Question

You are working with recessive mutations at two genes in Drosophila: the fused mutation fu causes fused wings and the garnet mutation g causes bright red eyes.

#1) You would like to determine how far the fu gene is from the Bar gene. The Bar mutation B acts dominantly to make a bar-shaped eye. You make a female that is a double heterozygote for the fu and B alleles. To what type of male should you cross this female in order to estimate the recombination distance between these two genes?

A) wildtype

B) fused wing, wild-type eye

C) wild-type wing, Bar eye

D) fused wing, Bar eye

Answer 1

B. Fusedwing, wild type eye

FufuBb x fufubb

fused wing and wild type eye(recessive) fufubb

Bar eye is dominant

To find recombinant distance we have to cross with this recessive alleles

We cross heterozygous fly with tester fly that is homozygous recessive fly

The purpose of this tester to ensure that allele provided by non tester fly parent is fully determine the phenotype or appearance of fly

By this we can directly read the genotype of each gamete through appearance of offspring

Example

FufuBb x fufubb

FufuBb, Fufubb, fufuBb, fufubb

So here, we can identify by appearance that wild type wing bar eye resembles mother genotype FufuBb

Fused wing, wild type eye resembles father genotype fufubb

Wild type wing wild type eye Fufubb and fusedwing and bar eye fufuBb are recombinants

So recombinant distance=no of recombinant offspring /total offspring

So by phenotype here we can identify recombinant offspring

That s why we use tester homozygous recessive genotype