Multigene Families
A multigene family is a group of similar, but not necessarily identical sequences, each sequence representing a gene, so that the gene is present in multiple copies. These multigene families can be broadly classified in atleast three groups : (i) The different members of a multigene family, if not identical, may function in different tissues of the organism at a particular time, or in the same tissue at different times. A mechanism will be available for tissue specific or temporal (time specific) regulation of activity of different members of such a multigene family (e.g. globin genes), (ii) In other cases all members of a multigene family may function in a specific tissue or at a specific time, but all members will function in the same cell due to heavy demand for the gene product, as in case of heat shock genes expressed on heat shock, or storage protein genes expressed during seed formation, (iii) In still other cases, there may be multigene families, members of which will ordinarily function in all living cells at all times, as in case of rRNA genes or SnRNA genes.