Microbial Genetics
The progress in modern genetics is mainly by the experiments done in microbial organisms like bacteria and virus. In these organisms, the nucleus is not separated from the cytoplasm. They have only haploid number of chromosomes. Hence each character is controlled by only one allele.

A bacterium

In addition to the bacterial chromosome, the cytoplasm contains another small circular DNA called plasmid or episome or fertility factor or sex factor. It contains some genes. A bacterium containing plasmid is called F+ and that lacks it is called F-. F+ cells are also called male cells and F- cells are called female cells. F+ cells have filamentous projections called pili (pilus-sing). During sexual process a F+ cell can donate a copy of plasmid to a F- cell. Thus the F- cell can be converted into a F+ cell. Sometimes the plasmid can get integrated with the bacterial chromosome. Such cells are called Hfr cells (high frequency cells).