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  Section: Genetics » Mendel's Laws of Inheritance
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Mendel's results, chromosome theory and linkage (present status)

Mendel's Laws of Inheritance
Gregor Mendel's life
Pre-Mendelian experiments
Mendel's experiments
Symbols and terminology
Principle of segregation (law of purity of gametes)
Principle of independent assortment
Mendel's results, chromosome theory and linkage
Molecular basis of Mendel's wrinkled seed character
The rules of probability (product rule and sum rule)
Mendelian genetics in humans
Deviations from Mendel's finding

Mendelian factors (later called 'genes') are located on chromosomes and that genes located on same chromosome, unless separated by long distances, will not exhibit independent assortment (in other words these will be linked). In view of this we can re-examine the seven pairs of genes used by Mendel and analyse, whether or not they exhibited independent assortment in all possible 21 combinations, when arranged in pairs. This is necessary, because often a parallelism is drawn between seven pairs of characters used by Mendel and the seven pairs of chromosomes found in pea. This is also used as an argument, albeit erroneously, for independent assortment observed by Mendel, by saying that had he taken more than seven characters, he would have had problems due to linkage. In this connection, we can examine the relationship between different characters with their standard gene symbols and the chromosomes on which each is located.
In his original paper, Mendel gave results of only two experiments showing independent assortment. One of them involved two characters (R-r, I-i) and the other involved three characters (R-r, I-i, A-a), such that they represented demonstration of independent assortment among only three of the seven characters in three possible combinations (out of a total of 21 combinations possible among seven characters). For other character combinations, experiments were conducted, but no results are available in Mendel's original paper. However, he claimed that independent assortment was available in all other combinations also.

Extensive linkage studies later conducted by H. Lamprecht (1961), however, demonstrated that seven genes used by Mendel belonged to only four linkage groups. It was also shown that the combinations i-a, v-fa and fa-le despite each belonging to same linkage group are not linked (due to separation by long distances) and exhibit independent assortment. However, a solitary combination, v-le showed linkage (13% recombination) suggesting lack of independent assortment. Therefore, it is obvious that either Mendel did not conduct an experiment for independent assortment between v and le or else overlooked the lack of independent assortment in this combination.


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