Variations within the DNA sequence of a particular gene affect its function, and may cause or predispose an individual a particular disease. Alterations in the genome may increase the frequency of disorder and disease with entire populations.
Although there are many types of genetic disorders, a specific disorder does not have to be inheritable to have a genetic basis. For example, non-heritable disorders can also arise from mutations in somatic cells resulting from exposure to mutagenic factors in the environment. Mutations, whether inherited mutations that appear in every cell of the body, or random mutations affecting a particular cell, can cause groups of cells to grow out of control, or inhibit the processes (contact inhibition processes) that normally prevent this from happening.
Some diseases and disorders are traced to the presence of a single form of a gene, to a mutation in a specific normal gene. Other common conditions, including not only some cancers but also some forms of heart disease and diabetes, are polygenic. Variations in a number of genes, in combination with environmental conditions that determine the extent to which these genes are expressed, affect the risk that an individual will develop such conditions. The risk calculations associated with many of the disorders commonly regarded as genetic diseases are often predictable as functions of relatively simple Mendelian inheritance.
There are many types of genetic diseases and disorders result from a few well-established mechanisms. Autosomal dominant disorders, in which one deleterious gene or allele expresses itself over a normal complementary allele is normal is the mechanism underlying
Crouzon disease. In contrast, phenylketonuria, is an autosomal recessive disorder, in which both deleterious alleles must be present. There are also sex-linked diseases and disorders wherein the deleterious gene or genes lie on sex chromosomes (X and Y chromosomes). There are X-linked dominant disorders (e.g., hypoplastic amelogenesis imperfecta), X-linked recessive disorders (Menkes' syndrome), and Y-linked disorders, in which the only mechanism of transmission is from father to son.
Not all genetic disorders depend on alterations to nuclear DNA. There are disorders, such as mitochondrial myopathy, that can result from alterations to mitochondrial DNA.
Genetic counseling deals with the problems associated with the diagnosis of a genetic disorder, the probable disease course, and possible treatments and management. Genetic testing used to assess the risks of genetic disorders and the risks of recurrence. Options for dealing with the risk of a genetic disorder and its recurrence sometimes involve methods of contraception, adoption, insemination by donor sperm, and prenatal diagnosis.
Bayes' theorem is used in genetic epidemiology in order to obtain the probability of disease in a group of people with some characteristic. In addition, Bayes' theorem is able to calculate unknown conditional probabilities (PVP) from known conditional probabilities (detection rate or sensitivity). For example, biochemical and ultrasound marker-based screening use a derivation of Bayes' theorem to select patients for whom further testing for a particular disease or disorder may be appropriate.
A variation of Bayes' theorem, termed the Bart's test, is very popular in the prenatal screening projects. Bart's test allows an adjustment of the probability of the disease (expressed as 1/total) for an appropriate factor named likelihood ratio, that is the ratio between the detection rate and the false positive rate.
Except for genes appearing on the X or Y chromosomes in males, there are usually two copies of each gene in humans. This redundancy provides a buffer to genetic diseases and disorders. In many cases, only one correctly functioning copy of a gene is necessary. Only when an individual has obtained two copies of an abnormal recessive gene will the corresponding disease manifest itself. Inheritance of this type is called homozygous recessive.
A heterozygous individual with one allele for such a condition may be completely unaffected. In other cases, the individual may even be at an advantage, which provides a clue as to why the mutation remains in the population. Sickle cell disease, relatively common among people of African descent, is an often-fatal condition in which red blood cells become sickle-shaped when the oxygen content of the blood decreases, as it does during physical exertion. The deformed blood cells block small blood vessels, causing tissue death (necrosis) in affected areas. Although only an individual with two alleles for sickle cell will have the disease, individuals with one sickle cell allele (type pf gene) have sickle cell trait. Trait carriers only experience disease-like symptoms at extreme low-oxygen conditions such as those found at very high altitudes. On the other hand, such an individual actually gains a significant advantage relative to malarial resistance. Malaria is endemic in Africa, and the evolutionary benefit of having a large population of people who are heterozygous for the trait overcomes the disadvantage of a fatal condition affecting homozygotes with two copies of the allele. Therefore this type of genetic disease may persist at a relatively high frequency in a population over a long period of time even if the actual disorder is serious or potentially fatal.
With dominant alleles, one copy of a defective gene is enough to produce a disease or disorder. Genetic disorders with dominant inheritance that are lethal at an early age do not remain in the population, because they kill the affected individual before he or she can reproduce. However, nonlethal dominant genetic disorders, such as the hand and foot malformation called camptobrachydactyly, do persist over time. Likewise, a lethal genetic disorder such as Huntington's disease that strikes after the
If the gene associated with a disorder is found on the X chromosome, typically males are afflicted more often and/or more severely than females. That is because in females who are heterozygous for such an X-linked trait, there is a normal version of the gene to compensate. Males have only one X chromosome, so if a X-linked gene is mutated, it usually has a severe effect. X-linked genetic disorders include hemophilia and red-green color blindness.
Chromosome abnormalities, such as the addition or deletion of a chromosome, may result from errors that occur when gametes (sperm and egg) are formed, during fertilization, or during the early development of the zygote. Most chromosome aberrations are lethal, resulting in spontaneous abortion (miscarriage), or death in infancy. Only a few, including the extra copy of chromosome 21 that results in Down syndrome, produces individuals who, although affected by mental and physical abnormalities, can survive into adulthood.
Abdel Hakim Ben Nasr, PhD