Red-eyed male . (WX)Y x (wX) (wX) white-eyed female Spermatozoa . WX.Y : wX . wX ova Red-eyed daughters . (WX)(wX) . (wX)Y white-eyed sons.
It is to be noted that during the process of the formation of the ova and spermatozoa the members of the pair of sex-chromosomes (as do all the rest) disjoin so that only one passes into each gamete.
In a particular sex-linked experiment Bridges got unexpected results : there were some red-eyed sons and white-eyed daughters. On cytological examination of such exceptional white-eyed females Bridges found that their cells displayed a Y-chromosome in addi tion to the normal pair of X's. This is what would occur if, during the maturation of the egg in which such an individual had had its origin, the X-chromosomes had failed to disjoin so that instead of coming to possess but one X it would contain two and if this non disjunctional egg had then been fertilised by a spermatozoon bearing a Y-chromosome. If a non-disjunctional female is used in experiments involving a sex-linked character such as white eye, the mode of inheritance of such characters will be obscured, as can be shown in the following scheme. She will elaborate four sorts of eggs instead of one. These can be fertilised either by the (WX)-bearing or by the V-bearing spermatozoon of the red eyed male.
The exceptional white-eyed daughters are white-eyed because they do not get one of their X-chromosomes from their red-eyed father: the exceptional red-eyed males are red-eyed because each gets his Y-chromosome from his mother and his X from his father.
The aberration in the chromosome distribution can explain the entire series of exceptional results. The evidence derived from breeding and from cytological investigation turned what seemed to be a direct contradiction of the chromosomal interpretation of sex-linked inheritance into a spectacular confirmation. The evidence confirms the conclusion that sex-linked characters are associated with the sex-determining mechanism because their fac tors are located in the sex-chromosomes and that sex is determined at the time of fertilisation by the mechanism. In
Drosophila certainly the Y-chromosome has no influence upon sex-determination, XXY and even XXYY individuals develop into females of normal appearance whilst X individuals lacking the Y develop into unexceptional looking males (but are sterile). XXX individuals either die or else grow up into sterile females and Y individuals without an X never appear—they must be non viable.
Gynandromorphism.—Another line of argument pointing to the same conclusion has been derived by Morgan and his as sociates from the study of gynandromorphism in Drosophila. A gynandromorph is an individual of a bisexual species which ex hibits a mosaic of male and female characters. Most specimens are lateral gynandromorphs, male on one side of the mid-line of the body, female on the other, with a sharp demarcation 'between the two kinds of tissue. In other instances one-quarter of the body is male, the other three-quarters female; in yet others the head is female and the rest of the body male. Morgan and Bridges (1919) have shown that if in the mating, which produces the gynandro morphs, sex-linked characters are involved and if the sex-linked characters of the two parents are dissimilar, then the sex-linked characters of the male parts are those either of the father or of the mother, whereas the sex-linked characters of the female parts are a combination of those of both parents. These facts point to the conclusion that gynandromorphism results from aberration in the distribution of the X-chromosomes. If it is assumed that the gynandromorph starts life as a female, XX in constitution, and that at some stage during the early divisions of the fertilised egg a daughter X-chromosome fails to enter one of the daughter cells, then this cell, unlike its sister, will contain one X instead of two, becoming XO instead of XX. It thus will come to possess the sex-chromosome constitution of male tissue. If the X-chromo some of paternal origin is lost the sex-linked characterisation of the male part of the gynandromorph will be like the mother's.