The second are chiefly due to retrograde modifications, and
will be the subject of the next lecture.
[276]
LECTURE X
MENDEL'S LAW OF BALANCED CROSSES
In the scientific study of the result of crosses, the most essential
point is the distinction of the several characters of the parents in
their combination in the hybrids and their offspring. From a theoretical
point of view it would be best to choose parents which would differ only
in a single point. The behavior of the differentiating character might
then easily be seen.
Unfortunately, such simple cases do not readily occur. Most species, and
even many elementary species are distinguished by more than one quality.
Varieties deviating only in one unit-character from the species, are
more common. But a closer inspection often reveals some secondary
characters which may be overlooked in comparative or descriptive
studies, but which reassume their importance in experimental crossings.
In a former lecture we have dealt with the qualities which must be
considered as being due to the acquisition of new characters. If we
[277] compare the new form in this case with the type from which it has
originated, it may be seen that the new character does not find its
mate, or its opposite, and it will be unpaired in the hybrid.
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