Download INHERITED DISEASES HANDOUT

INHERITED DISEASES HANDOUT
Sometimes the structure of an inherited gene contains an error. If the gene
controls the production of an important protein such as hemoglobin, the
hemoglobin will also have an error in its structure. Hemoglobin is the red
pigment in blood. In such a case, the hemoglobin may not do its job well. This is
an example of what scientists call an inherited disease.
People with sickle cell anemia have inherited at least one sickle cell gene. This
is because the gene for normal hemoglobin is codominant with the sickle cell
gene. (Remember that when two codominant genes are inherited, both are
expressed.) When both alleles are present, the person is said to be a carrier
(heterozygous) of the sickle cell trait. About half of a carrier’s hemoglobin is
normal. Carriers, therefore, show few of the harmful effects of sickle cell anemia.
When both sickle cell genes are present, however, the person has sickle cell
anemia and suffers all of the effects of the disorder.
In the United States, most carriers of sickle cell anemia are African Americans.
In fact, about 10 percent of African Americans carry the sickle cell trait. As many
as 40 percent of the population in some parts of Africa may be sickle cell carriers.
The frequency of sickle cell anemia in certain areas has to do with the relationship
between sickle cell anemia and malaria. Malaria is a disease that is common in
Africa and other tropical parts of the world. Malaria (like sickle cell anemia)
affects the red blood cells. Scientists have found that sickle cell carriers are
partially resistant to malaria. Thus the sickle cell trait probably developed as a
mutation that helped people who were carriers of the trait to resist malaria.
SEX-LINKED TRAITS
You will remember from earlier in the chapter that X and Y chromosomes are
sex chromosomes. The X and Y chromosomes are the only chromosome pairs
that do not always match each other. All body cells of normal human males carry
one X chromosome and one Y chromosome. Females have two matching X
chromosomes, or XX.
X chromosomes also carry genes for traits other than sex. However, Y
chromosomes carry few, if any, genes other than those for maleness. Therefore,
any gene -- even a recessive gene— carried on an X chromosome will produce a
trait in a male who inherits the gene. This is because there is no matching gene
on the Y chromosome. Such traits are called sex-linked traits because they are
passed from parent to child on a sex chromosome, the X chromosome. Because a
female has two X chromosomes, a recessive gene on one X chromosome can be
masked, or hidden, by a dominant gene on the other X chromosome.
An example of a disorder caused by a sex-linked trait is hemophilia (hee-mohFIHL-ee-uh). Hemophilia is an inherited disease in which the blood clots very
slowly or not at all. This disease was very common in the royal families of Europe.
During the nineteenth century, Queen Victoria of England had a son and three
grandsons with hemophilia. At least two of her daughters and four of her
granddaughters carried the gene for hemophilia on one X chromosome. But they
did not have the disease because they carried a gene for normal blood clotting on
their other X chromosome. Hemophilia spread through the royal families in
Europe as Victoria’s descendants married other royalty and passed the
hemophilia gene on.
Colorblindness is another sex-linked recessive trait. A person who is colorblind
cannot see the difference between certain colors, such as red and green.
Difficulty in distinguishing between the colors red and green is the most common
type of colorblindness. More males than females are colorblind. A colorblind
female must inherit two recessive genes for colorblindness, one from each
parent. But a colorblind male needs to inherit only one recessive gene. Why is
this so? Remember that males do not have a matching gene on the Y
chromosome that could mask the recessive gene on the X chromosome.
NONDISJUNCTION
During meiosis, or the process through which sex cells are formed,
chromosome pairs usually separate. But in rare cases a pair may remain joined.
This failure of chromosomes to separate from each other is known as
nondisjunction. When this happens, body cells inherit either extra or fewer
chromosomes than normal.
Look carefully at Figure 23-30. This is an example of a human karyotype. A karyotype
shows the size, number, and shape of chromosomes in an organism. Usually, people
have 46 chromosomes, or 23 chromosome pairs. But now look again at Figure 23-30.
How many chromosomes are there?
If you said 47, you are correct. The extra chromosome is found in what would
normally be the twenty-first pair. When a person has an extra chromosome on the
twenty-first pair, a condition called Down syndrome results. People with Down
syndrome may have various physical problems and some degree of mental retardation.
However, many people with Down syndrome hold jobs and make important
contributions to society.
Is there a way of determining before a child is born whether he or she will have Down
syndrome or another inherited problem? There are a number of ways. One way to
determine before a child is born whether the child will have an inherited disease is
called amniocentesis (am-nee-oh-sehn-TEE-sihs). Amniocentesis involves the removal of
a small amount of fluid from the sac that surrounds a baby while it is still in its mother.
This fluid contains some of the baby’s cells. Using special techniques, doctors and
scientist can examine the chromosomes in the cells. In this way, doctors can discover
whether or not an unborn child has Down syndrome. Various other tests can reveal the
presence of a variety of inherited disorders. Scientists hope that such tests will
eventually lead to the treatment of some disorders before babies are born.
FIGURE 23-30: Down
syndrome is a genetic disorder
in which all the body cells have
an extra twenty-first
chromosome. Although
people with Down syndrome
are mentally and physically
challenged many lead full,
active, and productive lives.