Sex Chromosomes and Sex-linked Inheritance

The first sound indication for a connection between chromosomes and sex came from H. v. HENKING (1891). He worked on spermatogenesis and its association with the egg development in the hemiptere Pyrrhocoris apterus and found out that half of the daughter cells contained one element more than the other during anaphase II. He was not sure whether it was an additional chromosome and thus called the structure X.

Similar observations were made in other insects and in 1902, C. E. McCLUNG postulated that the X-element was connected to sex determination. The proof came in 1905 from E. B. WILSON (Columbia University, New York), who counted 13 (= 2n) chromosomes in the male Protenor belfragei and 14 in the female and could assign one X-chromosome to the male and two to the female. He coined the term X-chromosome.

In 1909, he detected that the males of several other insect genera (Lygaeus, Tenebrio, Drosophila) possess a much smaller, differently shaped Y-chromosome in addition to the X-chromosome so that the genotype of the males is XY. Since X- and Y-chromosome differ structurally (and therefore cannot pair at meiosis), is it spoken of heterochromosomes in contrast to all other chromosomes, the autosomes.

A year later, T.H.MORGAN found a Drosophila-mutant with white instead of red eyes. The inheritance of the characteristic 'white eyes' (white,w) is sex-linked and could already in 1911 be correlated with the segregation of the sex chromosomes. The hereditary factor (the gene) for white is localized on the X-chromosome:

X^wX x XY > X^wX, X^wY, XX, XY or

XX x X^wY > XX^w, X^wX, XY, XY

The first sex-linked plant gene was found a year later, in 1912, by E. BAUR. He described a mutant of Silene alba (= Melandrium album) with narrow leaves and could prove that this feature is sex-linked.

The sex-linked inheritance of certain features became an important pillar of the chromosome theory. It has to be pointed out that features like 'white eyes' or 'narrow leaves' have a priori nothing to do with sex determination. The X- and the Y-chromosome are therefore not only responsible for the determination of the sexes but harbour also genes that exert their influence on features that are independent of the sex.

But how is the expression of the sex determined? No general answer exists. It was shown that the sex determination of Drosophila melanogaster and Silene alba (= Melandrium album, [White Campion]), is founded on different mechanisms and that the sex-determining genes are localized on different chromosomes. The sex determination of Melandrium album, for example, is decisively influenced by the Y-chromosome. The species is dioecious and the sex determination follows the classical scheme XX= female, XY= male. The Y-chromosome is very large. I. ONO , A. F. BLAKESLEE, H. WARMKE and M. WESTERGAARD (1948) analyzed the influence of the sex chromosomes on the autosomes in polyploid varieties.

At the same time, BRIDGES worked on Drosophila. His experimental data are collected in the following table.

The table shows that it is the balance between the determinants of the male sex and the female sex that settle the sex of Drosophila. The male determinants are located somewhere on the autosomes, the female on the X-chromosome. The situation is contrary in Melandrium album. Here, the presence of a Y-chromosome is decisive for the sex determination of the male. The Y-chromosome has in fact such a great influence on the expression of the male sex that it seems as if the autosomes and the X-chromosomes were of no importance at all. But in one case (number 22), it shows that even in Melandrium a balanced mechanism is at work. Obviously, the genes for the female sex are so weak that four X-chromosomes are necessary to compensate the influence of one Y-chromosome.

The condition in Drosophila and Melandrium should not lead to the conclusion that plants and animals differ completely in the way their sexes are determined. The sex determination of Rumex acetosa, for example, follows the Drosophila-scheme (I. ONO, 1935, Y. YAMAMOTO, 1938).