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Modern Genetic Analysis Chapter 7. Gene Mutations
Solved Problem

1.

A certain plant species normally has the purple pigment anthocyanin dispersed throughout the plant. This makes the green parts of the plant look brown (green plus purple) and the parts of the flower that lack chlorophyll (petals, ovary, anthers) look bright purple. The allele A is essential for anthocyanin production, and, in a homozygote, the recessive allele a results in a plant that lacks anthocyanin. An interesting new allele called au arises, which is unstable. The au allele reverts to A at a frequency thousands of times as great as regular (stable) a alleles do.
a. What phenotype would you expect in plants of genotype (1) au/au; (2) au/a; (3) A/au?
b. How can you confirm that the reversions are true mutations and not suppressors?

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Solution

a.(1) We have seen that mutations tend to be very rare; in the normal course of events, they are seldom observed unless they are seriously sought out. However, an allele that is reverting as frequently as au is likely to announce its reversion behavior prominently if a proper detection system is available. In a plant containing billions of somatic cells, many cells will undoubtedly revert at some stage of development. As development proceeds, each initial revertant cell will give rise to a clone of revertant cells that should be visible as a purple or brown sector. Therefore, a plant of au/au genotype should have white flowers with purple sectors of varying size and photosynthetic tissues that are basically green with brown sectors.

(2) Each sector is a clone derived from a single cell with au reverting to A. The allele au is expected to be dominant to a, because a is essentially inactive and will not prevent au from reverting. Hence, au/a plants will look the same as au/au, but with fewer purple or brown sectors because there are half the chances for reversion.

(3) Because A produces pigment in all cells, A/au will be indistinguishable from A/A. Even though au is reverting, the reversions will not show upa detection system is lacking. Such plants will be purple and brown throughout.
b. How can we prove that a pigmented sector is due to a mutation? It is worth mentioning that some spotted patterns (as in, for example, Dalmatian dogs) are not caused by mutant clones but rather by developmental effects. So spots do not necessarily indicate a mutationa skeptic could invoke some other cause.

The key is that genes are hereditary units and mutant alleles should be transmitted to subsequent generations of cells or individuals. How can we detect the transmission of revertant A alleles to progeny cells or individuals? If we were able to take or "scrape" a few cells out of a sector and grow them into a plant and that plant turned out to be purple, then the reversion hypothesis would be proved. Well, the technology for doing such an experiment is available. Is there any other way? If revertant sectors appear in the flower, then presumably the germinal tissue (anthers and ovaries) should sometimes be part of a sector. Therefore, we could collect pollen from such a flower and pollinate a plant of genotype a/a. If some of the pollen carries a revertant A, then the progeny from this pollen should be A/a and purple or brown throughout. Finding some progeny of this phenotype would prove that the reversion is a true mutation because the A gene could have come from nowhere else.

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© 1999 by W. H. Freeman and Company. All rights reserved.