Page 77 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.3 Extensions to Mendel for Multifactorial Inheritance 69

Penetrance and expressivity secondary effect. Modifier genes alter the phenotypes pro-
Retinoblastoma, the most malignant form of eye cancer, duced by the alleles of other genes. No formal distinction
arises from a dominant mutation in one gene, but only exists between major and modifier genes. Rather, a con-
about 75% of people who carry the mutant allele develop tinuum exists between the two, and the cutoff is arbitrary.
the disease. Geneticists use the term penetrance to de- Scientists sometimes call the set of unknown modifier
scribe the proportion of individuals with a particular geno- genes that influence the action of known genes the genetic
type who show the expected phenotype. Penetrance can be background.
complete (100%), as in the traits that Mendel studied, or Modifier genes influence the length of a mouse’s tail.
incomplete, as in retinoblastoma. For retinoblastoma, the The mutant T allele of the tail-length gene causes a short-
penetrance is ∼75%. ening of the normally long wild-type tail. But not all mice
In some people with retinoblastoma, only one eye is carrying the T mutation have the same length tail. A com-
affected, while in other individuals with the phenotype, parison of several inbred lines points to modifier genes as
both eyes are diseased. Expressivity refers to the degree or the cause of this variable expressivity. In one inbred line,
intensity with which a particular genotype is expressed in mice carrying the T mutation have tails that are approxi-
a phenotype. Expressivity can be variable, as in retinoblas- mately 75% as long as normal tails; in another inbred line,
toma (one or both eyes affected), or unvarying, as in pea the tails are 50% normal length; and in a third line, the
color (all yy peas are green). As we will see, the incomplete tails are only 10% as long as wild-type tails. Because all
penetrance and variable expressivity of retinoblastoma are members of each inbred line grow the same length tail, no
mainly the result of chance, but in other cases, it is other matter what the environment (for example, diet, cage tem-
genes and/or the environment that cause variations in perature, or bedding), geneticists conclude it is genes and
phenotype. Figure 3.25 summarizes in graphic form the not the environment or chance that determine the length
differences between complete penetrance, incomplete of a mutant mouse’s tail. Different inbred lines most likely
penetrance, variable expressivity, and unvarying expressivity. carry different alleles of the modifier genes that deter-
mine exactly how short the tail will be when the T muta-
tion is present; that is, these lines have different genetic
Modifier genes backgrounds.
Not all genes that influence the appearance of a trait con-
tribute equally to the phenotype. Major genes have a large
influence, while modifier genes have a more subtle, Environmental effects on phenotype
Temperature is one element of the environment that can
have a visible effect on phenotype. For example, tempera-
ture influences the unique coat color pattern of Siamese
cats (Fig. 3.26). These domestic felines are homozygous
Figure 3.25 Phenotypes may show variations in for one of the multiple alleles of a gene that encodes an
penetrance and expressivity. A genotype is completely enzyme catalyzing the production of the dark pigment mel-
penetrant when all individuals with that genotype have the same anin. The form of the enzyme generated by the variant
phenotype (green). Some genotypes are incompletely penetrant— Siamese allele does not function at the cat’s normal core
some individuals with the same genotype show the phenotype and
others do not. Genotypes may also show variable expressivity, body temperature. It becomes active only at the lower tem-
meaning that individuals with the same genotype may show the peratures found in the cat’s extremities, where it promotes
trait but to different degrees. the production of melanin, which darkens the animal’s
ears, nose, paws, and tail. The enzyme is thus temperature
sensitive. Under the normal environmental conditions in
Complete penetrance and unvarying expressivity temperate climates, the Siamese phenotype does not vary
much in expressivity from one cat to another, but one can
imagine the expression of a very different phenotype—no
dark extremities—in equatorial deserts, where the ambient
Incomplete penetrance and unvarying expressivity temperature is at or above normal body temperature.
Temperature can also affect survivability. In one type
of experimentally bred fruit fly (Drosophila melanogas-
ter), some individuals develop and multiply normally at
Complete penetrance and variable expressivity temperatures between 18°C and 29°C; but if the ther-
mometer climbs beyond that cutoff for a short time, they
become reversibly paralyzed; and if the temperature re-
mains high for more than a few hours, they die. These
Incomplete penetrance and variable expressivity insects carry a temperature-sensitive allele of the shibire