The Inheritance of Horns and Face Colour in Sheep

Extract

1. As far as the characters under observation are concerned it is immaterial which way the cross is made. Reciprocally bred first crosses are identical.

2. The inheritance of horns is closely connected with sex. Large horns are dominant in the male, recessive in the female.

3. The meaning of scurs is not yet settled. Two kinds of scurs were observed, small round firmly attached knobs and thin loose scurs. The fact, which unfortunately was not observed until the later stages of the experiment, that the appearance of scurs is sometimes delayed until the animal is two years old, has given rise to an additional complication.

4. A horned ram may be either pure horned or heterozygous as regards that character. His purity can readily be tested by mating with a number of horned ewes. If all his ram lambs are horued he is presumably pure, if any of them are hornless he is heterozygous.

5. A hornless ram must be pure hornless. His purity can be tested by mating with a number of pure hornless ewes, when all the progeny are found to be hornless.

6. A horned ewe must be pure horned. Her purity can be tested by mating her with a pure horned ram. All the ram lambs produced will be horned, for horns are dominant in the male. All the ewe lambs should be horned if she is pure. It may be several years before she bears enough ewe lambs to enable the experimenter to state with anything like certainty that she breeds true to horns. It is here that the chief difficulty of working with large animals on Mendelian lines is found. The females produce only one or two young in the year, so that several years must elapse before a female can be thoroughly tested.

7. A hornless ewe may be either pure hornless or heterozygous. She can be tested by mating with a hornless ram. The same difficulty again arises, in fact it must always arise in the case of testing slow breeding animals. The males are readily tested, but the testing of the females is so slow that it must often be uncertain. This is the explanation of the common and very true statement that the way to improve a flock is to use good males. Males are readily tested and their purity as regards desirable characters is therefore very soon assured. Several generations may have been bred from a female, and her blood diffused through the flock, before the breeder can be sure that she breeds true to the type he wants.

8. The occasional occurrence of scurs in Suffolks already referred to is probably explained by the dominance of the hornless condition in the female. A hornless ewe may be heterozygous. This can only be found out by a breeding test, and may easily be overlooked in practice. Her progeny would then mix with the flock, and a small proportion of their ram lambs would produce scurs.

9. There is no dominance of white face over black or vice versd. The first cross as regards face colour is intermediate between the two parental types. Pure white and black faces segregate in the second generation. The black face is not a simple character, since the number of speckled faces in Fa is far too large, and the speckled faces include several distinct types of pattern.

10. Woolly and bare heads appear to be a pair of characters which blend in the first cross but segregate again in later generations.

11. A number of striking instances of recombination have beenobserved. For instance, horns, woolly poll and face, and black face are combined in the ewe, Plate XII, Fig. 15. She has been shown to breed true to horns, but her purity as regards woolliness and blackness of face has not as yet been tested. Another example is the ram shown in Plate XII, Fig. 18, which combines the bare head and hornless character of the Suffolks with the white face of the Dorsets.

12. Finally attention should once more be drawn to the difficulties of experimental breeding with large animals. The slowness and lack of certainty in testing the females, and the troubles arising therefrom, have already been dilated upon. Another difficulty is the complicated nature of what might have been hoped to be simple characters. Points of economic importance such as would be likely to appeal to the butcher, the dealer or the wool merchant, are hardly likely to turn out less complicated than horns or face colour. The experiments described above have suffered greatly from the fact that it was impossible with the comparatively small area available to keep more than a very small proportion of the rams until they were old enough to show all their characters. The unsatisfactory state of the evidence given above as to the question of scurs is in part due to this. It is however a difficulty which would disappear with increased resources.