What are genetic disorders?
About one in 20 pregnancies results in a birth defect which may be a minor structural abnormality, mental retardation or a serious genetic disease. Genetic disorders are caused by defects in the genes or chromosomes.
What are genes and chromosomes and how do they develop defects?
Genes are hereditary material formed by segments of DNA present on thread-like structures called chromosomes. There are 23 pairs of chromosomes within the nucleus of each cell in the body. Sperm and egg cells however, have one set of 23 chromosomes each and are paired when they come together during fertilisation. Genetic disorders can arise during this process resulting in extra or missing chromosomes or damage to the chromosomes and genes. This can lead to physical, mental or metabolic defects.
What are the common genetic disorders?
Common genetic disorders include:
- Down syndrome, characterized by a flat facial profile, protruding tongue, poor muscle tone, slow growth and intellectual impairment.
- Cystic fibrosis, which affects the lining cells of internal organs such as the lungs, resulting in thick mucus and repeated infections.
- Tay-Sachs disease, which causes an absence of a protein in the brain affecting hearing, sight, movement and mental functions.
- Marfan syndrome, characterized by long fingers, arms and legs.
- Haemophilia, a condition that prevents blood clotting.
Down syndrome is caused by an extra chromosome. Conditions such as cystic fibrosis, sickle cell disease and Tay-Sachs disease occur only when faulty genes are inherited from both parents, termed recessive inheritance. Other conditions such as Marfan syndrome or achondroplasia require only one defective gene to manifest, termed dominant inheritance. Certain conditions such as colour blindness and haemophilia are linked to defects on the X chromosome and are transmitted from the mother to the son. Both genetic and environmental factors play a role in the expression of certain conditions such as cleft lip and palate or neural tube defects.
How are genetic disorders identified?
Genetic deformities can be detected early through genetic screening of the parents or through tests carried out during pregnancy. Genetic testing during pregnancy is usually performed in the first and second trimesters. It can involve testing the mother’s blood for certain disease markers or chromosomal deformities, ultrasound imaging techniques to view any abnormality in the growing foetus and amniocentesis to study foetal cells from a sample of amniotic fluid.