Psychology2016

Development across the Life Span 315

by those two genes, one gene from each parent. If both genes influence brown hair, the
person will obviously have brown hair, right? And if both influence blond hair, the per-
son’s hair will be blond.

But what if one gene influences brown hair and the other

blond hair?

The answer lies in the nature of each gene. Some genes that are more active in
influencing the trait are called dominant. A dominant gene will always be expressed in
the observable trait, in this case, hair color. A person with a dominant gene influencing
brown hair color will have brown hair, no matter what the other gene is, because brown
is the most dominant of all the hair colors.
Some genes are less active in influencing the trait and will only be expressed in
the observable trait if they are paired with another less active gene. These genes tend to
recede, or fade, into the background when paired with a more dominant gene, so they are
called recessive. Blond is the most recessive hair color and it will only show up as a trait
if that person receives a blond-hair-color gene from each parent.

What about red hair? And how come some people have a mixed

hair color, like strawberry blond?

In reality, the patterns of genetic transmission of traits are usually more compli-
cated. Almost all traits are influenced by more than one pair of genes in a process called
polygenic inheritance. (Polygenic means “many genes.”) Sometimes certain kinds of genes
tend to group themselves with certain other genes, like the genes influencing blond hair
and blue eyes. Other genes are so equally dominant or equally recessive that they com-
bine their traits in the organism. For example, genes involved in blond hair and red hair
are recessive. When a child inherits one of each from his or her parents, instead of one or
the other influencing the child’s hair color, the genes may blend together to form a straw-
berry-blond mix.

GENETIC AND CHROMOSOME PROBLEMS Several genetically determined disorders
are carried by recessive genes. Diseases carried by recessive genes are inherited when a
child inherits two recessive genes, one from each parent. Examples of disorders inher-
ited in this manner are cystic fibrosis (a disease of the respiratory and digestive tracts),
sickle-cell anemia (a blood disorder), Tay-Sachs disorder (a fatal neurological disorder),
and phenylketonuria (PKU), in which an infant is born without the ability to break
down phenylalanine, an amino acid controlling coloring of the skin and hair. If levels
of phenylalanine build up, brain damage can occur; if untreated, it can result in severe
intellectual disabilities. Figure 8. 2 illustrates a typical pattern of inheritance for domi-
nant and recessive genes using the example of PKU.

dominant

referring to a gene that actively

controls the eZRression of a trait.

recessive

referring to a gene that only influences

the eZRression of a trait when Raired

with an identical gene.