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Sex Types

	Sex Types -After the discovery of conjugation in E. coli by Lederberg and Tatum, W. Wolman and W. Hayes carried out detailed studies in E. en I. ,determine the kinetics of mating and transfer of genetic material using combination of genetic markers (nutritional, antibiotic antibioticsensivity etc.). When a strain auxotrophic for an amino acid (X-) and resistant to streptomycin (Sm') was mixed with a strain auxotrophic for another amino acid (Y-) and sensitive to streptomycin (Sm3) and plated on minimal medium containing streptomycin, no recombinants appeared.
However, in the reverse cross (X- y+ Sms x X+ y- Smr) recombinants of the type X+Y+ Smr were obtained. This showed that the genetic material was being transferred from the strain X- Y-f Sm3 to strain x+ Y- Smr. The former, was therefore acting as a donor while the other was a recipient. The donor cells were designated as F+ (fertile) and the recipient cells as F -. Further extensive mating studies with a variety of F+ and F- strains yielded the following information:
(1) In F+ X F- matings, recombinants appeared at a frequency of 10-5 - 10-6. (2) F- x' F- matings gave no recombinants and were sterile-and (3) F+ x F+ matings yielded recombinants at a very low frequency( < 10-6). Since these early studies, there has been much information accumulated about the process of genetic transfer by conjugation in E. coli. The ability to donate chromosomal DNA by conjugation however appears to be generally restricted to the Gram-negative bacteria, although transfer of extra chromosomal genetic elements from one cell to another by cell contact in some Gram-positive bacteria has been reported.

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