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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Somatic versus germinal mutation

Genes and chromosomes can mutate in either somatic or germinal tissue, and these changes are called somatic mutations and germinal mutations, respectively. These two different types are shown diagrammatically in Figure 15-3.

Figure 15-3. Somatic mutations are not transmitted to progeny, but germinal mutations may be transmitted to some or all progeny.

Figure 15-3

Somatic mutations are not transmitted to progeny, but germinal mutations may be transmitted to some or all progeny.

Somatic mutation

If a somatic mutation occurs in a single cell in developing somatic tissue, that cell is the progenitor of a population of identical mutant cells, all of which have descended from the cell that mutated. A population of identical cells derived asexually from one progenitor cell is called a clone. Because the members of a clone tend to stay close to one another during development, an observable outcome of a somatic mutation is often a patch of phenotypically mutant cells called a mutant sector. The earlier in development the mutation event, the larger the mutant sector will be (Figure 15-4). Mutant sectors can be identified by eye only if their phenotype contrasts visually with the phenotype of the surrounding wild-type cells (Figure 15-5).

Figure 15-4. Early mutation produces a larger proportion of mutant cells in the growing population than does later mutation.

Figure 15-4

Early mutation produces a larger proportion of mutant cells in the growing population than does later mutation.

Figure 15-5. Somatic mutation in the red Delicious apple.

Figure 15-5

Somatic mutation in the red Delicious apple. The mutant allele determining the golden color arose in a flower’s ovary wall, which eventually developed into the fleshy part of the apple. The seeds are not mutant and will give rise to red-appled (more...)

In diploids, a dominant mutation is expected to show up in the phenotype of the cell or clone of cells containing it. On the other hand, a recessive mutation will not be expressed, because it is masked by a wild-type allele that is by definition dominant to the recessive mutation. A second mutation could create a homozygous recessive mutation, but this event would be rare.

What would be the consequences of a somatic mutation in a cell of a fully developed organism? If the mutation is in tissue in which the cells are still dividing, then there is the possibility of a mutant clone’s arising. If the mutation is in a postmitotic cell—that is, one that is no longer dividing—then the effect on phenotype is likely to be negligible. Even when dominant mutations result in a cell that is either dead or defective, this loss of function will be compensated by other normal cells in that tissue. However, mutations that give rise to cancer are a special case. Cancer mutations arise in a special category of genes called proto-oncogenes, many of which regulate cell division. When mutated, such cells enter a state of uncontrolled division, resulting in a cluster of cells called a tumor. We shall look at some examples later in this chapter.

Are somatic mutations ever passed on to progeny? No. It is impossible, because somatic cells by definition are those that are never transmitted to progeny. However, note that, if we take a plant cutting from a stem or leaf that includes a mutant somatic sector, the plant that grows from the cutting may develop germinal tissue out of the mutant sector. Put another way, a branch bearing flowers can grow out of the mutant somatic sector. Hence, what arose as a somatic mutation can be transmitted sexually. An example is shown in Figure 15-6.

Figure 15-6. A mutation producing an allele for white petals that arose originally in somatic tissue but eventually became part of germinal tissue and could be transmitted through seeds.

Figure 15-6

A mutation producing an allele for white petals that arose originally in somatic tissue but eventually became part of germinal tissue and could be transmitted through seeds. The mutation arose in the primordium of a side branch of the rose. The branch (more...)

Any method for the detection of somatic mutation must be able to rule out the possibility that the sector is due to mitotic segregation or recombination (Chapter 6). If the individual is a homozygous diploid, somatic sectoring is almost certainly due to mutation.

Germinal mutation

A germinal mutation occurs in the germ line, special tissue that is set aside in the course of development to form sex cells. If a mutant sex cell participates in fertilization, then the mutation will be passed on to the next generation. An individual of perfectly normal phenotype and of normal ancestry can harbor undetected mutant sex cells. These mutations can be detected only if they are included in a zygote (Figures 15-7 and 15-8). Remember from Chapter 2 that the X-linked hemophilia mutation in European royal families is thought to have arisen in the germ cells of Queen Victoria or one of her parents. The mutation was expressed only in her male descendants.

Figure 15-7. Germinal mutation determining white petals in viper’s bugloss (Echium vulgare).

Figure 15-7

Germinal mutation determining white petals in viper’s bugloss (Echium vulgare). A recessive germinal mutation, a, arose in an A/A blue plant of the preceding generation, making its germinal tissue A/a. On selfing, the mutation was transmitted (more...)

Figure 15-8. A mutation to an allele determining curled ears arose in the germ line of a normal straight-eared cat and was expressed in progeny such as the cat shown here.

Figure 15-8

A mutation to an allele determining curled ears arose in the germ line of a normal straight-eared cat and was expressed in progeny such as the cat shown here. This mutation arose in a population in Lakewood, California, in 1981. It is an autosomal dominant. (more...)

The experimental detection of germinal mutation depends on the ability to rule out meiotic segregation and recombination as possible causes of phenotypic differences between parents and offspring.

MESSAGE

Before a new heritable phenotype can be attributed to mutation, both segregation and recombination must be ruled out as possible causes. This requirement is true for both somatic and germinal mutations.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21894

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