results would have been different.Genetic linkage, or the fact that genes on the same
chromosome tend to be inherited together, would have caused linked alleles and
corresponding traits to remain together rather than segregate independently. He did not
understand it at the time, but Mendel’s traits were each controlled by a single gene on
a completely different chromosome, which allowed them to segregate in the patterns
he observed.
There were numerous experiments on the crossing of different species or varieties of
plants during the eighteenth and nineteenth centuries; the primary intention was to
obtain new and improved varieties of fruits and vegetables. Knight (1799) and Goss
(1824) in the United Kingdom both worked on edible pea (Pisum sativum)—in fact,
made the same crosses as Mendel—and each observed the same general segregation pat-
terns, but did not record the numbers as did Mendel. Knight chose pea, because of its
short generation time, the numerous varieties available, and the self-fertilizing habit,
which obviated the need to protect flowers from insects carrying pollen. Presumably,
Mendel had the same goals and rationale.
Mendel’s laws (Bateson 1909) have served as the basis for all fields of genetics. Of
course, once DNA structure was described by Watson and Crick in 1953, the age of
modern genetics began (Watson and Crick 1953a, 1953b, Watson 1968). Even though
the mechanisms as to how DNA could store genetic information was not known,
Mendel’s principles still correctly described how genes were transferred between
generations. Mendel’s important work illustrates that comprehensive knowledge on a
Figure 2.6.A dihybrid crossing system involving a two-gene model where the alleles of two genes
independently assort from one another in the production of gametes: (a) dihybrid cross; (b) F 1 self-
fertilization.
2.2. MENDELIAN GENETICS 29