Figure 6.6: Sickle cell anemia causes red blood cells to become misshapen and curved (upper
figure) unlike normal, rounded red blood cells (lower figure). ( 6 )
Polygenic Traits and Environmental Influences
Another exception to Mendel’s rules ispolygenic inheritance, which is when a trait is
controlled by more than one gene. Often these traits are in fact controlled by many genes on
many chromosomes. Each dominant allele has an additive effect, so the resulting offspring
can have a variety of genotypes, from no dominant alleles to several dominant alleles. In
humans, some examples of polygenic traits are height and skin color. People are neither
short nor tall, as was seen with the pea plants studied by Mendel, which has only one gene
that encodes for height. Instead, people have a range of heights determined by many genes.
Similarly, people have a wide range of skin colors. Polygenic inheritance often results in a
bell shaped curve when you analyze the population (Figure6.8). That means that most
people are intermediate in the phenotype, such as average height, while very few people are
at the extremes, such as very tall or very short.
Most polygenetic traits are partially influenced by the environment. For example, height is
partially influenced by nutrition in childhood. If a child is genetically programmed to be
average height but does not get a proper diet, he or she may be below average in size.
Other examples of environmentally influenced traits are mental illnesses like schizophrenia
and depression. A person may be genetically predisposed to have depression, so when that
person’s environment contributes major stresses like losing a job or losing a close relative,
the person is more likely to become depressed.