Earth Science: Human – Genetically induced Diseases – Hereditary Diseases
Hereditary diseases are caused by gene mutations that manifest themselves as a disease of an organism. During reproduction, the mutated genes are passed on to the progeny according to the Mendelian laws of inheritance.
Hereditary diseases, which are also known as genetic disorders, can be genetically passed on to new generations of offspring by autosomal or gonosomal chromosomes (gonosomes are the X and Y sex chromosomes, while autosomes make up the remainder); beyond that, it is possible to distinguish them by whether they are dominant or recessive hereditary diseases.
Autosomal-dominant hereditary diseases
An autosomal-dominant hereditary disease occurs when a certain gene shows a change, and this change—in spite of a normal second copy of the gene—leads to the development of the disease. For the children of a person affected by such a disease, there is 50 percent risk that they too will become ill; if both parents have the disease, the probability increases to 75 percent.
With two damaged alleles (possible manifestation of the gene), an embryo normally dies prior to birth, so that most carriers of a dominant hereditary disease are heterozygous. A typical example of an autosomal-dominant hereditary disease is Marfan Syndrome, where a damaged allele leads to the formation of a defective structural protein, which in turn results in connective tissue weakness.
Autosomal-recessive hereditary diseases For an autosomal-recessive disease to affect a person, there must be a change in both alleles of a particular gene. If only one allele is affected, the second allele can compensate for the consequences. Therefore when this disease occurs in a child from apparently healthy parents, both parents are heterozygous: carrying one mutated and one healthy copy of the gene. Examples of autosomal-recessive hereditary disease are albinism and sickle-cell anemia.
X-chromosomal hereditary diseases
For X-chromosomal hereditary diseases, the gene location for the affected protein is on the female sex chromosome. X-chromosomal-recessive hereditary diseases can occur in the male progeny of healthy parents, when the mother was heterozygous.
A diseased father will pass the gene on to all of his daughters, but with a healthy homozygous mother, they will not get the disease, while their sons have a 50 percent risk of falling ill.
For instance, hemophilia and red-green blindness are sex-related chromosomal hereditary processes, and are both passed on chromosomal-reces-sively. An example of an X-chromosomal-dominant hereditary disease is genetically related nyctalopia (night blindness).
HEMOPHILIA
People who suffer from this incurable disease lack the blood-clotting sub- stance that normally prevents blood loss due to an injury becoming too severe. Blood clotting for someone with hemophilia begins significantly later than it does in healthy humans, therefore even small wounds can lead to serious and potentially dangerous problems.
This disease only affects men; women are only the carriers of this genetic disease. Nowadays there are possibilities to treat hemophilia selectively, for instance by administering cleaned blood-clotting factors.
BASICS
CHROMOSOMAL ANOMALIES A deviating number of chromosomes (numerical aberration) or a differing chromosomal structure (structural aberration) can also cause hereditary disease.
The most frequent numerical chromosomal aberration in humans is Trisomy 21, a defect that manifests itself in physical anomalies and mental retardation.