Mendel worked chiefly with peas, crossing different varieties. In his
methods of investigation he differed from all previous investigators in
concentrating his attention upon a single pair of alternative or
contrasted characters at a time, and observing how these alternative
characters are transmitted.
Thus when he crossed a tall with a dwarf, giving attention to this pair
of contrasted characters alone, he found that all the first hybrid
generation were talls, with no dwarfs and no intermediates. Accordingly
he called the tall character _dominant_, and the dwarf character
_recessive_, and a pair of contrasted characters which act in this way
are now called _factors_ or sometimes called _unit characters_. But on
allowing these hybrids to cross-fertilize one another in the usual way,
Mendel found that in the second generation of hybrids there were
_always_ _three talls to one dwarf_ out of every four. Further
experiments proved that these dwarfs of the second hybrid generation
_always bred true_, that is, one out of four; and that one out of the
remaining talls always bred true, making another quarter of the total;
while the remaining fifty per cent. proved to be mixed tails, always
acting as did the original hybrids, splitting up in the next generation
in the same arithmetical proportion as before.
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