Genetic disorder

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For a non-technical introduction to the topic, see Introduction to genetics.

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.[citation needed]

Contents

Single gene disorder

Prevalence of some single gene disorders [2]
Disorder Prevalence
Autosomal dominant
Familial hypercholesterolemia 1 in 500
Polycystic kidney disease 1 in 1250
Huntington disease 1 in 2,500
Hereditary spherocytosis 1 in 5,000
Marfan syndrome 1 in 20,000
Autosomal recessive
Sickle cell anemia 1 in 625
(African Americans)
Cystic fibrosis 1 in 2,000
(Caucasians)
Tay-Sachs disease 1 in 3,000
(American Jews)
Phenylketonuria 1 in 12,000
Mucopolysaccharidoses 1 in 25,000
Glycogen storage diseases 1 in 50,000
Galactosemia 1 in 57,000
X-linked
Duchenne muscular dystrophy 1 in 7,000
Hemophilia 1 in 10,000
Values are for liveborn infants

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.

Autosomal dominant

Main article: Autosomal dominant

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.

Autosomal recessive

Main article: Autosomal recessive

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

Main article: X-linked dominant

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.[citation needed]

X-linked recessive

Main article: X-linked recessive

X-linked recessive disorders are also caused by mutations in genes on the X chromosome. Males are more frequently affected than females, and the chance of passing on the disorder differs between men and women. The sons of a man with an X-linked recessive disorder will not be affected, and his daughters will carry one copy of the mutated gene. With each pregnancy, a woman who has an X-linked recessive disorder (XrXr) has a 50% chance of having sons who are affected and a 50% chance of having daughters who carry one copy of the mutated gene.Examples of this type of disorder Hemophilia A, Duchenne muscular dystrophy, Color blindness, Muscular dystrophy and Androgenetic alopecia.

Y-linked

Main article: Y linkage

Y-linked disorders are caused by mutations on the Y chromosome. Only males can get them, and all of the sons of an affected father are affected. Since the Y chromosome is very small, Y-linked disorders only cause infertility, and may be circumvented with the help of some fertility treatments. Examples are Male Infertility and hypertrichosis pinnae.

Mitochondrial

Main article: Mitochondrial disease

This type of inheritance, also known as maternal inheritance, applies to genes in mitochondrial DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial conditions to their children. Examples of this type of disorder are Human mitochondrial genetics, and Leber's Hereditary Optic Neuropathy.

Multifactorial and polygenic (complex) disorders

Genetic disorders may also be complex, multifactorial or polygenic, this means that they are likely associated with the effects of multiple genes in combination with lifestyle and environmental factors. Multifactoral disorders include heart disease and diabetes. Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of inheriting or passing on these disorders. Complex disorders are also difficult to study and treat because the specific factors that cause most of these disorders have not yet been identified.

On a pedigree, polygenic diseases do tend to “run in families”, but the inheritance does not fit simple patterns as with Mendelian diseases. But this does not mean that the genes cannot eventually be located and studied. There is also a strong environmental component to many of them (e.g., blood pressure).

See also

References

  1. ^ WGBH Educational Foundation
  2. ^ Table 7-1 in:Mitchell, Richard Sheppard; Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson. Robbins Basic Pathology. Philadelphia: Saunders. ISBN 1-4160-2973-7.  8th edition.
  3. ^ Wade, Nicholas (29 January 2006). "Japanese Scientists Identify Ear Wax Gene". New York Times. 

External links

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