380 Chapter 19Figure 19.10 People with polydactyly have extra digits on
their hands or feet. Usually the extra digits are duplicates.
These X-rays reveal two “middle” fingers on each hand.Courtesy of Irving Buchbinder, DPM, DABPS, Community Health Services, Hartford, CTFigure 19.11 Eye color and skin color are examples of polygenic
traits. In both cases, alleles of more than one gene interact to
produce and deposit melanin. A Some variations in the color of
the eye’s iris. B Skin color variations in a family. Both parents
are of mixed African and European descent.© Stan Sholik/Alamy© Bo Valentino/Shutterstock.com© MaxFX/Shutterstock.com© Paul Hakimata Photography/Shutterstock.com19.5
n Many phenotypes, such as eye color, can’t be predicted
with certainty. Biologists have uncovered several underlying
causes for these variations.A gene or multiple allele system
may have an all-or-nothing effect
on a trait. Either you have dimples,
or type A blood, or you don’t. But
in other cases, the expression of
a gene varies due to gene inter-
actions or nongenetic factors in
the environment.
The term penetrance refers to
the likely degree to which someone
who inherits an allele will have the
phenotype associated with it. For
example, the recessive allele that
causes cystic fibrosis is completely
penetrant. A full 100 percent of
people who are homozygous for it
develop CF. The dominant allele for having extra fingers
or toes (called polydactyly) is incom pletely penetrant. Some
people who inherit it have the usual ten digits, while oth-
ers have more (Figure 19.10). Camptodactyly is caused by
the abnormal attachment of muscles to bones of the little
finger. Some people who inherit the allele for it have a
stiff, bent little finger on both hands. Others have a bent
pinkie on one hand only. When an allele can produce a
range of phenotypes, its expression is said to be “variable.”
The camptodactyly allele also is incompletely penetrant. In
some people who inherit it, the trait doesn’t show up.polygenic traits come from several
genes combined
Polygenic traits result from the combined expression of
several genes. Examples are eye color and skin color, which
both reflect the activity of several genes involved in thestepwise production and distribution of melanin (Figure
19.11). For instance, black eyes have abundant melanin in
the iris. Dark-brown eyes have less melanin, and light-
brown or hazel eyes have still less. Green, gray, and blue
eyes don’t have green, gray, or blue pigments. Instead, they
have so little melanin that we readily see blue wavelengths
of light being reflected from the iris. Hair color also results
from the interactions of several genes. This explains our
real-world observation that there is lots of natural variation
in human hair color.
For many polygenic traits a population as a whole may
show continuous variation. That is, we can observe a
range of continuous differences in some trait. Earlobe
attachment is this way (Figure 19.12A). Continuous varia-
tion is especially evident in traits that are easily mea-
surable, such as height (Figure 19.12B). Height is also a
multifactorial trait. This term applies to complex pheno-
types that are shaped by more than one gene and also by
some aspect of the individual’s environment.© Gary Roberts/worldwidefeatures.comother Gene effects and interactions
continuous variation
Variation in a genetic trait
that shows up as a range of
phenotypes.
multifactorial trait
Phenotype shaped by mul-
tiple genes and one or more
nongenetic factors.
penetrance The degree to
which someone who inherits
an allele has the phenotype
associated with it.
polygenic trait
Phenotypes produced by
the combined expression of
several genes.
ABPhotoDisc/Getty ImagesCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).