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We can see if two genes are linked, and how tightly, by using data from genetic crosses to calculate the recombination frequency. By finding recombination frequencies for many gene pairs, we can make linkage maps that show the order and relative distances of the genes on the chromosome.
A genetic map shows the map distance, in cM, that separates any two loci, and the position of these loci relative to all other mapped loci. The genetic map distance is roughly proportional to the physical distance, i.e., the amount of DNA between two loci.
Researchers begin a genetic map by collecting samples of blood, saliva, or tissue from family members that carry a prominent disease or trait and family members that do not. The most common sample used in gene mapping, especially in personal genomic tests is saliva.
In Sturtevant's gene map, six traits are arranged along a linear chromosome according to the relative distance of each from trait B. Traits include yellow body (B), white eyes (C, O),...
Genetic maps provide an outline for the location of genes within a genome, and they estimate the distance between genes and genetic markers on the basis of recombination frequencies during meiosis.
Explain the basis of sequence tagged site ( STS) mapping, and list the various DNA sequences that can be used as STSs. Describe how radiation hybrids and clone libraries are used in STS mapping. The next two chapters describe the techniques and strategies used to obtain genome sequences.
Physical maps specify the distances between landmarks along a chromosome. Ideally, the distances are measured in nucleotides, so that the map provides a direct description of a chromosomal DNA molecule. The most important landmarks in physical mapping are the cleavage sites of restriction enzymes.
