Exploring Errors in Meiosis Activity
Occasionally, errors in meiosis (spermatogenesis or oogenesis) occur in humans,
which result in birth defects in offspring. Two such errors are explored below:
Cri-du-Chat Syndrome
http://learn.genetics.utah.edu/content/disorders/chromosomal/cdc/
The name of this syndrome is French for "cry of the cat," referring to the
distinctive cry of children with this disorder. The cry is caused by abnormal larynx
development, one of the many symptoms associated with this disorder. It usually
becomes less noticeable as the baby gets older, making it difficult for doctors to
diagnose cri-du-chat after age two. Cri-du-chat is caused by a deletion (the
length of which may vary) on the short arm of chromosome 5. Multiple genes are
missing as a result of this deletion, and each may contribute to the symptoms of
the disorder. One of the deleted genes is TERT (telomerase reverse transcriptase).
This gene is important during cell division because it helps to keep the tips of
chromosomes (telomeres) in tact.
How do people get
Cri-A deletion is caused by a break in the DNA molecule that makes up a
chromosome. In most cases, the chromosome break occurs while the sperm or
egg cell is developing. When this gamete is fertilized, the child will develop cridu-chat syndrome. The parent, however, does not have the break in any other
cells of the body and does not have the syndrome. In fact, breaks are so rare that
it is very unlikely to happen again if the parent has another child. It is possible
for a child to inherit a broken chromosome from a parent who also has the
disorder.
Babies with cri-du-chat are usually small at birth and
they may have respiratory problems. Often, the larynx
doesn't develop correctly, which causes the signature
cat-like cry.
People who have cri-du-chat have very distinctive
Several problems occur
features. They may have a small head (microcephaly), an
unusually round face, a small chin, widely set eyes, folds
inside the body, as well.
of
skin
over
their
eyes,
and
a
small
bridge
of
the
nose.
A small number of children have heart defects, muscular or skeletal problems,
hearing or sight problems, or poor muscle tone. As they grow, people with cridu-chat usually have difficulty walking and talking. They may have behavior
problems (such as hyperactivity or aggression) and severe intellectual disability. If
no major organ defects or other critical medical conditions exist, life expectancy
is normal.
Doctors most often identify cri-du-chat by the infant's cat-like cry. Other
signs are microcephaly, poor muscle tone, and mental retardation.
It is also possible to test for cri-du-chat (and other chromosomal abnormalities)
before the baby is born. Testing can be done either (1) on a tiny sample of tissue
from outside the sac where the baby develops (chorionic villus sampling (CVS)),
or (2) on a sample of the amniotic fluid that surrounds the baby (amniocentesis).
Although there is no real treatment for cri-du-chat syndrome, children with the
disorder can go through therapy to improve their language skills, motor skills,
and to help them develop as normally as possible.
Cri-du-chat is one of the most common syndromes caused by a
chromosomal deletion. It affects between 1 in 20,000 and 1 in 50,000 babies.
In 80 percent of the cases, the chromosome carrying the deletion comes from the
father's sperm rather than the mother's egg.
Deletions during the formation of an egg or sperm are caused by unequal
recombination during meiosis. Recombination normally occurs between pairs of
chromosomes while they are lined up at the metaphase plate. If the pairs of
chromosomes don't line up correctly, or if the chromosome breaks aren't
repaired properly, the chromosomes can gain or lose pieces. Unequal
recombination at a certain location on chromosome 5 causes cri-du-chat
syndrome.
Cri-du-chat--You Create the Model
Using the pop beads and magnetic centromeres you used for modeling meiosis
to create a model of chromosome 5. Create a homologous pair, using a different
color bead for the chromosome from each parent, and take chromosomes 5
through meiosis and explain how this mutation occurs during recombination due
to crossover.
Sketch your model in the box below. Include and label all of the steps of meiosis.
Summarize how an error in meiosis results in this genetic condition.
Turner Syndrome
http://learn.genetics.utah.edu/content/disorders/chromosomal/turner/
Turner syndrome is caused by a missing or incomplete X chromosome. People
who have Turner syndrome develop as females. Some of the genes on the X
chromosome are involved in growth and sexual development, which is why girls
with the disorder are shorter than normal and have incompletely developed
sexual characteristics.
