(4) _Unit-characters_ of which some, _when once arisen by Variation_,
are alternative to each other in the constitution of the gametes,
according to a definite system.
From the relations subsisting between these characters, it follows that
as each zygotic union of allelomorphs is _resolved_ on the formation
of the gametes, no zygote can give rise to gametes collectively
representing more than _two_ characters allelomorphic to each other,
apart from new variation.
From the fact of the existence of the interchangeable characters we
must, for purposes of treatment, and to complete the possibilities,
necessarily form the conception of an _irresoluble base_, though
whether such a conception has any objective reality we have no means as
yet of determining.
We have now seen that when the varieties _A_ and _B_ are crossed
together, the heterozygote, _AB_, produces gametes bearing the pure
_A_ character and the pure _B_ character. In such a case we speak of
such characters as _simple_ allelomorphs. In many cases however a more
complex phenomenon happens. The character brought in on fertilisation
by one or other parent may be of such a nature that when the zygote,
_AB_, forms its gametes, these are not individually bearers merely
of _A_ and _B_, _but of a number of characters themselves again
integral_, which in, say _A_, behaved as one character so long as its
gametes united in fertilisation with others like themselves, but on
cross-fertilisation are resolved and redistributed among the gametes
produced by the cross-bred zygote.
In such a case we call the character _A_ a _compound_ allelomorph,
and we can speak of the integral characters which constitute it as
_hypallelomorphs_. We ought to write the heterozygote (_A A′ A″_ ...)
_B_ and the gametes produced by it may be of the form _A_, _A′_, _A″_,
_A‴_,... _B_. Or the resolution may be incomplete in various degrees,
as we already suspect from certain instances; in which case we may have
gametes _A_, _A′ A″_, _A‴ A″″_, _A′ A″ A^v_,... _B_, and so on. Each
of these may meet a similar or a dissimilar gamete in fertilisation,
forming either a homozygote, or a heterozygote with its distinct
properties.
In the case of compound allelomorphs we know as yet nothing of the
statistical relations of the several gametes.