Download Linked Genes and Crossing Over

Linked Genes and Crossing Over
The facts:
1. Linked genes are genes that are inherited together because they are on the same chromosome.
*** Do not get this confused with sex-linked genes which are genes that are only carried on a single sex
chromosome.
2. Thomas Hunt Morgan discovered that the expected 9:3:3:1 phenotypic ratio in a dihybrid cross did
not always occur when examining some traits. When he mated wild type ( gray body and normal wings)
with a double mutant type (black body and vestigal wings(stumpy)), all of the offspring had normal body
and wing type. Then he mated the dihybrids with a double mutant male (this was really a test cross).
He expected to get a 1:1 phenotypic ratio if the genes were located on the same chromosome and if the
genes were on different chromosomes, he expected a 1:1:1:1 phenotypic ratio. He didn’t get either of
these. It was closest to the 1:1 ratio, but his results showed a small number of nonparental phenotypes.
The explanation for the nonparental phenotypes was that genetic recombination had occurred.
3. Another scientist named Sturtevant used these recombination frequencies to determine the location
of genes on a chromosome. He developed what is called a linkage map that is based on the
recombination frequencies. This is what we did with the Sordaria data. Map units do not correspond to
actual physcial distances because crossing over is not uniform over the entire chromosome, but it does
give the order and relative distances apart that genes are located on a chromosome. A map unit
corresponds to a 1% recombination frequency. The further apart genes are the more likely they are to
cross over and produce recombinations.
A cytogenic map of a chromosome based on staining can locate the gene exactly in the DNA and give
exact distances. The order of the linkage map and the cytogenic map is the same, but the spacing is
different.
In a series of breeding experiments, a linkage group composed of genes A, B, C, D, and E was found to
show approximately the recombination frequencies in the chart below. Using Sturtevants standard unit
of measure, “map” the chromosome.
Gene
A
B
C
D
E
A
---8
12
4
1
B
8
---4
12
9
C
12
4
----16
13
D
4
12
16
----3
E
1
9
13
3
------
Numbers represent recombination per 100 fertilized Eggs or % recombinations.
Start with the higest recombination percentages and establish the relative positions of those two genes.
For this chromosome it would be D & C. So you would mark your map with 16 units between D & C.
Draw the units on the line like a ruler and it will make it easier to place the other genes. The other genes
will lie between those two genes
Next, place the other genes in position relative to those genes. Keep your eraser handy and don’t be
afraid to start over when you see that your alignment does not work.
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