Genes control all features of an organism. Some times a gene may change or mutate either in the gamete or zygote. Mutated gene may not remain normal. Also, sometimes a defective gene present in the parent may not be expressed in the parent as the dominant normal member of its pair may mask the effect of the defective gene.
But if the child inherits the defective gene from both the parents, the presence of the defective pair of genes has a harmful effect. Such a disorder is termed hereditary or genetic disorder.
There are several kinds of hereditary disorders, some of which may be caused due to presence of only one defective gene which is dominant or sometimes by the presence of two defective recessive genes.
Genetic disorders cannot be cured by medicines. Scientists are trying to discover methods by which a defective gene occurring in an individual may be removed or replaced by a normal gene. This is called gene replacement therapy.
There are several genetic (hereditary) disorders. Three common hereditary disorders are Thallasemia, Haemophilia and Colour blindness.
The genes for both haemophilia and colour-blindness are located on X-chromosome, and hence, the disorder is passed down from mother to the son because a boy receives the X chromosome from the mother and Y chromosome from the father. In the mother, with two X-chromosomes, the defect does not show up.
Also in the daughter, the effect of defective gene on X-chromosome inherited from mother may be masked by a normal gene on the X-chromosome, inherited from her father. Since X chromosome bears the defective gene, the son suffers from the genetic disorder, as male has only one X chromosome and one Y chromosome and so the defective gene does not get masked.
Patients suffering from this disorder are unable to manufacture haemoglobin, the pigment present in red blood corpuscles which carries oxygen to tissues. This is because the pair of genes controlling haemoglobin production are defective. Thallasemics (persons suffering from thallasemia) require frequent blood transfusion in order to survive. The thallasemia gene is present on an autosome.
Those persons suffering from haemophilia have either a defective gene or lack genes, which control production of substance responsible for blood clotting. In the absence of such substance blood does not coagulate. Once bleeding starts, it does not clot easily.
Different kinds of colour-blindness have been detected but in the most common form of the disorder, a person is unable to distinguish the blue colour from green. This is due to the presence of a defective gene or absence of the gene, responsible for colour vision.
Thallasemia is an autosomal genetic disorder, while, haemophilia and colour blindness are sex-chromosomal or X-chromosomal disorders.
Marriages between close relatives termed consanguineous marriages are discouraged. In marriages between relatives, chances of inheriting defective genes by the offspring is enhanced as both parents, being related may possess the same defective genes. If recessive, the two defective genes express when passed on to offspring by both parents. Therefore, it is essential to know the probability of a genetic defect in the offspring and seek the advice of genetic counsellors who are professionals.
Genetic counselling helps one to know the chances of inheritance of a genetic disorder so that people can make informed decisions.