The
development of the embryo is dependent on genetic influences. However,
environmental conditions can also exert an important effect. It therefore
follows that congenital abnormalities may occur either as a result of genetic
or Enviromental defect or by combination of both (Foghl Anderson, 2000). Genetic factors may cause
many single anomalies and syndromes and may operate via simple Mendelian or
multifactorial
inheritance (Merck, 2003).
They are of chromosomal origins and
in some cases from external agents (Mutagens) influencing the chromosomes.
(Saladin, 1996). Environmental factors include toxins radiation, diet, drugs,
disease etc. This generally causes various anomalies with extreme consequences.
While genetic abnormalities will have well defined impact upon development.
Maternally derived effects can be harder to define and variable, depending on
different factors e.g. time, exposure and combination of other factors (Hills,
1999).
Genetic cause’s accounts for 51% of
major malformations were 9.5% are due to chromosomal abnormalities, 3.3% are
due to single mutant genes, 23.8% due to multifactorial inheritance and 14.3%
due to uncertain patterns of inheritance. Teratogens such as drugs and other
environmental factors were identified as causing about 32% of all
malformations. However, unknown cause accounted for 43% of all malformations
(McKeon, 1976).
In addition to genetic and
environmentally derived malformations, there is growing evidence that
intrauterine environment has a strong influence on later post-natal life and
neurologic development (Hills, 1999). This theory is based on the early
statistical analysis of disease/longevity in babies with low birth weights in England by
Barker and was initially called the “Barker hypothesis”.
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