Multigene Families in Eukaryotes
In many eukaryotic genomes, the nuclear DNA generally consists of (i) a high proportion in the form of repeated DNA, whose presence can not be easily explained, except for a small fraction that constitutes genes like rRNA genes and tRNA genes and (ii) a very small fraction as unique DNA (sometimes less than 1%), that accounts for structural genes mainly meant for proteins. The presence of lengthy intron sequences in split genes (see Organization of Genetic Material 3.  Split Genes, Overlapping Genes and Pseudogenes), accounts for upto as much as ten times the sequences (exons) coding for proteins, but this is still far short of the excess DNA that is found in eukaryotic genome. Therefore, the presence of extra repetitive DNA must be explained. A yet another part of this repetitive DNA (although still inadequate to explain the proportion of repeated DNA) can be explained by the presence of multiple copies of a number of genes or pseudogenes in eukaryotes. A set of such multiple copies of a gene descended by duplication and variation is popularly described as a 'gene family' or a 'multigene family'. Several such families have actually been discovered and studied in recent years. Some of these multigene families will be briefly described in this section in view of the significance they have achieved due to current research activity in this area utilizing recombinant DNA technology.