minant mutation.

The preceding cases have all involved rather great changes in some one
organ of the body. The following three cases involve slight changes, and
yet follow the same laws of inheritance as do the larger changes.

[Illustration: FIG. 32. Mutant race of fruit fly, called speck. There is a
minute black speck at base of wing.]

At the base of the wings a minute black speck appeared (fig. 32). It was
found to be a Mendelian character. In another case the spines on the thorax
became forked or kinky (fig. 52b). This stock breeds true, and the
character is inherited in strictly Mendelian fashion.

[Illustration: FIG. 33. Mutant race of fruit fly called club. The wings
often remain unexpanded and two bristles present in wild fly (b) are absent
on side of thorax (c).]

In a certain stock a number of flies appeared in which the wing pads did
not expand (fig. 33). It was found that this peculiarity is shown in only
about twenty per cent of the individuals supposed to inherit it. Later it
was found that this stock lacked two bristles on the sides of the thorax.
By means of this knowledge the heredity of the character was easily
determined. It appears that while the expansion of the wing pads fails to
occur once in five times--probably because it is an environmental effect
peculiar to this stock,--yet the minute difference of the presence or
absence of the two lateral bristles is a constant feature of the flies that
carry this particular factor.

In the preceding cases I have spoken as though a factor influenced only one
part of the body. It would have been more accurate to have stated that the
_chief_ effect of the factor was observed in a particular part of the body.
Most students of genetics realize that a factor difference usually affects
more than a single character. For example, a mutant stock called
rudimentary wings has as its principle characteristic very short wings
(fig. 34). But the factor for rudimentary wings also produces other effects
as well. The females are almost completely sterile, while the males are
fertile. The viability of the stock is poor. When flies with rudimentary
wings are put into competition with wild flies relatively few of the
rudimentary flies come through, especially if the culture is crowded. The
hind legs are also shortened. All of these effects are the results of a
single factor-difference.

[Illustration: FIG. 34. Mutant race of fruit fly, called rudimentary.]

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