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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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Gene interaction in petal color of foxgloves

Genetic variants of foxgloves (Digitalis purpurea) are excellent examples of gene interaction in the determination of the overall appearance of an organism. Three important genes interact to determine petal coloration. The first gene determines the ability of the plant to synthesize purple pigment (a type of anthocyanin). The M allele of this gene stands for ability to synthesize anthocyanin, whereas m stands for inability to synthesize this pigment, resulting in white petals. Figure 4-17 shows the phenotypes to be discussed here. The second gene is a modifier gene. One allele, D, determines the synthesis of large amounts of anthocyanin (dark purple), and d stands for low amounts (light purple). Possibly the D and d alleles regulate the synthesis of pigment by M. The third gene affects pigment deposition. The allele W prevents pigment deposition in all parts of the petal except in the throat spots, whereas the recessive allele w allows deposition of pigment all over the petal. Thus these three genes control the ability to synthesize, the amount synthesized, and the ability for the pigment to be deposited in specific petal cells. We shall consider a variety of dihybrid crosses and even a trihybrid cross.

Figure 4-17. Pigment phenotypes in foxgloves, determined by three separate genes.

Figure 4-17

Pigment phenotypes in foxgloves, determined by three separate genes. M codes for an enzyme that synthesizes anthocyanin, the purple pigment seen in these petals; m/m produces no pigment and produces the phenotype albino with yellowish spots. D is an enhancer (more...)

Consider the cross between the two genotypes M/M ; D/D ; w/w and M/M ; d/d ; W/W. The phenotype of the first genotype is dark purple because it has the D modifier and the ability to deposit pigment. The second phenotype is white with purple spots because, although the plant has the ability to synthesize pigment (conferred by the allele M), the W allele prevents deposition except in the throat spots. Let us consider the usual type of pedigree but eliminate the M allele because it will be homozygous in all individuals.

Image ch4e24.jpg

Overall, a 12:3:1 phenotypic ratio is produced. This kind of interaction is called dominant epistasis because, as can be seen from the F2 results, the dominant allele W eliminates the two alternatives expressed by D and d, dark and light purple, and replaces them with another phenotype, white with purple spots.

The other two dihybrids, in which the third gene is homozygous, result in 9:3:4 recessive epistasis ratios because both are affected by the m allele, which wipes out all pigment production. Hence the dihybrid M/m ; D/d ; w/w (for example) results in the following progeny ratio:

Image ch4e25.jpg

A trihybrid M/m ; W/w ; D/d would produce the following progeny ratio:

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When converted into 1/64ths, the ratio is:

Image ch4e27.jpg

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: NBK21774

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