Fig. 13. Cloning a Disease Gene by Chromosome Walking.

After a marker is linked to within 1_cM of a disease gene, chromosome walking can be used to clone the disease gene itself. A probe is first constructed from a genomic fragment identified from a library as being the closest linked marker to the gene. A restriction fragment isolated from the end of the clone near the disease locus is used to reprobe the genomic library for an overlapping clone. This process is repeated several times to walk across the chromosome and reach the flanking marker on the other side of the disease- gene locus. (Source: see Fig. 11.)

HUMAN GENETIC DIVERSITY:

The Ultimate Human Genetic Database

* Any two individuals differ in about 3 x 106 bases (0.1%).

* The population is now about 5 x 109.

* A catalog of all sequence differences would require 15 x 1015 entries.

* This catalog may be needed to find the rarest or most complex disease genes.