as any
meaning here.

[Illustration: FIG. 56. Group IV. (See text.)]

The other figure (b) represents "bent", so called from the shape of the
wings. This mutant is likewise very variable, often indistinguishable from
the wild type, yet when well developed strikingly different from any other
mutant.

This brief account of a few of the mutant races that can be most easily
represented by uncolored figures will serve to show how all parts of the
body may change, some of the changes being so slight that they would be
overlooked except by an expert, others so great that in the character
affected the flies depart far from the original species.

_It is important to note that mutations in the first chromosome are not
limited to any part of the body nor do they affect more frequently a
particular part. The same statement holds equally for all of the other
chromosomes. In fact, since each factor may affect visibly several parts of
the body at the same time there are no grounds for expecting any special
relation between a given chromosome and special regions of the body. It can
not too insistently be urged that when we say a character is the product of
a particular factor we mean no more than that it is the most conspicuous
effect of the factor._

If, then, as these and other results to be described point to the
chromosomes as the bearers of the Mendelian factors, and if, as will be
shown presently, these factors have a definite location in the chromosomes
it is clear that the location of the factors in the chromosomes bears no
spatial relation to the location of the parts of the body to each other.

LOCALIZATION OF FACTORS IN THE CHROMOSOMES

_The Evidence from Sex Linked Inheritance_

When we follow the history of pairs of chromosomes we find that their
distribution in successive generations is paralleled by the inheritance of
Mendelian characters. This is best shown in the sex chromosomes (fig. 57).
In the female there are two of these chromosomes that we call the X
chromosomes; in the male there are also two but one differs from those of
the female in its shape, and in the fact that it carries none of the normal
allelomorphs of the mutant factors. It is called the Y chromosome.

The course followed by the sex chromosomes and that by the characters in
the case of sex linked inheritance are shown in the next diagram of
Drosophila illustrating a cross between a white eyed male and a red eyed
female.

[Illustration: FIG