What is genetic variation?
Origins of genetic variation
Genetic variation is the difference in DNA sequences between individuals within a population. Variation occurs in germ cells i.e. sperm and egg, and also in somatic (all other) cells. Only variation that arises in germ cells can be inherited from one individual to another and so affect population dynamics, and ultimately evolution. Mutations and recombination are major sources of variation.
Mutations are the original source of genetic variation. A mutation is a permanent alteration to a DNA sequence. De novo (new) mutations occur when there is an error during DNA replication that is not corrected by DNA repair enzymes. It is only once the error is copied by DNA replication, and fixed in the DNA that it is considered to be a mutation (Figure 1). Mutations may be beneficial to the organism; deleterious (harmful) to the organism; or neutral (have no effect on the fitness of the organism).
Somatic mutations can accumulate in our cells and are mostly harmless. They can lead to local changes in tissues such as moles appearing on the skin, and can also have more serious effects - for example leading to cancer. To learn more about the role of somatic mutations in cancer have a look at this paper by Martincorena and Campbell 1. In this course we focus on heritable genetic variation, i.e. variation that occurs in germ cells.
Figure 1 Mutations are the original source of genetic variation. Image source: “The causes of mutations” Understanding Evolution. University of California Museum of Paleontology. 22 August 2008. http://evolution.berkeley.edu/evolibrary/article/evo_20 2.
Recombination is another major source of genetic variation Each of us has a mixture of genetic material from our parents. The mixing of this genetic material occurs during recombination when homologous DNA strands align and cross over. Recombination effectively ‘shuffles’ maternal and paternal DNA, creating new combinations of variants in the daughter germ-cells (Figure 2).
Figure 2 Recombination contributes to human genetic variation by shuffling parental DNA and creating new combinations of variants. Image source: Creation Wiki.
For a discussion on the sources of variation within a population see this chapter in An Introduction to Genetic Analysis. 7th edition 3.