Introduction
Human genetic diseases are due to any variation or mutation in the normal genes or chromosomes. These genes are passed down from the parents to the offspring and so human genetic diseases are hereditary. Some times the defected or mutated gene may not be expressed as it is recessive, in that case the disease doesn't develop. However, the gene may still be passed on to future generations in whom it develop expression. Heritable disorders are most common if the maternal age during conception is above thirty five years. One example of this is Down's syndrome, in which there is trisomy of chromosome 21, and is more common in infants of older mothers.
History
The genes can be mutated in several manners. Genes are made up of DNA and are present on the chromosome. A point mutation is when only a certain gene is defected. It may be deleted or inserted as an extra copy. Deletion may also be of a group of genes. If the whole chromosome is involved in such a way that a significant part of it is missing, is in excess, or both, that is known as chromosomal aberration. Trinucleotide repeat disorders are those in which the length of a gene is increased due to excess DNA. Human genetic diseases may be due to a single gene defect or multifactorial in which many genes are involved. A single gene disorder can be autosomal dominant or recessive. Normally there are two copies of the same gene. Autosomal dominant means even if one of those if mutated the disease will develop. Autosomal recessive disorders require both genes to be defective in order to be expressed. Sometimes the defective gene may be present on the sex chromosomes, X or Y, or in the mitochondria. When many genes are involved, the as in multifactorial or polygenic human genetic diseases, the expression and development of the disease also depends a lot on the environmental factors.
Features
Human genetic diseases affect the physical appearance, organ systems, IQ, mental well being and the quality of life of the person. Some examples of autosomal dominant human genetic diseases are Marfan's syndrome in which there is a disorder of the connective tissue and Huntington's disease which is neurodegenerative with cognitive and psychiatric abnormalities. Sickle-cell anemia, cystic fibrosis, and phenylketonuria are the most common examples of autosomal recessive diseases. Some common sex chromosome linked human genetic diseases are Rett syndrome, haemophilia, male infertility. Leber's hereditary optic neuropathy is a disease inherited through mitochondrial DNA from the mother. Multifactorial human genetic diseases tend of run in families. The most common of these are asthma, cancers, diabetes mellitus, inflammatory bowel disease, obesity and infertility. Hence some people are more prone to develop these than others.
Tips and comments
Individuals with heritable diseases often require special care and for those who have polygenic disorders running in their families it is necessary that they avoid triggers that may cause the disease to develop. Physical activity, balanced diet, and proper sleep goes a long way. Gene testing can be done to find out if someone is predisposed to cancer or not. In newborns a simple heel prick is required for detection of presence of abnormal genes and this can prevent a disabled future. Diet may need to be modified in genetic disorders that affect metabolism. Advancements are being made in order to prevent occurrence and to treat existing genetic disorders. Some of these are gene therapy, enzyme replacement therapy and screening of amniotic fluid of the older pregnant women.
