A gene is a small section of DNA on a chromosome, that codes for a particular sequence of amino acids, to make a specific protein. It is the unit of heredity, and may be copied and passed on to the next generation.
DNA is a large and complex polymer, which is made up of two strands forming a double helix. DNA determines the characteristics of a living organism. With the exception of identical twins, each person's DNA is unique.
DNA is made from base pairs which always come in the following combinations A-T, T-A, C-G and G-C. The order of these letters makes up an organism's genetic code.
An organism's genome is one copy of all of their DNA. With the exception of identical twins, no two people's genomes are the same.
Some characteristics are controlled by a single gene, such as fur in animals and red-green colour blindness in humans. Each gene might have different forms, and these are called alleles.
The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.
Chromosomes are found in the nucleus of a body cell in pairs. One chromosome of each pair is inherited from the mother and the other one is inherited from the father. The chromosome in each pair carries the same gene in the same location. These genes could be the same, or different versions:
Alleles are different versions of the same gene. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour. For any gene, a person may have the same two alleles, known as homozygous or two different ones, known as heterozygous.
The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.
A dominant allele is always expressed, even if one copy is present. Dominant alleles are represented by a capital letter, for example you could use a B. The allele for brown eyes, B, is dominant. You only need one copy of this allele to have brown eyes. Two copies will still give you brown eyes.
A recessive allele is only expressed if the individual has two copies and does not have the dominant allele of that gene. Recessive alleles are represented by a lower case letter, for example, b. The allele for blue eyes, b, is recessive. You need two copies of this allele to have blue eyes.
Homozygous alleles are both identical for the same characteristic, for example BB or bb.
Heterozygous alleles are both different for the same characteristic, for example Bb.
Most characteristics are a result of multiple genes interacting, rather than a single gene.