An organism’s genotype is its genetic
makeup, while its phenotypes are the physical traits determined by its genes.
Here, we see an example of how the genotype of a bacterial cell determines
its phenotype. The cell on the left contains a plasmid DNA with a gene, shown
in red, for resistance to the antibiotic ampicillin while the cell on the
right does not. Because this gene allows bacteria to grow in the presence of
ampicillin, the cell on the left can survive to multiply and form colonies on
the antibiotic-treated plates shown here. However, the cell on the right dies
when exposed to ampicillin, since it does not possess a gene for antibiotic
resistance. If we call the gene “amp,” for ampicillin resistance, then the
genotype of the cell containing it is AmpR
while that of the cell lacking the gene is AmpS,
for ampicillin sensitivity. The phenotype of the AmpR cells is survival, while that of the AmpS cells is death. However, the AmpR cell can mate with the AmpS cell by a process known as conjugation,
and can transfer a copy of its antibiotic resistance gene to the AmpS cell. As shown, the cell on the left
donates a plasmid containing the amp gene to the cell on the right through a
linkage formed between the membranes of the two bacteria. The genotype of
both cells is now AmpR, and the
phenotype of each is survival, since both can now form colonies in the presence
of ampicillin, as shown.
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