes in a cell of an organism make up the genome of that organism's species. The human genome contains about 1 billion nucleotides. In comparison, the genome of a bacterial cell contains about 4.5 million nucleotides. In 2001, researchers discovered that nucleotides in the human genome form about 30,000 genes, far fewer than had been previously estimated.

Some biologists believe that less than 5 per cent of the human genome is made up of genes with protein-coding information, and that half of the noncoding DNA consists of segments of DNA called transposons. Transposons can affect the transmission of traits. They sometimes change their location on the chromosome and, in doing so, can speed up, slow down, or block the function of specific genes. It is not known whether the properties of a given organism's transposons are inheritable.

Another characteristic of genetic material that may not be inheritable is called DNA methylation. In this process, a segment of DNA becomes inactivated when a methyl group (a compound formed by one atom of carbon and three atoms of hydrogen) becomes attached to it. Methylation plays an important role in regulating gene activity. Biologists believe defective methylation may play a role in the development of certain diseases, such as cancer.