Latent traits of the opposite sex of course immediately suggest recessive or unexpressed characters in the well-known Mendelian inheritance phenomena. In the bird-castration cases, we saw that to remove the inhibiting sex glands caused previously latent characters to act like dominant or expressed ones. The case of horns in sheep, investigated by Professor Wood[16], is so similar that it seems worth 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_ factors. In reuniting in fertilized eggs, the chance is just half and half that an _H_ will unite with another _H_ or with an _A_--that an _A_ will unite with an _H_ or another _A_. Thus we have two chances of getting _HA_ to each chance of getting either _AA_ or _HH_. Half the zygotes will be _HA_, one-fourth _HH_ and one-fourth _AA_. If we consider four average males, one will have two _A's_ (absence of the factor for horns) and will thus be hornless. One will have two _H's_, or the double factor for horns, and hence will exhibit horns--as will also the two _HA's_ since a single dose of horns expresses them in a male. So we have the three-to-one Mendelian ratio.