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.