Question

In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows:

Genotype            Number of offspring

Gg Ll Tt               628

Gg ll Tt                64

Gg Ll tt                12

Gg ll tt                 109

gg Ll Tt                103

gg Ll tt                 67

gg ll tt                  629

gg ll Tt                 14          

Total                    1600

If  the “t” gene is in the middle, which of the following equations best represents how you would calculate an accurate distance between gene G and gene L?

Distance = (64+67+109+103+2*26)/1600

Distance = (109+103)/1600

Distance = (64+67+109+103)/1600

Distance = (64+67+109+103+26)/1600

Distance = (64+67)/1600

Answer 1

Distance = (64+67+109+103+2*26)/1600

Given that the t gene is in the middle, the above represents the map distance between the G and L genes because Map Distance is defined as the number of recombination events between the G and L genes per 100 Progeny. Thus each recombination event between the G and T genes and the L and T genes would be counted separately.

The above formula represents the correct Map Distance as the double recombinant progeny each represent two recombination events between G and L genes and should therefore be counted twice.