Genetic inheritance

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

Genetic key terms

  • A gamete is a sex cell. In humans, gametes are sperm and eggs (ovums).
  • 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.
  • Chromosomes are contained inside the cell's nucleus. These are long threads of DNA, which are made up of many genes. Chromosomes in a body cell are found in pairs. One chromosome is inherited from the mother and one is inherited from the father. The chromosomes in each pair carry the same gene in the same location.
  • 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.

The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.

Diagram showing a chromosome and its DNA
  • Alleles or variants are different versions of the same gene (a variant). For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
  • The genotype is the collection of alleles that determine characteristics and can be expressed as a phenotype.
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.

Alleles may be either dominant or recessive:

  • A dominantallele is always expressed, even if only one copy is present. Dominant alleles are represented by a capital letter, for example, A. The allele for black fur in dogs is dominant. Only one copy of this allele is needed in order to have black fur. Two copies of the dominant allele will still produce black fur.
  • A recessiveallele 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 brown fur is recessive. A dog needs two copies of this allele to have brown fur.
  • A homozygous individual has identical alleles for the same characteristic, for example AA or aa.
  • A heterozygous individual has two different alleles for the same characteristic, for example Aa.

Some characteristics are controlled by a single gene, such as fur colour in animals and red-green colour blindness in humans. However, most characteristics are caused by the interaction of many different genes.