Complementation, Cis, Trans, Dominant, and Recessive
cis, trans, dominant, and recessive are commonly used genetic terms.
Their meaning can be clarified by considering a simple set of two genes that
are transcribed from a single promoter, an operon. We shall consider the lac operon of Escherichia coli. Denote the promoter by p and the three genes that code for proteins that diffuse through
the cell’s cytoplasm or membrane by lacZ,
lacY, and lacA (Fig. 8.5). The lacZ
gene codes for β-galactosidase.
This is an enzyme that hydrolyzes lactose to produce glucose and galactose. The
lacY gene codes for a membrane
protein that transports lactose into the cell, and lacA codes for an enzyme that transfers acetyl groups to some galac-tosides,
thereby reducing their toxicity.
heterozygous for genes may easily be constructed by genetic crosses in diploid
eukaryotes and even in prokaryotes by special tricks. If the genes are on phage
genomes, cells may be simultaneously infected with both phage types. Consider
the lac operon and possibilities for
mutations in genes Z and Y. If we introduce the operon p+Z-Y+ into cells that are p+Z+Y-,
the diploid p+Z-Y+/p+Z+Y-will
possess good β-galactosi-dase
and transport protein, and phenotypically will be able to grow on lactose. The Z+ gene complements the Z- gene and the Y+ gene comple-ments the Y- gene. Both Z+ and Y+ act in trans
and are trans dominant, or simply
dominant. Analogously, Z-
and Y- are recessive.
8.5 Genetic structure of thelacoperon.
mutation in the promoter generates more complexity. A p- muta-tion also appears to be Z- and Y-
even though the Z and Y genes retain their normal sequences.
This results from the fact that no β-galactosi-dase or lactose transport protein can be
synthesized if the lac promoter is
defective. In genetics terminology, such a strain is phenotypically Z- and Y- since, for growth purposes, it behaves as though it
lacks the Z and Y activities. However, it is genotypically
p-, Z+, and
Y+ since the Z and Y genes actually remain intact, as may be revealed in other types
of experiments. In our example, the p
element is cis dominant to the Z+ and Y+ genes, that is, its effect is only over genes on the
same piece of DNA. Also, p+
is not trans dominant to p- since a partial diploid of
the genetic structure p-Z+Y+/p+Z-Y+
would be unable to grow on lactose. The presence of the Z+ gene in a p-Z+
chromosome could be revealed by genetic recombination.