Position cloning

In positional cloning, disease genes are identified using only knowledge of their approximate of chromosomal location. This is a useful type of cloning for deletions in the genetic code. The DNA sequence is extended from an identified linked marker region toward the gene chromosome by the processes “walking” and “jumping”. In “walking” a segment of genomic DNA containing the identified marker is sequenced from the marker region. Another variation is "jumping," in which the genomic DNA containing the marker sequence is circularized with the marker region on one end and sequenced across the ligated region, thus obtaining the sequence information some distance away from the marker (10).
 

Karyotype

A karyotype is a picture of the 22 pairs of autosomes and the pair of sex chromosomes that have been isolated from a cell in metaphase.  The banding patterns due to the staining of the chromosomes can indicate gross chromosomal abnormalities.

The image below shows chromosome 13 and the site on the q arm where RB1 is found is hi-lighted in red.

RFLP (restriction fragment length polymorphism)
A polymorphism which causes a change in length of a restriction fragment. It is typed by Southern hybridization. It can arise in two ways: (1) as a result of restriction site polymorphism (2) the number of repeat units change in a moderately long VNTR.