Understanding Genetic and Chromosomal Disorders
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Genetic Disorders and Their Causes
Genetic disorders: Most genetic disorders are caused by mutations in one or more genes or by abnormal numbers of chromosomes.
Genetic Disorders Caused by Mutations
Some disorders are caused by mutations in autosomal genes. In the table below, there is a list of genetic disorders caused by a mutation in a single gene.
| Genetic Disorder | Direct Effect of Mutation | Signs and Symptoms of the Disorder | Mode of Inheritance |
|---|---|---|---|
| Marfan Syndrome | Defective protein in connective tissue | Heart and bone defects and unusually long, slender limbs and fingers | Autosomal dominant |
| Sickle Cell Anemia | Abnormal hemoglobin protein in red blood cells | Sickle-shaped red blood cells that clog tiny blood vessels, causing pain and damaging organs and joints | Autosomal recessive |
| Vitamin D-Resistant Rickets | Lack of a substance needed for bones to absorb minerals | Soft bones that easily become deformed, leading to bowed legs and other skeletal deformities | X-linked dominant |
| Hemophilia A | Reduced activity of a protein needed for blood clotting | Internal and external bleeding that occurs easily and is difficult to control | X-linked recessive |
Dominant and Recessive Alleles
Few genetic disorders are controlled by dominant alleles. A dominant allele is expressed in every individual who inherits even one copy of it. If it is a serious disorder, most people die at a young age and fail to reproduce, making it difficult for the gene to stay in the population.
A mutant recessive allele is not expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but they carry the mutant allele and can pass it to their offspring. Thus, the allele is likely to pass on to the next generation rather than die out.
Chromosomal Disorders
These result from the failure of replicated chromosomes to separate during meiosis (nondisjunction). Some of the resulting gametes will have an extra or missing chromosome. The table below lists several genetic disorders that are caused by abnormal numbers of chromosomes. Most chromosomal disorders involve the X chromosome.