In approximately 1% of the human population, genetic disorders can be linked to a single altered protein. In most cases, they provide evidence of a single-gene, single-polypeptide association.
However, multifactorial disorders are far more common. Such disorders or disease are caused by the many interactions of numerous genes and proteins, also linked in conjunction with the environment.
Biologists label individuals as either normal (wild-type) or abnormal (mutant). However, the sum of our genetic makeup is what determines, for example, those individuals who are able to consume a high fat diet and not experience heart disease, and those who would yield to disease when exposed to particular pathogenic microorganisms.
It has been suggested that up to 60% of the human population are affected by diseases and disorders to which they are genetically predisposed.
In human genetics, the alleles which cause genetic diseases may be inherited in either a dominant or recessive pattern, and may also be carried on either autosomes or on sex chromosomes. Furthermore, some human diseases are caused by more wide-ranging chromosomal abnormalities. When genetic diseases are traced and tracked over several human generations, different inheritance patterns can be seen. This process of genetic tracing is of vital importance when ascertaining whether an individual has an inherent risk of developing a genetic disease or disorder.
Autosomal Recessive Pattern
In relation to disorders and disease where autosomal recessive mutant alleles are evident, usually both parents of an affected individual are carriers, with a normal phenotype and a heterozygous genotype. Each time such carrier parents conceive a child, they have a 25% probability of having an affected son or daughter. Examples of diseases caused by autosomal recessive traits are PKU, sickle-cell disease and cystic fibrosis.
Autosomal Dominant Pattern
This trait is caused by an abnormal autosomal dominant allele. In this case, the presence of only one mutant allele is sufficient to produce the clinical phenotype. In autosomal dominance, direct transmission from an affected parent to their child is the rule. Familial hypercholesterolemia, for example, is caused by an abnormal autosomal dominant allele.
X-Linked Recessive Pattern
The gene responsible for causing disease via an x-linked recessive pattern is contained on the X chromosome. Thus a son who inherits a mutant allele on the Y chromosome will inherit the disease, as his Y chromosome does not contain a normal allele. However, a daughter who inherits one mutant allele will be an unaffected heterozygous carrier, since she has two X chromosomes and thus, two alleles. The most common pattern of inheritance is from carrier mother to son and all rare X-linked diseases are much more common in males than females. Haemophilia is an x-linked recessive condition, only affecting males. Females carry the disease if affected and do not suffer from any ill-effects. However, it follows that a female carrier can pass the mutant allele to her child.
Chromosomal abnormalities also cause human diseases. Such abnormalities include the gain of or loss of one or more chromosomes (known as aneuploidy), the loss of a portion of chromosome (deletions) and the transfer of a section of chromosome to another chromosome (translocation). Whilst some of these abnormalities are inherited, many are the result of meiotic events such as nondisjunction. An example of a disease caused by chromosomal abnormalities is fragile-X syndrome, which affects approximately one male in every 1,500 and one female in every 2,000. Such individuals are likely to have a constriction near the tip of the X chromosome which tends to break during preparation for microscopy, giving the condition its name. Although the basic pattern of inheritance is that of an X-linked recessive trait, there are deviations from this pattern.
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