Transcript 7.3 Gene Linkage and Mapping

7.3 Gene Linkage and Mapping
KEY CONCEPT
Genes can be mapped to specific locations on
chromosomes.
7.3 Gene Linkage and Mapping
Gene linkage was explained through fruit flies.
• Thomas Hunt Morgan found that linked traits are on the
same chromosome and can be inherited together.
• Chromosomes, not genes, assort independently during
meiosis.
Wild type
Mutant
Fig. 3.1 – The wild type fruit fly shows the most common
phenotype. The mutant fruit fly has white eyes, no wings and a
different body color.
7.3 Gene Linkage and Mapping
• Because the linked genes were not inherited together
every time, Morgan also concluded that chromosomes
must exchange homologous genes during meiosis
(crossing over).
7.3 Gene Linkage and Mapping
Linkage maps estimate distances between genes.
• The closer together two genes are, the more likely they will
be inherited together.
• Cross-over frequencies are related to distances between
genes.
• Linkage maps show the relative locations of genes.
7.3 Gene Linkage and Mapping
Linkage maps estimate distances between genes.
Summarize: How can a linkage map be made from observations
of traits?
7.3 Gene Linkage and Mapping
• Cross-over frequencies can be converted into map units. The
order of genes on a chromosome can be determined if all of
their cross-over frequencies are known.
– gene A and gene B cross over 6.0
percent of the time
– gene B and gene C
cross over 12.5 percent
of the time
– gene A and gene C cross over 18.5 percent of the
time
A = body color
B = eye color
C = wing size
7.3 Gene Linkage and Mapping
1. Based on the above linkage map, what is the frequency
that gene A and gene B cross over together? Gene D and
gene C?
2. Draw a linkage map based on the following cross-over
percentages for three gene pairs: A-B = 8%, B-C = 10%
and A-C = 2%