400 Chapter 20
eXpLore on your own
In autosomal dominant inheritance, a dominant allele
usually is expressed to some degree.
section 20.6 Many genetic disorders
are X-linked—the mutated gene occurs on
the X chromosome. Males, who inherit only
one X chromosome, typically are affected.
sections 20.8, 20.9 A chromosome’s
structure can be changed by deletions,
duplications, or translocations. Such changes
usually lead to harmful changes in traits.
Chromosome number can be altered by
nondis junction, in which one or more pairs
of chromosomes do not separate during meiosis or mitosis.
In trisomy, an individual inherits an extra copy of one type of
chromosome. If one copy of a given chromosome is missing,
the condition is called monosomy.reView Questions
- How do X and Y chromosomes differ?
- What is a “carrier” of a genetic trait?
- What evidence indicates that a trait is coded by a dominant
allele on an autosome? - Explain the difference between an X-linked trait and
a sex-influenced trait. - Explain what nondisjunction is, and give two examples of
phenotypes that can result from it.
summary
sections 20.1, 20.2 A gene has a specific location
on a specific chromosome. The genes on a chromosome
are physically linked. Those that are closest together
usually end up in the same gamete.
Autosomes are the same in males and females. They
are roughly the same in size and shape and carry genes
for the same traits. Sex chromosomes (X and Y) differ
from each other in size, shape, and the genes they carry.
section 20.3 A child’s gender i s
determined by the father’s sperm, which can
have either an X or a Y chromosome. Males have
an XY genotype; females are XX. Genes on the X
and Y chromosomes are X-linked and Y-linked,
respectively. In a female, X inactivation shuts
down the expression of genes carried on one of
her X chromosomes. Sex-influenced traits, such
as pattern baldness, appear more frequently in one sex. They
may reflect the varying influences of sex hormones.
section 20.4 A pedigree chart can help
establish inheritance patterns and track genetic
abnormalities through several generations of a
family. Table 20.1 lists some common genetic
disorders and abnormalities.
section 20.5 Genetic disorders
provide information about patterns of
gene inheritance. In disorders that involve
autosomal recessive inheritance, a person
who is homozygous for a recessive allele
has the recessive phenotype. Heterozygotes
generally have no symptoms.Several mutant genes are known to be associated with
neurobiological disorders (NBDs) such as schizophrenia,
which affects one of every one hundred people worldwide.
Schizophrenia is characterized by delusions, hallucinations, disorganized
speech, and abnormal social behavior. Another facet of schizophrenia and
other NBDs (such as bipolar disorder) is that affected people often are
exceptionally creative. One example is John Nash (Figure 20.20A), the brilliant
mathematician and Nobel Prize winner whose battle with schizophrenia was
portrayed in the film A Beautiful Mind. Another was the writer Virginia Woolf
(Figure 20.20B), who committed suicide after a long mental breakdown.
Evidence suggests to some researchers that a number of other highly
creative, distinguished historical figures, possibly including Abraham Lincoln,
suffered from some type of NBD. To explore this topic further, do an Internet
search and make a list of ten well-known people from the past who may
have had an NBD. What behaviors or other characteristics have been cited
to support the hypothesis that each individual was affected by an NBD?
Figure 20.20 Some highly creative people have
suffered from a genetic neural disorder. Two
famous cases are A mathematician John Nash
and B writer Virginia Woolf.A B
A: © Reuters/Corbis; B: Hulton-Deutsch Collection/Historical Premium/CorbisCopyright 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).