BIOLOGY – 9th GRADE
MYP YEAR 5
Read the following article about
genetic disorders and create a table in
your notebook with the information
on the five diseases explained. Include
as much information as possible in
the table.
CRITERION EVALUATED: B.
1. Level 1 Single-gene disorders: These disorders result when a mutation causes the protein product
of a single gene to be altered or missing. Over 10 000 human diseases are caused by defects in
single genes. These single gene disorders, which are also described as unifactorial or monogenic
diseases, are individually very rare but they affect about 1 per cent of the population as a whole.
Examples of single gene disorders include cystic fibrosis, Huntington's disease, sickle cell anemia,
Tay-Sachs disease, myotonic dystrophy, Duchenne and Becker muscular dystrophies, Fragile X
syndrome and spinal muscular atrophy, to name a few.
2. Level 2 Chromosome Abnormalities: In these disorders, entire chromosomes, or large segments of
them, are missing, duplicated, or otherwise altered. While some chromosome abnormalities are
harmless variations, most are associated with clinical disorders of one kind or another. Half of all
spontaneous abortions are due to chromosome abnormalities but the incidence in live births falls to
less than 1 per cent. Examples: Down Syndrome, Klinefelter Syndrome, Turner Syndrome.
3. Level 3 Multifactorial Disorders: Multifactorial disorders result from mutations in multiple genes,
often coupled with environmental causes. Examples: Hypothyroidism, Colon cancer, Breast/Ovarian
cancer, Alzheimer's disease.
SINGLE-GENE-DISORDERS
What do we know about heredity and cystic fibrosis? Cystic fibrosis (CF) is the most common,
fatal genetic disease in the United States. About 30,000 people in the United States have the disease.
CF causes the body to produce thick, sticky mucus that clogs the lungs, leads to infection, and blocks
the pancreas, which stops digestive enzymes from reaching the intestine where they are required in
order to digest food.
Cystic Fibrosis is a Single Gene Disease: Mutations in a single gene - the Cystic Fibrosis
Transmembrane Regulator (CFTR) gene - causes CF. The gene was discovered in 1989. Since then,
more than 900 mutations of this single gene have been identified.
What do we know about heredity and Huntington's disease? Huntington's disease (HD) is an
inherited neurological illness causing involuntary movements, severe emotional disturbance and
cognitive decline. In the United States alone, about 30,000 people have HD. In addition, 35,000
people exhibit some symptoms and 75,000 people carry the abnormal gene that will cause them to
develop the disease. There is no cure for this fatal disease.
A single abnormal gene produces HD. In 1993, scientists finally isolated the HD gene on chromosome
4. The gene codes for production of a protein called "huntingtin," whose function is still unknown. But
the defective version of the gene has excessive repeats of a three-base sequence, "CAG." In the
normal huntingtin gene, this sequence is repeated between 11 and 29 times. In the mutant gene, the
repeat occurs over and over again, from 40 times to more than 80.
This defect causes the resulting huntingtin protein to be malformed, prone to clumping in the brain
and causing the death of nearby nerve cells. Cells of the basal ganglia, a brain area responsible for
coordinating movement, and of the cortex, which controls thought, perception and memory, are most
often affected.
Since the gene that causes HD is dominant, each child of an HD parent has a 50-50 chance of
inheriting the HD gene. The child needs only one copy of the gene from either parent to develop the
disease. A person who inherits the HD gene, and survives long enough, will sooner or later develop
the disease. If the child does not inherit the defective gene, the child will not get the disease nor pass
the gene on to subsequent generations. Symptoms of HD generally appear in mid-life.
CHROMOSOME ABNORMALITIES
What is Down syndrome? Down syndrome is a
chromosomal condition related to chromosome 21. It
affects 1 in 800 to 1 in 1000 live born infants.
What are the symptoms of Down syndrome? People who
have Down syndrome have learning difficulties, mental
retardation, a characteristic facial appearance, and poor
muscle tone (hypotonia) in infancy. Individuals with
Down syndrome also have an increased risk for having
heart
defects,
digestive
problems
such
as
gastroesophageal reflux or celiac disease, and hearing
loss. Some people who have Down syndrome have low
activity of the thyroid gland (hypothyroidism) - an organ
in the lower neck that produces hormones.
Down syndrome can be diagnosed in infancy based on
the characteristic clinical findings. When Down syndrome is suspected in a person, a genetic test
called a chromosome analysis is performed on a blood or skin sample to look for an extra chromosome
21 (trisomy 21). Trisomy 21 means that each cell in the body has three copies of chromosome 21
instead of the usual two copies. Having an extra number 21 chromosome interrupts the normal course
of development, causing the characteristic clinical features of Down syndrome.
