except in the case of identical twins, the occurrence of genetically identical persons is
estimates based on many repetitions. For any one offspring, any one of the
possibilities can occur.
Often, there is no clear-cut dominance or recessiveness within a pair of genes.
Also, most human traits are influenced by more than one pair of genes.
a. Incomplete Dominance. In incomplete dominance, the heterozygous
condition (Aa) produces a phenotype partially resembling both the homozygous
dominant condition (AA) and the homozygous recessive condition (aa). An example is
Wolman's disease, a homozygous recessive condition leading to the accumulation of
lipids in the body. Persons who are heterozygous for this trait tend to have a high level
of cholesterol in their serum.
b. Complementary Inheritance. In complementary inheritance, two
independent pairs of genes affect a trait. Both must be present for a trait to occur.
c. Multifactorial Inheritance. Most human characteristics are affected by a
number of gene pairs.
14-10. CLINICAL IMPLICATIONS
Genes can be affected and changed by a number of circumstances. Some
changes may be beneficial. Other may be harmful. In either case, the effects will be
transmitted to one's offspring.
a. A gene may be lost, for example, by a gamete. The resulting off-spring may
then not have a certain trait. For example, some individuals are unable to produce a
specific enzyme because they do not have the appropriate gene. A metabolic process
using that enzyme may be impossible for that individual.
b. Some individuals may have an excessive number of genes. Examples are
individuals with an extra X or Y chromosome. This can substantially affect both
anatomy and personality.
c. Genetic charts and genetic counseling are sometimes used to advise
prospective parents of genetic problems they may expect with their offspring.
d. Technical advances in the biological sciences have made genetic
engineering possible. Thus, we see the rise of an industry devoted to altering the
genetic makeup of microorganisms for the purpose of producing certain chemicals. The