Linkage
Linkage is defined as the tendency of two or more genes to remain together in the original combination in the same chromosome during the process of inheritance for a number of generations. All the genes on a chromosome are said to be linked to one another.
Linkage was discovered by T.H Morgan.
Linkage was found in Drosophila, sweet peas, maize, man, etc.
In Drosophila, the genes of body colour and nature of wings are linked and located on the same chromosome.
In sweet pea, the genes of flower colour and size of pollen grain are linked.
In maize, the genes for colour and shape of seeds are linked. In man, the genes for haemophilia and colour blindness are linked.
T.H Morgan(1911) proposed the theory of linkage based on his experiments in Drosophila. The theory of linkage explains the main principles of linkage. They are the following:
- A chromosome contains many genes.
- The genes are arranged in a linear fashion on the chromosomes.
- The genes present in a chromosome are linked.
- The linked genes are inherited together from the parents to the offspring.
- Linkage is a rare phenomenon when compared to crossing over.
- The strength of linkage depends on the distance between the linked genes. Closely located genes shows strong linkage. Widely located genes show weak linkage.
- Mendel's law of independent assortment is not universal.
- The linked genes do not show independent assortment. The genes located on different chromosome only show independent assortment.
- When dominant genes are located on one homologous chromosome and the recessive genes on the other homologous chromosome, the arrangement of linked genes is called cis arrangement. When the dominant and recessive genes are linked in one chromosome, the arrangement is known as trans arrangement.
- All the linked genes of a homologous pair of chromosome constitute one group called linkage group. The number of linkage group in an organism is equal to the number of pairs of chromosomes. Thus Drosophila has four linkage groups as it has four pairs of chromosomes. Pea plants has 7 linkage groups as it has 7 pairs of chromosomes. Man has twenty three linkage groups as he has twenty three pairs of chromosomes.
- Linkage is of two types, namely complete linkage and incomplete linkage. In complete linkage, the chromosomes do not break and the linked genes inherit together for many generations.