Normally, a girl inherits
one X chromosome
from her mother and
one X chromosome
from her father. But girls
who have Turner
syndrome are missing
one
of theirwhen
X
Nondisjunction
happens
chromosomeschromosomes.
are distributed
incorrectly during egg and
sperm formation. The gametes
above either are missing or have
an extra X chromosome.
Turner syndrome is typically caused by what is
The abnormality is not inherited
from an affected parent (not
passed down from parent to
child), because women with
Turner syndrome are usually
called nondisjunction during meiosis. Nondisjunction happens when a pair of
sex chromosomes fails to separate during the formation of a sperm (or egg).
When sperm with no X chromosome unites with a normal egg to form an
embryo, that embryo will have just one X chromosome (X rather than XX). As
the embryo grows and the cells divide, the X chromosome will be missing
from every cell of the baby's body.
Turner syndrome affects growth and sexual development. Girls with this disorder
are shorter than normal and may not start puberty when they should. This is
because the ovaries (which produce eggs, as well as the sex hormones estrogen
and progesterone) don't develop properly.
Turner syndrome usually does not affect intelligence. Common physical
symptoms of Turner Syndrome include a stocky build, arms that turn out slightly
at the elbow, a receding lower jaw, a short webbed neck, and a low hairline at the
back of the neck.
Medical symptoms can include lymphedema (swelling of hands and feet), heart
and/or kidney defects, high blood pressure, and infertility (inability to have
children).
What are the symptoms of Turner
A girl with Turner Syndrome:
One of the missing genes on the X
chromosome is the SHOX gene,
which is responsible for long bone
growth. The missing SHOX gene is
the reason girls who have the
disorder are unusually short. Other
missing genes regulate ovarian
development, which influences
syndrome?
How do doctors diagnose Turner syndrome?
About half of the cases are diagnosed within the first few months of a girl's life by
the characteristic symptoms (swelling of the hands and feet, heart defect). Other
patients are diagnosed in adolescence because they do not grow normally or go
through puberty.
When a doctor suspects Turner syndrome, a blood sample can be used to make a
karyotype (a chromosome analysis), and the diagnosis can be confirmed.
Turner syndrome may be diagnosed during pregnancy with chorionic villus
sampling (CVS) or amniocentesis. Alternatively, an ultrasound (a machine that
uses sound waves to look inside a mother's uterus) can identify the disorder by
its physical symptoms before the baby is born.
Hormone replacement therapy is the best way to treat this disorder. Teenagers
are treated with growth hormone to help them reach a normal height. They may
also be given low doses of androgens (male hormones that females also produce
in small quantities) to increase height and encourage hair and muscle growth.
Some patients may take the female hormone estrogen to promote sexual
development.
Turner Syndrome affects 60,000 females in the United States. This disorder is
seen in 1 of every 2000 to 2500 baby girls, with about 800 new cases
diagnosed each year.
In 75-80% of cases, the single X chromosome comes from the mother's egg; the
father's sperm that fertilizes the egg is missing its sex chromosome.
Turner syndrome is named for Dr. Henry Turner, who in 1938 published a report
describing the disorder.
The average height of an untreated woman with Turner syndrome is 4 feet 8
inches.
A female fetus (normally XX) that is missing one of its X chromosomes can
survive, but a male fetus (normally XY) cannot. The X chromosome is a long DNA
molecule with many genes that are needed for cells to function; it is essential for
life. In contrast, the Y chromosome carries few genes and is not essential for life.
Turner Syndrome--You Create the Model
Using the pop beads and magnetic centromeres you used for modeling meiosis
to create a model of a male’s sex chromosomes. Use a different color bead for
the X and Y chromosome, and take the sex chromosomes through meiosis and
explain how this mutation occurs as a result of nondisjunction.
Sketch your model in the box below. Include and label all the steps of meiosis.
Summarize how an error in meiosis results in this genetic condition.