one sex, _e.g._ the male, then these determinants would be modified in
association with the sex-chromosomes of that sex, and thus though
after reduction and fertilisation they would be present in the female
zygote also, they would not develop in that sex. Thus supposing _M_ to
represent a modification acquired in the male and _m_ the absence of
the modification, such as the feathered antenna of a moth, and the
sex-chromosomes to be _X_ and _Y_, then we should have in the
gametocytes--

Male Female

_MM mm_

_XX XY_

Gametes _MX, MX: mXmY_

Zygotes _MmXX male, MmXY female_,

and the character _M_ would only appear in the male because it only
develops in association with _XX_ in the somatic cells descended from the
male zygote. This would be the result in the first generation in which a
somatic modification affected the factors in the chromosomes. In the next
generation _m_ in the male would be affected, and the male for the sake of
simplicity might be supposed to become _MMXX_. When the female gametes
segregated, some would always be _mY_, and some zygotes therefore _MXmY_.
Others might be _MMXY_. On this theory, therefore, there would always be
some females heterozygous for the male character.

Geoffrey Smith, one of the many promising young scientific investigators