We all have studied in our junior standards about cell. Cell is the the structural and functional unit of life, from them the origin of life takes place. The cell contains a nucleus which contains th genetic material for the survival of the organism DNA. DNA contains the chromosomes. There are .any several genes present on the chromosomes. As the chromosomes are the carriers of heredity they have the tendency to be inherited together, such genes are called linked genes. The tendency of two or more genes present on the same chromosomes that are inherited together is known as linkage. This phenomenon was discovered in plants by Bateson and Punnett and in animals by T.H. Morgan.
Morgan used Drosophila melanogaster for his experiments. He carried out many dihybrid cross experiment on drosophila melanogaster to study genes that are sex linked; for example, he and his group crossed yellow-bodied, white eyed female to the wild type with brown-bodied, red eyed males and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and F2 ratio deviated very significantly from 9:3:3:1 ratio. Morgan and his group knew that the genes are located on X chromosome and stated that when when two genes in a dihybrid cross are situated on the same chromosomes then the proportion of the parental combination is much higher than non parental type. This occurs due to physical association or linkage of the two genes, he also found out when genes are grouped on the same chromosomes some genes are strongly linked and some are loosely linked.
The linked genes which are closely located on the chromosome do not separate and inherited together are called completely linked genes and the inheritance is called complete linkage.
When the linked genes which are loosely located on the same chromosome and have chances of seperation by crossing over are called as incompletely linked and their inheritance is called as incomplete linkage.