Sperm. Egg. { Large [Male] "idiochromosome"} I. { or } + Large [Female] sex chromosome = a [Female] { Odd chromosome. }
II. { Small [Female] "idiochromosome"} { or } + Large [Male] sex chromosome = a [Male] { No sex chromosome }
Here we know that such a combination of gametes must occur to give the observed results, but we are not certain that we have a right to attribute the sex characters to these particular chromosomes or in fact to any chromosomes. It seems, however, a reasonable assumption in accordance with the observed conditions. The scheme also assumes either selective fertilization or, what amounts to the same thing, infertility of gametic unions where like sex chromosomes are present. It also assumes that the large female sex chromosome is dominant in the presence of the male sex chromosome, and that the male sex chromosome is dominant in the presence of the small female sex chromosome. Or, it might rather be said that these are not really assumptions, but inferences as to what must be true if the heterochromosomes are sex chromosomes. This theory of sex determination brings the facts observed in regard to the heterochromosomes under Castle's modification of Mendel's Law of Heredity ('99).
The question of dominance is a difficult one, especially in parthenogenetic eggs and eggs which are distinctly male or female before fertilization. It may be possible that the sex character of the egg after maturation is always dominant in the fertilized egg, as appears to be the case in these insects (see scheme). Conditions external to the chromosomes may determine in certain cases, such as Dinophilus, which sex character shall dominate in the growing oöcyte, and maturation occur accordingly. It is evident that this reasoning would lead to the conclusion that sex is or may be determined in the egg before fertilization, and that selective fertilization, or infertility of gametic unions containing like sex characters, has to do, not with actual sex determination, but with suitable distribution of the sex characters to future generations. If both sex characters are present in parthenogenetic eggs, as appears to be the case in aphids and phylloxera, dominance of one or the other must be determined by conditions external to the chromosomes, for we have both sexes at different points in the same line of descent without either reduction or fertilization.