How to Calculate Cotransformation Frequency with Examples?

Gene transfer in bacteria occurs by conjugation, transduction and transformation.

Transformation involves the uptake of genetic material from the surrounding medium and its incorporation into bacterial chromosome or plasmid.

Transformation is used to map bacterial genes. Cells that receive genetic material through transformation are called transformants. Genes can be mapped by looking at the rate at which two or more genes are co transformed in transformation. if two genes are close together on the same chromosomal fragment the chance of co transformation is more. 

Higher cotransformation frequency indicates a shorter distance between the two genes on the donor’s chromosome.

Cotransformation Frequency Problems

Problems 1

In a transformation experiment, donor DNA from an E.coli strain with the genotype Z’Y’ was used to transform a strain of genotype of genotype Z Y. The frequencies of transformed classes were:

Z+Y+               200

Z+Y                 400

Z-Y+                400

total               1000

what is the frequency (%) with which Y locus is co transformed with the Z locus?

Answer:

Cotransformation frequency=no. of transformed cells with Z+Y+ genotype/total number of transformed cells

that is 200/1000=0.2 or 20%

The frequency of co-transformation is 20%

Problem 2

DNA from a strain of Bacillus subtilis with the genotype trp+ tyr+ is used to
transform a recipient strain with the genotype trp tyr . The following numbers of
transformed cells were recovered:
Genotype Number of transformed cells

trp+ tyr 154
trp tyr+ 312
trp+ tyr+ 354
What do these results suggest about the linkage of the trp and tyr genes?
Answer:

Cotransformation frequency=no. of transformed cells with trp+ tyr+ genotype/total number of transformed cells

that is 354/820=0.43 or 43%

The frequency of co-transformation of trp and tyr genes are 43%

The high level of cotransformation indicates that these two genes are
closely linked.

Problem 3

DNA from a ‘c+ d+’ bacterial strain was used to transform a ‘c- d-’ strain.

The following transformants were obtained:
c+ d- : 300
c- d+ : 377
c+ d+: 223
Calculate the transformation and cotransformation frequencies ?

Answer:

Cotransformation frequency (%): Cotransformation frequency=no. of transformed cells with c+d+ genotype/total number of transformed cells x100

223/(300+377+223) X100 = 24.8%