In some plants, a red pigment is synthesized enzymatically from a colorless prec

Question

In some plants, a red pigment is synthesized enzymatically from a colorless precursor. Delphinidium, a purple pigment, may be made from cyaniding by the enzymatic addition of one -OH group to the cyaniding molecule. In one cross where these pigments were involved, purple X purple produced 81 purple, 27 red, and 36 white. a. What epistasis ratio is approximately by these offspring? b. How many pairs of genes are involved? c. What are the genotypes of the purple parents? d. Give the genotypes of all the offspring?

Explanation / Answer

The ratio of purple: red: white pigments = 81: 27: 36 = 9: 3: 4

This is an example of dominent epistasis.

a). The ratio of purple: red: white pigments = 81: 27: 36 = 9: 3: 4

This is an example of recessive epistasis. Means, the recessive allele masks the phenotypic expression of another allele.

b). Two pairs of alleles are involved.

C & d). Enzymatic addition and pigment production are controlled by two differrent genes, these two genes must be expressed to produce purple pigment. For example, gene P regulates its production and gene R regulates its incorporation. Mutation in either of the genes produces different pigments. For example, let the gentype of purple X purple is, PpRr and PpRr.

Cross between these two will have the offpsring with the following genotypes PpRr* PpRr =

PPRR ----------> 1/16 (purple)

PPRr -----------> 2/16 (purple)

PpRR -----------> 2/16 (purple)

PPrr ---------------> 1/16 (White)

PpRr -----------> 4/16 (purple)

Pprr ------------> 2/16 (white)

ppRr ------------> 2/16 (red)

ppRR --------------> 1/16 (red)

pprr ----------------> 1/16 (white)

Thus, the resulting phenotypic ratios are, purple: red: white = 9: 3: 4

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