If the changes in a genome are restricted to a single gene, a single- gene defect ensues. Usually this involves a point mutation and leads to an altered amino acid sequence in the proteins that are coded in this section of the DNA. A human being has ca. 40-60,000 genes that directly influence characteristics such as hair, skin and eye coloration as well as the development and growth of our bodies.
In addition, regulatory regions and the also play important roles in turning genes on and off as well as in their correct transcriptions and translations.
Normally, every child obtains half of its genetic material from each parent. The inheritance of monogenetic diseases occurs in accordance with Mendel's laws. In this, one distinguishes among dominant, codominant and recessive genes (). Today, such single-gene defects can be diagnosed using DNA analyses.
Diseases exist, though, that are not inherited in accordance with Mendel's laws. To these belong defects of the mitochondrial genome as well as illnesses that are due to alterations of various genes (polygeny) or in which environmental factors are involved. (see genome: more complex than the sum of its genes).
Autosomal dominant inheritance
The disorder already appears when only one of the two genes of a chromosome pair is defective. The risk of a mutation carrier transferring his disease to his offspring amounts to 50%.
Examples of such diseases are:
- Marfan's syndrome
- von Recklinghausen's disease (neurofibromatosis)
Autosomal recessive inheritance
If the inheritance is recessive the corresponding genes on both chromosomes must exhibit the defect.
Examples for such diseases are:
- Familiar hemochromatosis (the most frequent autosomal recessively inherited disease)
- Cystic fibrosis
- Sickle cell anemia
- Tay-Sachs disease (infantile gangliosidosis)
- Various mucopolysaccharidoses (MPS)
X chromosomal inheritance
Between 200-300 X chromosome-linked diseases are known. With X chromosomal recessively inherited diseases the affected gene is transferred by phenotypically healthy female carriers. X-chromosomal-dominant inherited diseases are usually lethal.
Examples for X chromosome-linked recessive diseases are:
- Duchenne's muscular dystrophy
- Hemophilia A and B
- Red-green blindness