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Gene Linkage / Gene Mapping HW v2/04
Name: _____________________
Reading: CMR, pp. 174 – 176 (Sec 9.18 - 9.20) You MUST do the book reading for this module.
Additional background: Sturtevant expressed the distances between genes in “map units,”
defining one map unit as equivalent to a 1% recombination frequency. You will find this
terminology used in the following problems. Note that map units do not give actual distances
between genes on a chromosome, but rather relative distances that can be used to determine the
order of multiple genes on a chromosome.
Additional Vocab
Locus (pl. loci): A specific location on a chromosome.
Ex) Homologous chromosomes have corresponding genes (that control the same traits) at all loci.
1. A wild-type fruit fly (heterozygous for gray body color and normal wings) was mated with a
black fly with vestigial wings. The offspring had the following phenotypic distribution: wild-type,
778; black-vestigial, 785; black-normal, 158; gray-vestigial, 162. (use key given in reading)
KEY:
a. What are the genotypes of the parents? _______________________
b. How many offspring were there total? ________________
c. Which offspring phenotypes were recombinant? _______________________________
d. How many offspring were recombinants? ______________
e. What is the recombination frequency between the genes for body-color and wing-type?
(give formula and show work)
2. Determine the sequence of genes along a chromosome based on the following recombination
frequencies: A—B, 8 map units; A—C, 28 map units; A—D, 25 map units; B—C, 20 map
units; B—D, 33 map units.
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3. What is the maximum relative map “distance” possible between two linked genes? ______ m.u.
Why is this the case? (if you cannot answer these questions, go on to #4 and then come back to these q’s)
4. Assume certain genes “A” and “B” are linked and are 50 map units apart. Each locus
has one other allele which is recessive to “A” and “B” respectively (represented by
letters “a” and “b”).
a. How many map units apart must “a” and “b” be then? _________________
An animal heterozygous at both loci is crossed with one that is homozygous recessive
at both loci.
b. What are the genotypes of these parents? (use letters A,a,B,b): ______________________
c. If genes A and B were not linked, what genotypes and phenotype ratios would you expect
from this cross? (show cross)
d. Now, if given that alleles A and B are linked and are 50 m.u. apart, what percentage of the
offspring will show phenotypes resulting from crossovers (recombinant phenotypes)?
e. What are the parental phenotypes for this cross? ______________________________
What are the recombinant phenotypes for this cross? ______________________________
f.
If you did not know genes A and B were linked, how would you interpret the results of this
cross? What would you assume is the physical relationship between gene A and gene B?
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5. In another cross, a wild-type fruit fly (heterozygous for gray body color and red eyes) was
mated with a black fruit fly with purple eyes (genotype: ggrr). The offspring were as follows:
wild-type, 721; black-purple, 751; gray-purple, 49; black-red, 45.
Key:
G=
g=
R=
r=
a. What are the genotypes of the parents? _______________________
b. How many offspring were there total? ________________
c. Which offspring phenotypes were recombinant? _______________________________
d. How many offspring were recombinants? ______________
e. What is the recombination frequency between the genes for body-color and eye-color?
(show work)
f.
What is the distance between the genes for body-color and eye-color? _________ m.u.
Using your calculations from problem 1, what is the distance between the genes for bodycolor and wing-type? ________ m.u.
g. Using information (and key) from problem 1, what fruit flies do you need to mate to
determine the sequence of the body-color, wing-shape, and eye-color genes on the
chromosome? Give the genotypes of the flies you would cross and say what information
you would hope to learn from this cross.
L=
l=
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