Hemizygote
 
The notion of hemizygosity has historical importance in bacterial genetics. After the early demonstration by Lederberg of genetic recombination in Escherichia coli and the extension of genetic analysis to include a much wider range of unselected markers, a number of complicated anomalies of chromosome segregation began to appear showing that this bacterial system is unconventional. A particular segment of the chromosome of one of the parents, identified by a pair of linked markers, was virtually missing from the heterozygotes generated initially and failed to appear in their haploid progeny. These initial diploids were incomplete (hemizygous). It was first wrongly thought that the missing segment was eliminated from them after their formation. The phenomenon was even given the name of postzygotic exclusion . Analysis of this phenomenon led to the discovery of sexuality in bacteria and to the notion of circular chromosome, which resolved the difficulties. To avoid confusion, the term merozygote should be used instead of hemizygote for these incomplete and transient heterozygotes.
The term hemizygote should be restricted to a diploid cell or individual in which one of the two copies of a given gene has been lost either by deletion or loss of a chromosome. Heterogametic species, in particular human males, are said to be hemizygous for genes located on the X-chromosome and not on the Y-Chromosome (and vice versa).