alleles confer dark skin pigment, and their eﬀects are ad-
alleles confer pale skin tone. An individual
genotype would be about as dark-skinned as a
human can get, while an
person would be very fair. How-
ever, when individuals heterozygous for at least one of these gene
pairs mate, a broad range of pigmentation is possible in their oﬀ-
spring. Such polygene inheritance results in a distribution of ge-
notypes and phenotypes that, when plotted, yields a bell-shaped
Check Your Understanding
Why does a male always express an X-linked recessive allele?
How can you explain why parents of average height can
produce very tall or very short offspring?
For answers, see Appendix H.
testosterone receptors are present on X but not on Y. A gene found
only on the X chromosome is said to be
Only relatively short regions at either end of the Y chromo-
some (corresponding to about 5% of the Y’s DNA) code for
nonsexual characteristics corresponding to those on the X.
Tose regions are the only areas that can participate in cross-
overs with the X.
When a male inherits an X-linked recessive allele—for ex-
ample, for hemophilia or for red-green color blindness—its
expression is never masked or damped, because there is no
corresponding allele on his Y chromosome. Consequently, the
recessive gene is always expressed, even when present only in
single dose. In contrast, females must have two X-linked reces-
sive alleles to express such a disorder, and as a result, very few
females exhibit any X-linked conditions.
X-linked traits are typically passed from mother to son,
never from father to son, because males receive no X chromo-
some from their father. Of course, the mother can also pass the
recessive allele to her daughter, but unless the daughter receives
another such allele from her father, she will not express the trait.
Explain how polygene inheritance differs from that
resulting from the action of a single pair of alleles.
So far, we have considered only traits inherited by mechanisms
of classical Mendelian genetics, which are fairly easy to under-
stand, and such traits typically have two, or perhaps three, alter-
nate forms. However, many phenotypes depend on several gene
pairs at diﬀerent locations acting in tandem. Such
variation between two extremes and explains many human
characteristics. Examples of polygene traits in humans include
skin color, height, metabolic rate, and intelligence.
Skin color, for instance, is controlled by three separately inher-
ited genes, each existing in two allelic forms:
Human sex chromosomes (14,100
Proportion of second-generation population
Simpliﬁed model for polygene inheritance of
skin color based on three gene pairs.
Alleles for dark skin are
incompletely dominant over those for light skin. Each dominant
A, B, C
) contributes 1 “unit” of darkness (indicated by a dark
dot) to the phenotypes.
Here the parents are homozygotes at opposite ends of the
phenotype range. Each child inherits 3 “units” of darkness (is a
heterozygote) and has intermediate pigmentation. When they grow
up, if they mate with people with the same alleles, their offspring
(second generation) may have a wide variation in pigmentation, as
shown by the plotted distribution of skin colors.