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CHAPTER OUTLINE
24.1 Gene Linkage
All the alleles on one chromosome form a linkage group because they tend to be inherited
together and do not show independent assortment.
24.2 Sex-Linked Inheritance
Out of 23 pairs of chromosomes, 22 are called autosomes and one pair is the sex chromosomes,
so named because they differ between the sexes. In humans, males have the sex chromosomes X
and Y, and females have two X chromosomes. Traits controlled by genes on the sex
chromosomes are said to be sex-linked. The Y chromosome from the father often does not carry
an allele for a trait found on the X chromosome.
Sex-Linked Alleles
When considering X-linked traits, the allele on the X chromosome is shown as a
letter attached to the X chromosome.
Pedigrees for Sex-Linked Disorders
Most sex-linked disorders are carried on the X chromosome. More males than females
have the disorder because recessive alleles on the X chromosome are always expressed in
males. The Y chromosome lacks an allele for the disorder.
X-Linked Recessive Disorders of Interest
Color blindness, Duchenne muscular dystrophy, fragile X syndrome, and
hemophilia are examples of X-linked recessive disorders.
24.3 Changes in Chromosome Number
Sometimes individuals are born with either too many or too few chromosomes, most likely due to
nondisjunction during meiosis. Nondisjunction occurs during meiosis when either the
homologous pair or the sister chromatids fail to separate.
Down Syndrome
The most common autosomal trisomy seen among humans is trisomy 21, also
called Down syndrome. This syndrome has particular characteristics that are
easily recognized. Persons with Down syndrome usually have three copies of
chromosome 21 because the egg had two copies instead of one.
Changes in Sex Chromosome Number
An abnormal sex chromosome number is the result of inheriting too many or too
few X or Y chromosomes.
Turner Syndrome
Females with Turner syndrome have only one sex chromosome, an X.
Klinefelter Syndrome
Klinefelter syndrome results when a male inherits two X chromosomes and one
Y chromosome and develops severe symptoms.
Poly-X Females
A poly-X female has more than two X chromosomes.
Jacobs Syndrome
XYY males, who have Jacobs syndrome, can only result from
nondisjunction during spermatogenesis.
24.4 Changes in Chromosomal Structure
Chromosomal mutations occur when chromosomes break and fail to reunite correctly.
Deletions and Duplications
A deletion occurs when a single break causes a chromosome to lose an end piece or
when two simultaneous breaks lead to the loss of an internal chromosomal segment. In a
duplication, a chromosomal segment is repeated in the same chromosome or in a
nonhomologous chromosome.
Translocation
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A translocation is the exchange of chromosomal segments between two
nonhomologous chromosomes.
Inversion
An inversion occurs when a segment of a chromosome is turned 180 degrees.
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