38 PRETEND GENESblended in the meeting of eggs and sperm. Th at meant that most varia-
tion would be continuous in nature, like height and weight.
Subsequent discoveries in ge ne tics, from Gregor Mendel’s experiments
on pea plants in the 1860s to others around 1915, suggested other wise.
Th ey indicated that the hereditary material passed across generations as
particles rather than in “plasm” or quasi- liquid form. At fi rst, too, those
studies appeared to implicate one gene governing the development of
each bodily trait. For example, diff er ent versions of one gene would de-
termine whether eyes were blue or brown; those of another gene would
determine whether hair was dark or blonde; and so on. But things turned
out to be much more complicated.
Mendel had chosen to study pea plants for special reasons. Th ey pres-
ent a number of traits with categorically distinct variations in which the
frequencies of each variety can easily be counted. Moreover, pea seeds
were readily available from merchants and they reproduce quickly with
great fecundity. More importantly, the plants can be experimentally
self- pollinated or cross- pollinated across or within categories. In fact,
Mendel spent the fi rst two years of his study establishing pure lines that
faithfully reproduced with the chosen traits. For example, there were
purple fl owers or white fl owers, smooth or wrinkled seeds, green or yellow
pods, or long or short stems. Th ese were the diff er ent phenotypes (Greek
for “the form that is shown”) showing phenotypic variation.
Th is is what Mendel found. Crossing purple fl owers with white fl ow-
ers produced off spring with only purple fl owers: there was no blending,
or production of intermediate colors. However, when the fl owers of the
fi rst generation (F1) were allowed to self ( were self- fertilized), they pro-
duced off spring (the F2 generation) with a mixture of purple and white
fl owers in the ratio of 3:1. Breeding with the other phenotypes produced
closely similar results, with 3:1 ratios in the F2.
Th us was posed the obvious question: how can it be that the one (the
recessive) phenotype appeared to be totally eclipsed by the other (the
dominant) phenotype in the F1 generation, but then reappear in the F2?
Although all the F1 fl owers were purple, the plants evidently still carried
the potential to produce white fl owers. Th e theory of blending inheritance
cannot explain such results.
This content downloaded from 139.184.14.159 on Tue, 17 Oct 2017 13:51:26 UTChttp://www.ebook3000.com