summarizing, by way of illustration.

Both sexes in Dorset sheep have well-developed horns; in the Suffolk
breed both sexes are hornless. If the breeds are crossed, all the rams
in the first (hybrid) generation have horns and all the ewes are
hornless. If these hybrids are mated, the resulting male offspring
averages three horned to one hornless; but the females are the reverse
of this ratio--one horned to three hornless. This is an example of
Mendel's principle of segregation--factors may be mixed in breeding, but
they do not lose their identity, and hence tend to be sorted out or
segregated again in succeeding generations.

In the horned Dorsets, we must suppose that both males and females carry
a dual factor for horns--technically, are _homozygous_ for horns. The
hornless Suffolks, on the contrary, are homozygous for _absence_ of
horns. Thus the dual factor in the zygotes or fertilized eggs at the
basis of the first filial (hybrid) generation consists of a single
factor for horns and a single factor for their absence. If we represent
horns by _H_ and absence of horns by _A_, Dorsets have a factor _HH_,
Suffolks _AA_ and the hybrids _HA_.

All the males in this generation have horns, which means that a single
"dose" of the factor _H_ will produce horns in a male, or that they are
_dominant_ in males. But a single dose will not produce horns in a
female--that is, horns are _recessive_ in females--the factor is present
but unexpressed.

Mating two _HA_ hybrids, the _H_ and _A_ of course split apart in the
formation of the gametes, as the _HH_ and _AA_ did in the previous
generation; so that we get an equal number of single _H_ and _A_