A conjugation experiment is performed to map the location of three genes involve

Question

 A conjugation experiment is performed to map the location of three genes involved in the ability to synthesize three amino acids, histidine (His), tryptophan (Trp), and leucine (Leu). An Hfr strain with genotype met-his+leu+trp+ is mated with a met+his-leu-trp- recipient. The met marker, which (confers the ability to synthesize the amino acid methionine) is known to be transferred very late. After a short mating the cells are broken apart and plated on four different growth media. The amino acids present in the growth medium and the number of colonies observed on each are the following:  

 His, Trp: 250  

 His, Leu: 50  

 Leu, Trp: 500  

 His only: 10  

 A) What is the order of injection of the genes?  

 Please provide explanation. 

Explanation / Answer

Conjugation happens linearly. So, the time taken for genes to enter recepient is directly proportional to the order of the genes. If in a particular time, number of colonies represent high or less frequency of the respective gene transfer. In the given example, when medium has 'His only', it has only 10 colonies. So, it means that in a given time His gene takes more time to get transferred and thus it is on the rare end of the gene lot. 'Leu, Trp' has a high colony number of 500. It means that in a given time they are transferred with high frequency, followed by His, Trp (250) and His, Leu (50). Trp has a high colony number, it means that it gets transferred quickly. So, it is in the first position of the lot. So, the gene sequence would be Trp - Leu - His.

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