Genetic inheritance

A short video explaining gene inheritance and demonstrating how to use a Punnett square

Genetic key terms

  • DNA is a natural polymer made of nucleotides. It forms two strands in a double helix. It carries the genetic code, which controls how an organism develops and functions. With the exception of identical twins, each person's DNA is unique.
  • Chromosomes are contained inside the cell's nucleus. These are long, thin, threadlike structures made from molecules of DNA that store genetic information. Chromosomes are found in the nucleus of body cells in pairs - one chromosome is inherited from the mother and 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 variants.
  • A gene is a region of DNA containing the instructions for a cell that give the order of amino acids in a protein. Some characteristics are controlled by a single gene, such as earwax texture in humans or pea shape in peas. Each gene might have different forms, and these are called alleles.
Diagram showing the relationship between the cell, its nucleus, chromosomes in the nucleus, and genesThe diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes
  • A gamete is a sex cell. Male and female gametes fuse during fertilisation. In humans, the male gamete is the sperm and the female gamete is the ovum (egg cell). Both the egg and the sperm have 23 chromosomes, which combine to make a fertilised egg (zygote) of 46 chromosomes.
  • Genetic variants are different versions of the same gene. For example, the type of earwax you have is determined by a single gene. One variant causes wet earwax and the other variant causes dry earwax.
  • The genotype is the collection of alleles that determine an organism's characteristics. When these interact with the environment they are expressed as a phenotype.
  • Alleles are the two copies of a gene on a pair of chromosomes. They may contain exactly the same or different information.
A: Allele blue , allele brown heterozygous dominant, brown eyes. B: Two alleles brown, homozygous dominant, brown eyes.  Individual C: Two blue alleles, homozygous recessive, blue eyes.The diagram shows the chromosomes of three different people. Individuals A and B both have brown eyes, even though A is heterozygous and B is homozygous (dominant). Only people with a homozygous recessive genotype can have blue eyes as shown by individual C.

Alleles may be either dominant or recessive:

  • A dominant allele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter, for example, A. The allele for wet earwax is dominant. You only need one copy of this allele to have wet earwax. Two copies will also give you wet earwax.
  • 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 dry earwax is recessive. You need two copies of this allele to have dry earwax.
  • Homozygous alleles are both identical for the same characteristic, for example AA or aa. A homozygote is an individual who has identical alleles for a particular gene.
  • Heterozygous alleles are both different for the same characteristic, for example Aa. A heterozygote is an individual who has different alleles for a particular gene.

Most characteristics are a result of multiple genes interacting, rather than a single gene.