Many individuals are born with defects due to genetic factors. Some such defects occur because the child inherits a defective gene, or genes from the parents. Others are caused through mutations- spontaneous changes that occur to a gene or chromosome. Environmental factors affecting the foetus during the uterine development may also cause defects. If, during pregnancy, a woman smokes, consumes alcohol or other drugs, or suffers dietary deficiencies, the developing foetus may be adversely affected.
About one in every seven hundred births is Down syndrome baby. Down syndrome is now more frequently referred to as trisomy- 21, as new individuals with the disorder have three of the chromosome numbers twenty-one. Characteristics of a baby with Downs Syndrome include a small, round head with a large tongue and a broad skull; a short stature and stubby fingers; malformation of the heart, ears, hands and feet; and mental retardation. Sexual maturity is rarely attained.
The risk of having a child with Down syndrome increases sharply if a woman is reaching the end of her child-bearing period. For women aged over 45 or over the risk is around one in fifty, whereas for women in their middle reproductive years- around 20-35 years- the risk is only about one in a hundred.
Down syndrome can be detected in cells taken from the amniotic fluid of a pregnant woman, in a procedure called amniocentesis. Women having children late in life are strongly advised to have such a test. If the condition is detected early in the pregnancy, the parents have the option of a therapeutic abortion, whereas such practices are legal.
About two in every thousand babies born in Australia have a neural tube defect and about half of these have spina bifida. Spina bifida occurs when the vertebrae of the spinal column fail to form a complete bony arch around the spinal cord. In about fifty percent of cases it is the lower back region of the spine that is affected.
In serious cases, the coverings of the spinal cord and the wall of the spinal cavity itself may protrude, resulting in the opening of the neural canal to the exterior. Surgery to repair this is condition often results in paralysis of the lower limbs and associated lack of control of bowel and bladder functions. In less severe cases the spinal deformity may be small, and when it is repaired the child is able to walk and function normally.
The incidence of spina bifida varies considerably between countries. Researchers have been led to believe that there is a strong environmental influence in spina bifida. In recent years a link has been established between inadequate intake of folic acid before and during pregnancy and the incidence of neural tube defects like spina bifida.
A genetic link to spina bifida is also likely. All women who have a history of neural defects or spina bifida in the family are at an increased risk of themselves producing a baby with a neural tube defect. However, over ninety five percent of neural tube defects occur in families where there has never been an infected child.
In some cases, the factor with the greater effect can be identified; whether it be environmental factors or heredity, but never the factor that is most important. Many of the characteristics displayed by humans are due to the interaction of both heredity and environment. The height to which an individual grows for example, is a case where heredity plays a role in the determination of the end result.
If a person has tall parents, it is more likely that he or she too, will also be tall. In addition, growth is affected by environmental factors like nutrition. The question of that of the two factors- heredity or environment is the more important has no simple answer. .