A mutation occurs in the DNA of a single cell. In single-cell organisms, that mutation is passed on directly to its descendants, typically through the process of mitosis. In multicellular animals, there is a partitioning early in development into somatic cells, which form the body cells, and germline cells, which form the gametes for the next generation. Mutations may be passed on to somatic cells via mitosis and to gametes via meiosis. In plants, this somatic/germline separation occurs later in the cells that form the flower.
Somatic cells form the tissues of the organism and are not passed on as gametes. Any mutations in somatic cells will only affect the individual in which they occur, not its progeny. If mutations occur in somatic cells, its mutant descendants will exist alongside other non-mutant (wild type) cells. If the mutation occurs at a very early stage of development, the mutation will be present in more cells. This gives rise to an individual composed of two or more types of cells that differ in their genetic composition. Such an individual is said to be a mosaic. An example is shown in Figure 6.2.1 Cancer cells are another example of mosaicism.
Germline cells are those that form the eggs or sperm cells (ovum or pollen in plants), and are passed on to form the next generation. Therefore, mutations in germline cells will be passed on to the next generation but won’t affect the individual in which they occur.
In animals, somatic cells are segregated from germ line cells. In plants, somatic cells become germline cells; so somatic mutations can become germline mutations.
Take a look at the video below, 2H – Somatic & germline mutations, presented by Professor Redfield of UBC (Useful Genetics, 2015) on YouTube, which explains Somatic and Germline mutations.
Haploid vs. Diploid Organisms
Haploid organisms, have only one copy of a gene, thus a mutation will directly affect the organism’s phenotype. Therefore, the phenotype can be used to directly infer the genotype of the organism.
However, in diploid organisms, there are two copies of each gene. The phenotype depends upon an interaction between the two alleles. Thus, any mutation may not have a direct impact on the organism’s phenotype. The interaction of the two alleles can show complete dominance, incomplete dominance, co-dominance, or recessiveness. Therefore, inferring the genotype based upon its phenotype is not as simple as in diploids.
Watch the video below, The Life Cycle of Yeast – Professor Rhona Borts, from University of Leicester (2010) on YouTube.
Redfield, R./UBC [Useful Genetics]. (2015, August 10). 2H – Somatic & germline mutations (video file). YouTube. https://www.youtube.com/watch?v=bSPBow616f8
University of Leicester. (2010, November 19). The life cycle of yeast – Professor Rhona Borts (video file). YouTube. https://youtu.be/XxDlzMARURE
- Figure 6.1.1 A pair of human eyes. Left eye colour is blue-grey and right eye colour is shown to be mosaic, with two colours shown – blue-grey and orange. This is an example of segmental heterochromia. [Back to Figure 6.2.1]