John Smith is a pig farmer. For the past 5 years, Smith has been adding vitamins and low doses of antibiotics to his pig food; he says that these supplements enhance the growth of the pigs. Within the past year, however, several of his pigs died from infections of common bacteria, which failed to respond to large doses of antibiotics. Can you offer an explanation for the increased rate of mortality due to infection in Smith's pigs? What advice might you offer Smith to prevent this problem in the future?

Rarely, conjugation of Hfr and Fr cells produces two Hfr cells. Explain how this occurs.

A strain of Hfr cells that is sensitive to the antibiotic streptomycin (str8) has the genotype gal+ his+ bio+ pur+ gly+. These cells were mixed with an Fr strain that is resistant to streptomycin (st/) and has genotype galr hisr bior purr glyr. The cells were allowed to undergo conjugation. At regular intervals, a sample of cells was removed and conjugation was interrupted by placing the sample in a blender. The cells were then plated on medium that contains streptomycin. The cells that grew on this medium were then tested for the presence of genes transferred from the Hfr strain. Genes from the donor Hfr strain first appeared in the recipient Fr strain at the times listed here. On the basis of these data, give the order of the genes on the bacterial chromosome and indicate the minimum distances between them:


3 minutes


14 minutes


35 minutes


36 minutes


A series of Hfr strains that have genotype m+ n+ o+ p+ q+ r+ are mixed with an Fr strain that has genotype mr nr or pr qr rr. Conjugation is interrupted at regular intervals and the order of appearance of genes from the Hfr strain is determined in the recipient cells. The order of gene transfer for each Hfr strain is:

What is the order of genes on the circular bacterial chromosome? For each Hfr strain, give the location of the F factor in the chromosome and its polarity.

*18. Crosses of three different Hfr strains with separate samples of an F1 strain are carried out, and the following mapping data are provided from studies of interrupted conjugation:

Appearance of Genes in F1 cells

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