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# Genetic map of 3 linked genes found on the same chromosome

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In corn, the following recessive genes have been identified on chromosome 3:

yg = yellow green seedlings
wx = waxy endosperm
bz = bronze color

From a three point testcross between plants heterozygous for all three genes and plants homozygous recessive for all three genes, the following data were obtained:

yellow green 6
yellow green, waxy 720
wild type 56
bronze, waxy 13
bronze 681
yellow green, bronze 25
waxy 31
yellow green, bronze, waxy 42

a. Which gene is in the middle?
b. What is the distance between genes yg and wx?
c. What is the distance between genes wx and bz?
d. What is the distance between genes yg and bz?

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#### Solution Summary

When genes are found on the same chromosome they are called linked genes. By examining the recombination frequencies between these genes during a three-point test cross you can determine how far apart the genes are from each other. This is called a gene map. This explanation will walk you through how to examine the data provided by a three point cross to determine what the distances are between each gene. Once the distances are known the explanation then describes how to put the genes in the correct order they would be found in on the chromosome.

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## Genetic problems regarding gene mapping of linked genes

Problem 1

A female mouse with yellow-mottled fur, eye-ear reduction and regular teeth was mated to a male mouse with dark gray fur, normal eye-ear size and irregular teeth. The F1 females were all wild type, that is, dark gray fur, normal eye-ear size and regular teeth. The F1 males had yellow-mottled fur, eye-ear reduction and regular teeth. A cross of F1 females to F1 males produced the following offspring in the F2 generation.

Phenotype for Number of
Fur color Eye-Ear size Teeth Males Females
Dark Gray Normal Regular 11 142
Yellow Mottled Reduced Irregular 18 0
Dark Gray Reduced Irregular 47 0
Yellow Mottled Normal Regular 54 57
Dark Gray Reduced Regular 3 51
Yellow Mottled Normal Irregular 4 0
Yellow Mottled Reduced Regular 135 150
Dark Gray Normal Irregular 128 0

a. Draw a genetic map showing the distances between these genes.
b. Express the differences in map units.
c. What is the coefficient of coincidence?
d. What is the degree of interference?

Problem 2

A female rabbit with hemophilia, rickets, and a tail was mated to a male rabbit that did not have hemophilia or rickets but lacked a tail. The F1 females were all wild type, that is, no having hemophilia or rickets but having a tail. The F1 males had hemophilia, rickets, and tails. A cross of F1 females to F1 males produced the following offspring in the F2 generation:

Hemophilia Rickets Tails Males Females
Yes No No 5 0
No Yes Yes 3 84
Yes Yes No 30 0
No No Yes 27 160
Yes No Yes 86 91
No Yes No 81 0
Yes Yes Yes 128 165
No No No 140 0

a. What are the genotypes of the original parents?
b. Write Each genotype to show which alleles are linked together on the same chromosome.
c. What are the genotypes of the F1 males and females? Write each genotype to show which alleles are linked together on the same chromosome.
d. Draw a genetic map showing the distances between these genes. Express the distances in map units.
e. What is the degree of interference?

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