Mendel, while enunciating, his law of independent assortment, ascertained that two or more elements, that constitute a hybrid, undergoes independent segregation without being influenced by the other.
Mendel’s principles of independent assortment apply not only to genes but to chromosomes as well, so that paternal and maternal chromosomes are segregated to separate gametes during meiosis.
It is for this reason that genes carried on different chromosomes undergo complete segregation. Fortunately for Mendel, all the seven characters that he chose for study were on different chromosomes (there are seven pairs of chromosomes in pea plants).
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There are hundreds of characters in organisms and if there is a gene for every character as Mendel visualized (which is a fact also) there must be hundreds and indeed thousands of genes, and if genes are situated on the chromosomes there must be hundreds or thousands of chromosomes.
But this is not so; while there could be thousands of genes, the number of chromosomes in most of the individuals hardly exceeds hundred; in fact it is much less.
In Drosophila, where hundreds of characters have been studied there are only four pairs of chromosomes. In human beings with thousands of characters, there are only 23 pairs of chromosomes.
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If genes are situated on the chromosomes, it means then several genes responsible for so many characters must be situated on a single chromosome.
When this is so, genes or characters on a single chromosome cannot segregate. They cannot be assorted independently instead they tend to be inherited together.
It was Sutton (1903) who first pointed out the anomalous situation in believing total independent assortment for all the characters, when the number of chromosomes is far less than the number of genes.
For e.g.: if we identify two genes located on a chromosome and perform a dihybrid cross, the chances are the two genes will tend to go together, instead of being assorted independently.
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The two traits controlled by these will appear together in an individual, instead of segregating to two individuals. In a dihybrid cross of this type, Mendel’s 9:3:3:1 ratio will be modified.
This phenomenon of genes (present on a single chromosome) showing the tendency to be inherited together is called linkage.
However until Morgan’s work in 1910, Genetics did not have a satisfactory explanation for the tendency of the genes to assort together in the offspring. We shall consider some of the hypotheses concerning the above mentioned behaviour of genes.
As has been pointed out already, Sutton (1903) opined that several genes are present on the chromosome and they tend to be inherited together. However he did not visualize the concept of linkage.
Instead he said that there are groups of genes, and these groups in an individual correspond to the number of chromosomes. Sutton did not experimentally confirm his views.