In this way, for the first time the link between a strictly defined
chromosome and a given hereditary feature has been shown. Besides, it
was clarified why male and female individuals are normally appearing in a
ratio approximately 1:1.
The problem of sex determination has been provoking a great interest
but it is not sufficiently clear up to day. The presented scheme is not
applicable to all species even within one and the same genus. Most often
supersex individuals appear displaying three X-chromosomes (XXX) or
intersex ones with a triploid (3n) instead of diploid (2n) chromosome
number. The intersex ones also called gynandromorphs (Fig. 2–15) are
characterized by that one half of them bears the features of the male sex
while the other half the ones of the female sex. In many plants and animals
the diferentiated sex is replaced by hermaphroditism, i.e. the combination
of the male and female sex in one organism. Grasshoppers, bugs, some
butterflies are deprived of the Y-chromosome, the heterogamete male or
female sex being (XO) and nevertheless, in some cases the ratio between
male and female individuals is roughly 1:1.
image a
image b
image c
image d
Figure 2–15. Gynandromorphs (intersexuals) in Drosophila melanogaster
A — first division of the egg; the elimination of the X-chromosome is
shown. B — a gynandromorph formed after the elimination; the left side
corresponds to the female sex (XX) and the right one — to the male sex
(XO); C — the head of the fly; the elimination of the X-chromosome in
the course of one of the last somatic mitosis has led to the emergence of
a red spot in the eye (After De Robertis et al., 1973). D — a bilateral
gynandromorph. The zygote had two X-chromosomes and should have
developed into a female. It was heterozygotic for the gene for white eyes
(w) and for shorter wings (m) with the structure ++/wm. In the first
division the X-chromosome with genes + and + has been eliminated in
one of the blastomers. As a result the left side of the fly has formed as a
male displaying mutant properties. The right side had two X-chromosomes
and has developed into a female (After Dubinin, 1976)