Chromosomes and human GenetiCs 395wHat are some inHeritance patterns for
tHe X cHromosome?- A trait that shows up most often in males and that a son can
inherit only from his mother is most likely passed on through
X-linked recessive inheritance. - A few rare mutant traits are passed to offspring via X-linked
dominant inheritance. A heterozygous mother will pass the
allele to half her offspring. An affected father will pass the
allele only to his daughters.
taKe-home messaGesome X-linked abnormalities are quite rare
Amelogenesis imperfecta is one of a few known examples
of a trait that may be caused by a dominant mutant allele
on the X chromosome. With this disorder, the hard, thick
enamel coating that normally pro-
tects teeth doesn’t develop prop-
erly (Figure 20.12).
On rare occasions, someone
whose sex chromosomes are XY
develops as a female. The result is
testicular feminizing syndrome,
or “androgen insensitivity.” In
people who have this syndrome,
a gene mutation on the X chromo-
some produces defective receptors
for male sex hormones (andro-
gens), including testosterone. Nor-
mally, cells in the testes and other
male reproductive organs bind one
or more of the hormones and then
develop further. With defective receptors, however, they
can’t bind the hormones. As a result, the embryo develops
externally as a female but has no uterus or ovaries.many factors complicate genetic analysis
This chapter’s examples of autosomal and X-linked traits
give a general idea of the clues that geneticists look for.
Genetic analysis is usually a difficult task, however. These
days few people have large families, so it may be neces-
sary to pool several pedigrees. Typically the geneticist will
make detailed analyses of clinical data and keep abreast
of current research, in part because more than one gene
may be responsible for a given phenotype. For example,
we know of dozens of conditions that can arise from a
mutated gene on an autosome or from a mutated gene on
the X chromosome. Also, some genes on autosomes are
dominant in males but recessive in females—so they may
initially appear to be due to X-linked recessive inheritance,
even though they are not.
Appendix VII at the back of this textbook provides more
information on genetic disorders and a database of human
genetic traits that you can access online.anything that causes bleeding, even a bruise. The blood
of a heterozygous female clots normally, because the nor-
mal gene on her normal X chromo some makes enough
factor VIII. (Hemophilia B, which involves clotting
factor IX, has similar symptoms.)
Hemophilia A affects about 1 in 7,000 males. Among
nineteenth-century European royal families, however, the
frequency was unusually high because close relatives often
married. Queen Victoria of England and two of her daugh-
ters were carriers. In a pedigree developed some years ago,
more than fifteen of her sixty-nine descendants at that time
were affected males or female carriers (Figure 20.11).
You may recall from Chapter 6 that diseases lumped
under “muscular dystrophy” involve progressive wast-
ing of muscle tissue. Duchenne muscular dystrophy
(DMD) is X-linked. It affects 1 in 3,500 males. As muscles
degenerate, affected boys become weak and unable to
walk. They usually die by age 20 from cardiac or respira-
tory failure. The gene that is mutated in DMD normally
encodes the protein dystrophin, which gives structural
support to muscle fibers. In DMD, muscle fibers lack
dystrophin, so they can’t withstand the physical stress of
contraction and break down. In time the whole muscle
is destroyed.
Red–green color blindness is an X-linked recessive
trait. About 8 percent of males in the United States have
this condition. It arises from mutation of an allele that
codes for the protein opsin, which binds visual pigments
in cone cells of the retina. Females also can have red–green
color blindness, but this occurs rarely because a girl must
inherit the recessive allele from both parents. The genes
for opsin and blood clotting factor VIII are closely linked,
so often hemophilia and red–green color blindness are
inherited together.
The Russian royal family members. All were executed near the
end of the Russian Revolution. They were recently exhumed from
their hidden graves. DNA analysis was used to confirm that the
remains were indeed the Czar and his family. (Bettmann/Corbis)
Alexei
(hemophilic
son)Czarina
Alexandra
(a carrier;
descendant
of Queen
Victoria)Czar Nicholas II
(free of allele for
hemophilia A)BFigure 20.12 Discolored,
abnormal tooth enamel is
typical of a person with
amelogenesis imperfecta.
(“Amelogenesis imperfecta” by Peter
JM Crawford, Michael Aldred, and
Agnes Bloch-Zupan. Orphanet Journal
of Rare Diseases © 2007 Crawford et
al.; licensee BioMed Central Ltd. 2:17.)Copyright 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).