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, smooth
36 red, pointed
10 white, pointed
14 red, smooth
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: wwss X ++ ++ giving progeny: w+ s+ (red, pointed)
The second cross is: w+s+ X wwss giving progeny where the genotypes can be unambiguously
interpreted from their phenotypes.
The parental gamete types will be: ws and ++ , resulting in [white, smooth] and [red, pointy]
progeny.
The recombinant gamete types will be: w+ and +s , resulting in [white, pointy] and [red, smooth]
progeny.
Therefore, the recombination frequency is:
(10+14)
= 24cM
(10 + 14) + (40 + 36)
1) In Drosophila, the allele for miniature wing (m) is recessive to the wild type (normal+), and the gene
for vermilion eyes (e) is recessive to the wild type (red eyes+). A female heterozygous for vermilion
eye and miniature wing was mated to a vermilion-eyed, miniature-winged male. The following
offspring were collected:
140 wild type wing, red eye
3 wild type wing, vermilion eye
6 miniature wing, red eye
151 miniature wing, vermilion eye
What genes are contained in the parental type gametes?
What genes are contained in the recombinant gametes?
Which offspring exhibit parental phenotypes?
Which offspring exhibit recombinant phenotypes?
Calculate the frequency of recombination between the vermilion and miniature loci.
2) In guinea pigs, black (B) is dominant to brown (b), and solid color (S) is dominant to spotted (s). A
heterozygous black, solid-colored pig is mated with a brown, spotted pig. The total offspring for several
litters are:
16 black solid
5 black spotted
5 brown solid
14 brown spotted
What genes are contained in the parental type gametes?
What genes are contained in the recombinant gametes?
Which offspring exhibit parental phenotypes?
Which offspring exhibit recombinant phenotypes?
Calculate the frequency of recombination between the vermilion and miniature loci.
3) In Drosophila, there is a dominant gene for gray body color and another gene dominant for normal
wings. The recessive alleles of these two genes result in black body color and vestigial wings,
respectively. Flies homozygous for gray color and normal wings were crossed with flies with black
bodies and vestigial wings. The F1 progeny were then test crossed with the following F2 results.
Gray body, normal wings
Black body, vestigial wings
Gray Body, vestigial wings
Black body, normal wings
236
253
50
61
Calculate the frequency of recombination for these two genes
4) Which statement is true about the individuals in the F1 generation?
a) There are no linked alleles
b) The linked alleles are GG and VV
c) The linked alleles are Gv and gV
d) The linked alleles are GV and gv
5) Which of the following is true about the 61 black, normal flies in the F2 generation?
a) There are no linked alleles
b) the linked alleles are gV and gv
c) The linked alleles are the same as in the F1 generation
d) The linked alleles cannot be determined without performing another cross
6) If some of the black bodied, normal winged flies were crossed with black bodied, vestigial winged
flies, the offspring would probably be
a) like those obtained in the F2 generation
b) unpredictable
c) about 50% black vestigial, 50% black normal
d) about 50% gray vestigial, 50% black normal
7) Diagram a linkage map for genes A, B, C, and D based on the following crossover frequencies
A and B 40%
B and C 20%
C and D 10%
C and A 20%
D and B 10%