Linkage Problems

Solving Linkage Problems

Tip: The most important part is to determine which progeny resulted from parental type gametes, and which from recombinant types.

In a plant, leaf color and leaf shape are controlled by two linked genes. Leaves of the wild-type plant are red. A recessive mutation in this gene causes white leaves. Wild-type leaves are pointed, and a recessive mutation in this gene causes them to be smooth. The following crosses were performed:

pure breeding white, smooth X pure breeding wild type

gives F1: all red, pointed

Now, the next cross:

red, pointed X pure breeding white, smooth

gives F2:
40 white, curly
36 red, pointed
10 white, pointed
14 red, curly

What is the recombination frequency between the gene for color and for shape?

Solution:

First, assign genotype symbols. Since the mutations are recessive to wild-type, use + for the wt allele and lower case letters for the mutant alleles:
w = recessive color allele for white
s = recessive shape allele for smooth
The first cross is: X giving progeny: (red, pointed)

The second cross is: X giving progeny where the genotypes can be unambiguously interpreted from their phenotypes.
The parental gamete types will be: and , resulting in [white, smooth] and [red, pointy] progeny.
The recombinant gamete types will be: and , resulting in [white, pointy] and [red, smooth] progeny.

Therefore, the recombination frequency is: