Definition Of A Physical

A physical map is the linear alignment of genes or segments of DNA as they reside on a chromosome in positions based on units of DNA nucleotides or chromosomal

Chromosome Pairing

B1 B2

Crossingover and Recombination

Crossingover and Recombination

Haploid Gametes Produced 10% 10%

r

\

1

B1 1

A

Al 1

1

V

J

r

A2

B2

V

J

r

V

B1

y

Fig. 1 Diagram of a cell prepared to enter meiosis. Each chromosome has been replicated but the sister chromatids are still attached at the centromere. The black chromosome was contributed by the father and contains the A1 and B1 alleles at marker loci A and B, respectively. Likewise, the mother contributed the gray chromosome with marker alleles A2 and B2. The maternal and paternal chromosomes pair at the beginning of meiosis. Next, one paternal chromatid crosses over one maternal chromatid, the two chromosomes break at the point of the crossover, and the segments are then fused to the other chromatid. After two cycles of cell division, four haploid gametes are produced. Two of the gametes are identical to a gamete contributed by one of the parents (parental gametes) and two gametes have one allele from the maternal chromosome and one allele from the paternal chromosome (recombinant ga metes). After observation of many gametes, the percentage of recombinant gametes is determined to be 20% (10% contain A2 and B1 alleles and 10% contain A1 and B2 alleles), indicating that these two markers are located 20 centimorgans apart.

bands. The resolution of a physical map depends on the technique used and the status of available information for the species of interest.

The first techniques developed could only assign genes to chromosomes; in situ hybridization technology permit ted assignments to specific chromosome bands, and now for the human and mouse genomes, assignments can be based on the actual number of base pairs.

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