It is caused by changes or mutations that occur in the DNA sequence of a single gene, also called Mendelian disorder. There are more than 6,000 known single- gene disorders, which occur in about 1 out of every 200 births.
Some examples of monogenetic disorders include:
Cystic Fibrosis, Sickle Cell Anaemia, Marfan syndrome, Huntington’s disease, and Hemochromatosis. Single-gene disorders are inherited in recognisable patterns: autosomal dominant, autosomal recessive, and X-linked.
Example: Sickle cell anaemia is a disease passed down through families in which red blood cells form an abnormal crescent shape (Red blood cells are normally shaped like a disc.) Sickle cell anaemia is caused by an abnormal type of haemoglobin called haemoglobin S. Haemoglobin is a protein inside red blood cells that carries oxygen. Haemoglobin S changes the shape of red blood cells, especially when the cells are exposed to low oxygen levels. Then the red blood cells become crescent shaped or sickles. The sickling occurs because of a mutation in the haemoglobin gene. The haemoglobin beta(HBB) gene is found in region 15.5 on the short (p) arm of human chromosome 11. In sickle cell haemoglobin (HbS) the glutamic acid in position 6 is mutated to valine in a beta chain. This change allows the deoxygenated form of the haemoglobin to stick to itself and
become crescent shape.
The fragile, sickle shaped cells deliver less oxygen to the body’s tissues. They can also get stuck more easily in small blood vessels, and break into pieces that interrupt healthy blood flow.
It is called complex or polygenic disease. Complex diseases are caused by a interaction of environmental factors and mutations in multiple genes. Some common chronic diseases are multifactorial in nature. Examples of complex diseases include:
Cardio Vascular diseases, high blood pressure, Alzheimer’s disease, arthritis, diabetes, cancer, and obesity.
Lethal gene: Gene which causes death of individual in early stage when it comes in homozygous condition called lethal gene. Lethal gane may be dominant or recessive both.
Semilethal gene:….. Click here to continue studying