(i) Multiple Alleles : Most genes occur in two alternative forms, both controlling the same character and occupy the same locus in homologous chromosomes. These different form of the same gene are called alleles. However, some genes may occur in more than two allelic forms and they are called multiple alleles. A set of such multiple alleles may contain 3 to 20 or even more members which occupy the same locus in homologous chromosomes. In such a set of multiple alleles one member is always dominant and one recessive to all others. An individual carry only two such alleles e.g., ABO blood groups.

(ii) Artificial method to control population : 

The unreliable the natural methods of contraception and are replaced by artificial methods :

(a) Contraceptive pills : Birth control pills which contain hormones, prevent a woman from getting pregnant if used regularly. Contraceptive pills also help women to have regular menstrual cycles and reduce the chances of anemia.

(b) Barrier contraception : Most popular barriers are the condom and the diaphragm that keep the sperm coming in contact with eggs in female reproductive system.

(c) Intra uterine device (IUD) : It is small device placed in the uterus to prevent pregnancy. Once the medicated IUD is in place, it can provide birth control for 5 to 10 years.

(d) Injection for birth control: Vaccination is another method of birth control. This vaccine is effective for three months and has to be applied four times a year.