DNA, genes and chromosomes

Key terms used in genetics

  • A gamete is a sex cell. In humans, gametes are sperm and eggs. 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.
  • Inside the cell’s nucleus are the chromosomes. Chromosomes are found in the nucleus of a body cell, and occur in pairs. One chromosome is inherited from the mother, and one is inherited from the father. These are long threads of DNA, which are made up of many genes.
  • A gene is a small section of DNA on a chromosome, which 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.
  • The chromosome in each pair carries the same gene in the same location. These genes could be the same, or different versions of the same gene.
  • 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.
  • The phenotype is the visible characteristics of an organism which occur as a result of its genes. For example, blue eyes or brown hair.
  • A dominant allele is expressed in the heterozygote. Dominant alleles are represented by a capital letter, for example A. The allele for brown eyes is dominant. You only need one copy of this allele to have brown eyes. Two copies will also 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 a. The allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.
  • Homozygous alleles are both identical for the same characteristic, for example AA or aa.
  • Heterozygous alleles are both different for the same characteristic, for example Aa.