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The fertilisations are therefore one X red X red, one X red X white, one X red Y white, and one X white Y white. These last are the white-eyed males. The two different crosses are represented diagrammatically below, the dark rod representing the X red chromosome, the clear rod the X white chromosome, and the bent clear rod the Y white chromosome.
When the division in the gametes of the recessive individual takes place its gametes all contain the recessive character. Thus, if we indicate the dominant character by D and the recessive by d, the constitution of the two individuals is Dd and dd. The gametes they produce are D+d and d+d, and the fertilisations are therefore Dd, Dd, dd, dd,
The fertilisations are thus XX which develops into a female fly, and XY which develops into a male. Drosophila therefore is an example of one of the cases described by Wilson. Dr. He remarks that the X chromosome must be a male-determining factor since in many cases it is the only sex-chromosome in the males, yet its introduction into the egg establishes the female condition.
If we represent the character of femaleness by F and maleness or the recessive by f, we have the ordinary sexual union represented by Ffxff; the gametes will then be F+f and f+f and the fertilisations Ff and ff, or males and females in equal numbers, as they are, at least approximately, in fact.
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