Primary abnormalities are divided into three subgroups:
- Gene aberrations
- Chromosomal aberrations
- Multifactorial anomalies
Gene aberrations account for roughly 7.5% of congenital abnormalities. Either monogenetic mutations or polygenetic mutations are involved that can be further inherited in accordance with Mendel's laws.
One also distinguishes here two kinds: structural and quantity aberrations. They comprise roughly 0.5 % of the congenital abnormalities.
They can be traced back to several genes and can be influenced by environmental factors (medications, chemical products). To this group belong all abnormalities of the neural tube, harelips and cleft palates, as well as cardiac-circulation-disorders, dysplasia of the hips, and cryptorchism.
They are due to the influence of teratogenic factors on an individual who was originally normal. Secondary abnormalities depend on the health of the mother, on the moment at which the violation occurred, on the nature of the responsible agent and on the genetic predisposition of the child.
There are numerous teratogenic factors that can be put into the following order:
- Infectious agents
- Medications, hormones and chemical products
- Physical agents (ionizing radiation)
- Other factors (metabolites, toxic substances)
Teratology (teras: monster) is concerned with congenital abnormalities-
Teratogenesis is the area of embryology that studies the causes, the mechanisms and the models of developmental anomalies. One of the concepts of teratogenesis is that certain periods during the development are more susceptible to teratogenic agents than others.
In order to examine a potentially teratogenic substance one has to pay attention to several points:
- The vulnerable phase of the forming organ
- The dose of the teratogenic substance and how it is applied
- The genotype of the embryo
- The environment
Studies of potentially teratogenic substances can be performed in two ways. In the first method epidemiologic criteria are involved. Here one examines the relationship between the frequency of the anomalies that occur and a prenatal exposure to an agent.
As an alternative, based on animal experiments, substances can also be tested concerning their teratogenic potential. The results cannot, though, always be transferred to humans directly (e.g., thalidomide).
The examination of the teratogenic potential of a substance is made more difficult by the fact that most congenital abnormalities are multifactorial. For the resulting pathology the genetic structure of the individual also plays an important role.This is why a teratogenic substance can have catastrophic consequences for one individual while for another there are no effects.