Human Chromosome Analysis and Sorting

Flow cytometry has provided the cytogeneticist with a fast and accurate method of measuring the quantity of DNA in each human chromosome (1 ). Almost all the chromosomes in the human complement can now be resolved and abnormal chromosomes and aneuploidies (13,21, and X) recognized. A flow karyotype shows a pattern of peaks and troughs that is unique for each individual or cell line because of the variation in heterochromatic regions of the chromosomes (2 ). When combined with family studies, flow cytometry has been able to resolve homologues differing in DNA content by as little as 1/2000 of the human genome (3 ,4 ), less than a metaphase band. In addition, the sorting capabilities of most flow machines have provided a method for the purification of small but useful quantities of individual chromosomes, for example, 2�10⁶ average sized human chromosomes are equivalent to 500 ng of DNA. Using recombinant DNA techniques, this material can be used to generate a large number of DNA probes to produce a chromosome-specific library, which can be used for the molecular analysis of genetic disease (5 ,6 ). More recently, molecular biologists have experimented with gene mapping by sorting small quantities of individual chromosomes onto filters for spot-blot hybridization with DNA probes (7 ).