Genetic linkage map of Mendel's seven characters in Garden Peas (Pisum sativum)
Mendel worked with seven
characters, which he assigned to seven elementen
(genes). Pisum has seven pairs of
chromosomes.
It is highly improbable that seven randomly-chosen genes would
each occur on a separate chromosome. Dihybrid crosses
involving two elementen on the same chromosome would
have shown linkage
and a departure from the expected 9:3:3:1 ratio, which
would have been unexplainable by Mendel's rules. [Remember that chromosomes had not
yet been observed in the 1860s]. Therefore, it is concluded, he
must have thrown out some dihybrid results that didn't fit his
expectations.
In fact, linkage analysis shows that the seven genes map to only six chromosomes, and those that are on the same chromosome (three on III: Fas, Le, & V; two on V: R & Gp) are not close enough to show linkage disequilibrium. As there are (7)(7-1)/2 = 21 possible dihybrid crosses, and Mendel reported results from only a few of these (not including the linked pair), there is no basis on which to doubt the accuracy and completeness of his dihybrid results.
In fact, linkage analysis shows that the seven genes map to only six chromosomes, and those that are on the same chromosome (three on III: Fas, Le, & V; two on V: R & Gp) are not close enough to show linkage disequilibrium. As there are (7)(7-1)/2 = 21 possible dihybrid crosses, and Mendel reported results from only a few of these (not including the linked pair), there is no basis on which to doubt the accuracy and completeness of his dihybrid results.