Page 78 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

70 Chapter 3 Extensions to Mendel’s Laws

Figure 3.26 In Siamese cats, temperature affects coat color. (a) A Siamese cat. (b) Melanin is produced only in the cooler
extremities. This phenomenon occurs because Siamese cats are homozygous for a mutation that renders an enzyme involved in melanin
synthesis temperature sensitive. The mutant enzyme is active at lower temperatures but inactive at higher temperatures.
a: © Renee Lynn/Science Source

Warmer
temperature
Warmer Colorless No Light
temperature precursor melanin fur
Enzyme
nonfunctional

Cooler
Cooler Colorless temperature Melanin Dark
temperature precursor fur
Enzyme
functional
(a) (b)

gene, which specifies a protein essential for nerve as phenylketonuria, or PKU, will develop a range of neu-
cell transmission. This type of allele is known as a rological problems, including convulsive seizures and
conditional lethal because it is lethal only under certain mental impairment, unless they are put on a special diet.
conditions. The range of temperatures under which the Homozygosity for the mutant PKU allele eliminates the
insects remain viable are permissive conditions; the le- activity of a gene encoding the enzyme phenylalanine hy-
thal temperatures above that are restrictive conditions. droxylase. This enzyme normally converts the amino acid
Thus, at one temperature, the allele gives rise to a pheno- phenylalanine to the amino acid tyrosine. Absence of the
type that is indistinguishable from the wild type, while at enzyme causes a buildup of phenylalanine, and this
another temperature, the same allele generates a mutant buildup results in neurological problems. Today, a reli-
phenotype (in this case, lethality). Flies with the wild- able blood test can detect the condition in newborns.
type shibire allele are viable even at the higher tempera- Once a baby with PKU is identified, a protective diet that
tures. The fact that some mutations are lethal only under excludes phenylalanine is prescribed. The diet must
certain conditions clearly illustrates that the environment also provide enough calories to prevent the infant’s body
can affect the penetrance of a phenotype. from breaking down its own proteins, thereby releasing
Even in genetically normal individuals, exposure to the damaging amino acid from within. Such dietary
chemicals or other environmental agents can have phe- therapy—a simple change in the environment—now
notypic consequences that are similar to those caused by enables many PKU infants to develop into healthy adults.
mutant alleles of specific genes. A change in phenotype Finally, two of the top killer diseases in the United
arising in such a way is known as a phenocopy. By defi- States—cardiovascular disease and lung cancer—also
nition, phenocopies are not heritable because they do illustrate how the environment can alter phenotype by
not result from a change in a gene. In humans, ingestion influencing both expressivity and penetrance. People may
of the sedative thalidomide by pregnant women in the inherit a propensity to heart disease, but the environmental
early 1960s produced a phenocopy of a rare dominant factors of diet and exercise contribute to the occurrence
trait called phocomelia. By disrupting limb development (penetrance) and seriousness (expressivity) of their condi-
in otherwise normal fetuses, the drug mimicked the ef- tion. Similarly, some people are born genetically prone to
fect of the phocomelia-causing mutation. When this lung cancer, but whether or not they develop the disease
problem became evident, thalidomide was withdrawn (penetrance) is strongly determined by whether they choose
from the market. to smoke.
Some types of environmental change may have a pos- Thus, various aspects of an organism’s environment,
itive effect on an organism’s survivability. In the follow- including temperature, diet, and exercise, interact with its
ing example, a straightforward application of medical genotype to generate the phenotype, the ultimate combina-
science artificially reduces the penetrance of a mutant tion of traits that determines what a plant or animal looks
phenotype. Children born with the recessive trait known like and how it behaves.