Genetic inheritance

Genetic key terms

Greg Foot explains gene inheritance using cats

  • A gamete is a sex cell. In humans, gametes are sperm and eggs (ova).
  • 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.
  • 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.

Some characteristics are controlled by a single gene, such as fur colour 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.

Diagram showing the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes

Chromosomes are found in pairs in the nucleus of a body cell. One chromosome is inherited from the mother and one is inherited from the father. The chromosome in each pair carries the same genes. These genes could be the same, or different versions.

Alleles or variants 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: 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 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 brown eyes 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, 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. Inheriting brown eyed genes from both parents is called homozygous (the same) dominant (because brown dominates blue). Inheriting blue eyed genes from both parents is called homozygous (the same) recessive (because blue is recessive to brown).
  • Heterozygous alleles are both different for the same characteristic, for example Aa. Inheriting a brown eyed gene from one parent and a blue eyed gene from the other is called heterozygous.

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