Is Down syndrome inherited? Most cases of Down syndrome are not inherited, but occur as random
events during the formation of reproductive cells (eggs and sperm). An error in cell division called
nondisjunction results in reproductive cells with an abnormal number of chromosomes. For example,
an egg or sperm cell may gain an extra copy of chromosome 21. If one of these atypical reproductive
cells contributes to the genetic makeup of a child, the child will have an extra chromosome 21 in each
of the body's cells.
What is Klinefelter syndrome? Klinefelter syndrome is a condition that occurs in men as a result of
an extra X chromosome. The most common symptom is infertility.
Humans have 46 chromosomes, which contain all of a person's genes and DNA. Two of these
chromosomes, the sex chromosomes, determine a person’s gender. Both of the sex chromosomes in
females are called X chromosomes. (This is written as XX.) Males have an X and a Y chromosome
(written as XY). The two sex chromosomes help a person develop fertility and the sexual
characteristics of their gender. Most often, Klinefelter syndrome is the result of one extra X (written as
XXY). Occasionally, variations of the XXY chromosome count may occur, the most common being the
XY/XXY mosaic. In this variation, some of the cells in the male's body have an additional X
chromosome, and the rest have the normal XY chromosome count. The percentage of cells containing
the extra chromosome varies from case to case. In some instances, XY/XXY mosaics may have
enough normally functioning cells in the testes to allow them to father children.
Klinefelter syndrome is found in about 1 out of every 500-1,000 newborn males. The additional sex
chromosome results from a random error during the formation of the egg or sperm. About half of the
time the error occurs in the formation of sperm, while the remainder ones are due to errors in egg
development. Women who have pregnancies after age 35 have a slightly increased chance of having a
boy with this syndrome.
Males who have Klinefelter syndrome may have the following symptoms: small, firm testes, a small
penis, sparse pubic, armpit and facial hair, enlarged breasts (called gynecomastia), tall stature, and
abnormal body proportions (long legs, short trunk).
What is Turner syndrome? Turner syndrome is a chromosomal condition that alters development in
females. Women with this condition tend to be shorter than average and are usually unable to
conceive a child (infertile) because of an absence of ovarian function. Other features of this condition
that can vary among women who have Turner syndrome include: extra skin on the neck (webbed
neck), puffiness or swelling (lymphedema) of the hands and feet, skeletal abnormalities, heart defects
and kidney problems. This condition occurs in about 1 in 2,500 female births worldwide, but is much
more common among pregnancies that do not survive to term (miscarriages and stillbirths).
Turner syndrome is a chromosomal condition related to the X chromosome. Researchers have not yet
determined which genes on the X chromosome are responsible for most signs and symptoms of Turner
syndrome. They have, however, identified one gene called SHOX that is important for bone
development and growth. Missing one copy of this gene likely causes short stature and skeletal
abnormalities in women with Turner syndrome.
What are the symptoms of Turner syndrome? Girls who have Turner syndrome are shorter than
average. They often have normal height for the first three years of life, but then have a slow growth
rate. At puberty they do not have the usual growth spurt.
Non-functioning ovaries are another symptom of Turner syndrome. Normally a girl's ovaries begin to
produce sex hormones (estrogen and progesterone) at puberty. This does not happen in most girls
who have Turner syndrome. They do not start their periods or develop breasts without hormone
treatment at the age of puberty. Even though many women who have Turner have non-functioning
ovaries and are infertile, their vagina and womb are totally normal.
In early childhood, girls who have Turner syndrome may have frequent middle ear infections.
Recurrent infections can lead to hearing loss in some cases. Girls with Turner Syndrome are usually of
normal intelligence with good verbal skills and reading skills. Some girls, however, have problems with
math, memory skills and fine-finger movements.
Is Turner syndrome inherited? Turner syndrome is not usually inherited in families. Turner syndrome
occurs when one of the two X chromosomes normally found in women is missing or incomplete.
Although the exact cause of Turner syndrome is not known, it appears to occur as a result of a
random error during the formation of either the eggs or sperm. In Turner syndrome, the girl does not
have the usual pair of two complete X chromosomes. The most common scenario is that the girl has
only one X chromosome in her cells. Some girls with Turner syndrome do have two X chromosomes,
but one of the X chromosomes is incomplete.
Information taken from:
1. National Human Genome Research Institute (2011). Specific Genetic Disorders
http://www.genome.gov/10001204
2. University of Utah (2012). Genetic Disorders Library.
http://learn.genetics.utah.edu/content/disorders/whataregd/letsou/