1 2 3 4 5 6 7 8 9 10 423 bp 322 bp 234 bp 198 bp 11 12 13 14 15 16 17 18 19 20 423 bp 322 bp 234 bp 198 bp -- 25 26 27 28 29 30 21 22 23 24 423 bp 322 bp 234 bp 198 bp 31 32 33 34 35 36 37 38 39 40 423 bp 322 bp 234 bp 198 bp
The distance between two molecular markers that are linked along
the same chromosome can be determined by analyzing the outcomes
of crosses. This can be done in humans by analyzing a family’s
pedigree. However, the accuracy of linkage mapping
with human pedigrees is fairly limited because the number of people in most
families is relatively small. As an alternative, researchers can analyze a population of sperm, produced from a single male, and compute
linkage distance in this manner. As an example, let’s suppose a male
is heterozygous for two polymorphic STSs. STS-1 exists in two sizes:
234 bp and 198 bp. STS-2 also exists in two sizes: 423 bp and 322
bp. A sample of sperm was collected from this man, and individual
sperm were placed into 40 separate tubes. In other words, there was
one sperm in each tube. Believe it or not, PCR is sensitive enough to
allow analysis of DNA in a single sperm! Into each of the 40 tubes
were added the primers that amplify STS-1 and STS-2, and then the
samples were subjected to PCR. The following results were obtained:
A. What is the arrangement of these two STSs in this individual?
B. What is the map distance between STS-1 and STS-2?
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