Genetic Disorders
Recessive Alleles
• Most of the time a genetic disorder will be recessive.
• The cause of many diseases were discovered by
comparing the chromosomes of individuals with the
disease to the chromosomes of normal individuals. If
you could find a difference between the two
karyotypes, you could tell which chromosome
controls that trait.
Disorders Inherited as Recessive Traits
Over a thousand human genetic disorders are known to have Mendelian
inheritance patterns. Each of these disorders is inherited as a dominant or
recessive trait controlled by a single gene. Most human genetic disorders are
recessive.
A particular form of deafness is
inherited as a recessive trait.
Recessive Alleles
• Phenylketonuria (PKU)
• Phenylalanine is an amino acid found in
many foods.
• People with PKU don’t make the enzyme
that is needed to break down the amino
acid phenylalanine.
• If a newborn with PKU is given milk they
would not be able to break down the
phenylalanine and it would build up in
their body tissue.
• The buildup of phenylalanine could cause
mental retardation
• There is no cure, but if someone with PKU
avoids ingesting phenylalanine they won’t
show any symptoms.
• PKU is caused by a recessive allele on
chromosome 12.
• How do such small changes in DNA cause such major disorders?
• Cystic Fibrosis is caused by a recessive allele on chromosome 7. (autosomal
recessive)
• The mutation is usually a deletion of 3 bases in the middle of a gene.
• This gene usually makes a protein that allows Chloride ions to pass through cell
membranes. Since the protein is not made the cells don’t transport the protein to
the cell membrane correctly.
• Since the cell can no longer transport chloride ions, the tissues in the body don’t
work properly.
• This disorder causes digestive problems and difficulty breathing. Breathing is made
difficult by a heavy mucus that blocks their lungs.
• Only 50% of children born with this make it to their 20’s.
Tay-Sachs Disease
This disease results in the breakdown of
the nervous system and results in death
within the first few years of life.
There is no treatment for Tay-Sachs
disease.
There is a test for the allele. By having
this test, parents could tell what the
chance is of them having a child with
the disorder.
Dominant Alleles
• Not all disorders are recessive. Some only need one
copy of an allele that causes the disorder to express
the disorder.
Dominantly Inherited Disorders
Achondroplasia, a form of dwarfism with an incidence of one case among
every 10,000 people. Heterozygous individuals have the dwarf phenotype.
Is it possible for a couple with dwarfism to have a normal size child?
Dominant Alleles
• Huntington’s disease, a degenerative
disease of the nervous system, is
caused by a lethal dominant allele that
has no obvious phenotypic effect until
the individual is about 30 to 45 years
old.
• This disease causes loss of muscle control
and loss of mental function.
Codominant Alleles
• Sickle cell disease is caused
by a codominant allele.
• The blood cells of someone with
sickle cell disease are sickle shaped and rigid.
• Because of their shape the blood cells get stuck in the
capillaries, which stops blood from flowing through. This
damages tissue beyond these capillaries. The result is
physical weakness and heart, brain, and spleen damage.
• Sickle cell disease is caused by changing a single base in
DNA. This substitution changes the amino acid glutamic
acid to valine. These cells with valine are less soluble and
they stick together when oxygen is low.
Codominant Alleles
• Why would such an allele exist if it causes a disorder?
• This disorder is most common in people of African
descent. In Africa malaria is common. Malaria is a
parasitic disease that infects red blood cells.
• People who are heterozygous for the allele for sickle
cell disease seem to be resistant to malaria. This is
because when the body destroys the sickle cells it
destroys the parasite at the same time.