PRETEND GENES 39Mendel went on to do other experiments of breeding and counting
in F2 generations using crosses and backcrosses. Of course, we now know
that off spring inherit two forms (or alleles) for each gene, one from each
parent. One form tends to be dominant over the other (recessive) form,
and the pairs of alleles may be identical or diff er ent. Taken together, those
conclusions helped explain the peculiar phenotypic ratios and established
the basic princi ples of Mendelian ge ne tics. Note that the environment did
not enter into consideration in Mendel’s experiments, because, above a
very basic threshold common to all plants, it did not make any diff erence
to the result.
NON- MENDELIAN TRAITSSuch Mendelian traits have been identifi ed in humans. Th ink of eye color;
hair color; blood type; having (or not having) a widow’s peak; being color
blind; and some diseases, such as Huntington’s disease or muscular dys-
trophy. It was eventually realized, however, that most traits are not like
those specially selected by Mendel, with their phenotypes falling into neat
categories (and even those just mentioned are now known to be infl u-
enced by other factors).
For example, in 1908, Swedish plant breeder Herman Nilsson- Ehle
worked with the red and white kernels of wheat seeds. Instead of off spring
falling into one or other of the same discrete (red or white) categories, he
found that they exhibited a range of intermediate colors, from deep red
to pure white. Th e ratios of these hues did not support the idea of single
genes for each trait. Instead, they suggested that variants (or alleles) of
three genes were involved as if their diff er ent eff ects just added together
in diff er ent proportions. It was subsequently found that environmental
factors also infl uenced color, so that in practice a continuous range of
hues could be observed.
In fact, it was already obvious that most bodily traits and functions
are, like height and weight, expressed in continuous values, not discrete
ones. And far greater (usually unknown) numbers of variant genes are
involved. Trait variation is more obviously aff ected by the environment,
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