Human Chromosomes
Section 14–2
This section describes the structure of
human chromosomes. It also describes
genetic disorders that are sex-linked, as
well as disorders caused by
nondisjunction.
Human Genes and
Chromosomes
Circle the letter of each sentence that is true
about human genes and chromosomes.


Chromosome 22 contains long stretches of
repetitive DNA that do not code for
proteins.
Human genes located close together on
the same chromosome tend to be
inherited together.
Sex-Linked Genes
What are sex-linked genes?
• They are genes that are located on the
X or Y chromosomes.
Is the following sentence true or false?
The Y chromosome does not contain
any genes at all.
False
Complete the compare-andcontrast table for sex-linked genes.
Is the following sentence true or false?


All X-linked alleles are expressed in
males, even if they are recessive.
true
Punnett Square below illustrates
color blindness inheritance.
X-Chromosome Inactivation


How does the cell “adjust” to the
extra X chromosome in female cells?
In female cells, one X chromosome
is switched off.
What is a Barr body?

It is a dense region in the nucleus
that is formed by the turned-off X
chromosome.
Is the following sentence true or
false?

Barr bodies are found only in males.


If you saw a white cat with orange and
black spots, is it most likely a male or a
female?
It’s probably a female. In cells in some
parts of the body, one X chromosome that
has the allele for orange spots is switched
off, whereas in cells in other parts of the
body, the other X chromosome with the
allele for black spots is switched off.
Chromosomal Disorders
What occurs during nondisjunction?

Homologous chromosomes fail to
separate during meiosis.
Chromosomal Disorders
Is the following sentence true or false?


If nondisjunction occurs, gametes
may have abnormal numbers of
chromosomes.
True
Chromosomal Disorders

The condition in which an individual
has three copies of a chromosome is
known as
trisomy , which means “three
bodies.”
Chromosomal Disorders



Is the following sentence true or
false?
Down syndrome occurs when an
individual has two copies of
chromosome 21
False

Circle the letter of the characteristic
of Down syndrome.
b. mental retardation
Chromosomal Disorders
Why does an extra copy of one
chromosome cause so much trouble?

Scientists do not exactly know for
sure.
Chromosomal Disorders
Circle the letter of each sentence that is
true about sex chromosome disorders.


A female with the karyotype 45,X has
inherited only one X chromosome and
is sterile.
The Y chromosome contains a sexdetermining region that is necessary for
male sexual development.