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A genetic disorder is an illness caused by abnormalities in genes or chromosomes. While some diseases, such as cancer, are due in part to a genetic disorders, they can also be caused by environmental factors. Most disorders are quite rare and affect one person in every several thousands or millions. Some types of recessive gene disorder confer an advantage in the heterozygous state in certain environments.[1]A haploid cell has only one set of chromosomes. A diploid cell has two sets of chromosomes. In human, the somatic cells are diploid, and the gametes are haploid.
Genetic diseases are typically diagnosed and treated by geneticists. Genetic counselors assist the physicians and directly counsel patients. The study of genetic diseases is a scientific discipline whose theoretical underpinning is based on population genetics and molecular genetics.[citation needed]
Where the disorder is the result of a single mutated gene, it can be passed on to subsequent generations in several ways. Genomic imprinting and uniparental disomy, however, may affect inheritance patterns. The divisions between recessive and dominant types are not “hard and fast” although the divisions between autosomal and X-linked types are (since the latter types are distinguished purely based on the chromosomal location of the gene). For example, achondroplasia is typically considered a dominant disorder, but children with two genes for achondroplasia have a severe skeletal disorder that achondroplasics could be viewed as carriers of. Sickle-cell anemia is also considered a recessive condition, but heterozygous carriers have increased immunity to malaria in early childhood, which could be described as a related dominant condition.
Only one mutated copy of the gene will be necessary for a person to be affected by an autosomal dominant disorder. Each affected person usually has one affected parent. There is a 50% chance that a child will inherit the mutated gene. Conditions that are autosomal dominant often have low penetrance, which means that although only one mutated copy is needed, a relatively small proportion of those who inherit that mutation go on to develop the disease. Examples of this type of disorder are Huntington’s disease, Neurofibromatosis 1, Marfan Syndrome, Hereditary nonpolyposis colorectal cancer, and Hereditary multiple exostoses,which is a highly penetrant autosomal dominant disorder. Birth defects are also called Congenital anomalies.
Two copies of the gene must be mutated for a person to be affected by an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers). Two unaffected people who each carry one copy of the mutated gene have a 25% chance with each pregnancy of having a child affected by the disorder. Examples of this type of disorder are Cystic fibrosis, Sickle cell anemia (Also Partial Sickle Cell Anemia), Tay-Sachs disease, Niemann Pick disease, Spinal muscular atrophy, and Dry (otherwise known as “rice-brand”) earwax[3]
X-linked dominant disorders are caused by mutations in genes on the X chromosome. Only a few disorders have this inheritance pattern. Males are more frequently affected than females, and the chance of passing on an X-linked dominant disorder differs between men and women. The sons of a man with an X-linked dominant disorder will not be affected, and his daughters will all inherit the condition. A woman with an X-linked dominant disorder has a 50% chance of having an affected daughter or son with each pregnancy. Some X-linked dominant conditions, such as Aicardi Syndrome, are fatal to boys, therefore only girls have them (and boys with Klinefelter Syndrome). Other examples of this type of disorder are X-linked hypophosphatemia (hypophosphatemic rickets, vitamin D-resistant rickets), Aicardi Syndrome, Fragile X, and Rett’s syndrome.
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