When is a cold not just a cold?
At some point in your life, you're bound to catch a cold. And there are ways to prevent catching a cold. But what if you couldn't prevent an illness? What if you were born with a disease? What if having a disease was actually due to your DNA? These are genetic diseases, and they can be very serious.
Human Genetic Disorders
Some human genetic disorders are also X-linked or Y-linked, which means the faulty gene is carried on these sex chromosomes. Other genetic disorders are carried on one of the other 22 pairs of chromosomes; these chromosomes are known as autosomes or autosomal (non-sex) chromosomes.
Autosomal Recessive Disorders
Some genetic disorders are caused by recessive alleles of a single gene on an autosome. An example of an autosomal recessive genetic disorder is cystic fibrosis. Children with cystic fibrosis have excessively thick mucus in their lungs, which makes it difficult for them to breathe. The inheritance of this recessive allele is the same as any other recessive allele, so a Punnett square can be used to predict the probability that two carriers of the disease will have a child with cystic fibrosis. Recall that carriers have the recessive allele for a trait but do not express the trait. What are the possible genotypes of the offspring in the following table (Table below)? What are the possible phenotypes?
According to this Punnett square, two parents that are carriers (Ff) of cystic fibrosis gene have a 25% chance of having a child with cystic fibrosis (ff).
Another example is sickle-cella anemia. If a person receives two recessive alleles, the wrong amino acid is created (glutamate instead of valine) which changes the person's ability to create healthy blood cells (with hemoglobin) to carry oxygen in the blood. Without the proper amount of hemoglobin in blood, tissues don't receive the proper amount of oxygen and die. Sickle-cell anemia is found on chromosome 11. Sickle cells usually live about 15 days, whereas healthy blood cells last on average 120 days.
Automsomal Dominant Disorders
Huntington’s disease is an example of an autosomal dominant disorder. This means that if the dominant allele is present, then the person will express the disease.
The disease causes the brain’s cells to break down, leading to muscle spasms and personality changes. Unlike most other genetic disorders, the symptoms usually do not become apparent until middle age. You can use a simple Punnett square to predict the inheritance of a dominant autosomal disorder, like Huntington’s disease. If one parent has Huntington’s disease, what is the chance of passing it on to the children? If you draw the Punnett square, you will find that there is a 50 percent chance of the disorder being passed on to the children.
- autosome: Any chromosome that is not a sex chromosome.
- carrier: Person that carries the recessive allele for a trait but does not express the trait.
- cystic fibrosis: Autosomal recessive genetic disorder characterized by excessively thick mucus in the lungs.
- Huntington’s disease: Autosomal dominant genetic disorder characterized by muscle spasms and personality changes.
- sickle-cell anemia: Autosomal recessive genetic disorder characterized by not enough hemogolbin in blood cells
- Autosomal recessive genetic disorders, such as cystic fibrosis, are caused by recessive alleles of a single gene on an autosome.
- Autosomal dominant genetic disorders, such as Huntington's disease, are caused by dominant alleles of a single gene on an autosome.
Use the resource below to answer the questions that follow.
- What are Genetic Disorders? at http://learn.genetics.utah.edu/content/disorders/whataregd/
- What are multifactorial disorders? What is an example of a multifactorial disorder?
- What are single-gene disorders? What is an example of a single-gene disorder?
- What causes Colon Cancer? How is it diagnosed? How is it treated?
- Can you be a carrier of an autosomal dominant recessive disorder?
- One parent is a carrier of the cystic fibrosis gene, while the other parent does not carry the allele. Can their child have cystic fibrosis?