Question

You are interested in the genetics of flower colour determination in daffodils. The wild type has yellow flowers. In a genetic screen, you isolate two mutants with white flowers. Suggest and explain experimental strategies to answer the following questions:

I. Is each of the mutants defective in a single or multiple genes?

II. Assuming that the mutants are defective in a single gene, are the mutations recessive or dominant to the wild type gene?

III. Assuming both mutants are affected in a single recessive gene, are the same or different genes affected in the two mutants?

Answer 1

1. Assuming that it is a recessive mutation,
Cross the mutant with WT and obtain F1.
Allow selfing of the F1 to produce the F2.

If the phenotype is due to a mutation in a single gene, the mutant phenotype appears in a 3:1 ratio in the F2 progeny i.e. 1/4 = 25% of the F2 progeny exhibits the mutant phenotype.

If the phenotype is due to mutations in two genes, 1/16 = 6.25% of the F2 progeny exhibits the mutant phenotype.

If the phenotype is due to mutations in three genes, 1/64 = 1.56% of the F2 progeny exhibits the mutant phenotype.

If the mutant phenotype is due to a dominant allele, the above-mentioned percentage of progeny would exhibit the WT phenotype in each case.

2. Cross the mutant with WT and obtain F1.
If the phenotype is due to a recessive mutation, all the F1 progeny exhibits the WT phenotype.
If the phenotype is due to a dominant mutation, all the F1 progeny exhibits the mutant phenotype.

3. Cross mutants with each other.
i.e. take one mutant as male and the other as female and perform a cross between them.

If both the mutations are located in the same gene, the F1 progeny of the above cross exhibits the mutant phenotype. i.e. they are allelic to each other.

If both the mutations are located in different genes, the F1 progeny of the above cross exhibits the WT phenotype. i.e. they are not allelic to each other.