The genetic and the operative evidence relating to secondary sexual characters

by Thomas Hunt Morgan

(2) In the F₂ generation the F₁ type is not as frequent as would be expected on the view that the heterozygotes could not be distinguished.

E. Back-Cross of F₁ ♀ to Sebright ♂.

It is possible to add, now, while this paper is passing through the press (June 1919), the results of a back-cross of 4 F₁ females to a Sebright male carried out during the summer of 1918. The birds being now mature their permanent colors are evident. Making the back-cross in this direction is much less advantageous than the reciprocal described above, because the Sebright contains most of the dominant color factors. The group of birds obtained appeared to be less variable in color than those from the other back-cross, and one can see at a glance that more of them approach the Sebright type; some quite closely.

All of the males are hen-feathered, as expected. No evidence was found that two types of males exist, which would have been expected if the two types noted in F₂ had any hereditary significance. If, then, as the F₂ results suggest, two factors for hen-feathering are present both are dominant, and no genetic distinction is found between individuals in which one or both of the dominant factors are duplex or simplex.

There were 9 adult hens and 10 hen-feathered cocks. An attempt is made below to refer them to their corresponding F₂ classes.

F. Review of the Heredity of the Color of the Plumage of Poultry.

In poultry there are perhaps more different colors and color-patterns than in any other species of domesticated animals. The genetic work has advanced far enough to show that many of the differences depend on Mendelian factors. It is probable that, in addition to the main factors, there are many contributory, minor, or modifying factors that give the finer details to “show birds.”

It is generally supposed that the wild bird from which some at least of the domesticated races have come is Gallus bankiva of India and Indo-China, or else one or another of its subspecies. In any case, the wild type of coloration is approximately known, since the known wild races are colored alike in all essential respects. Even were the color of the wild type not known, the original plumage could be deduced with some degree of probability from the atavism that appears when some of the races are hybridized. It is interesting to find that many of the new plumage characters are dominant to the wild type. The same relation also holds rather generally for other characters of poultry, such as the comb, etc.

Amongst the uniform or single-colored races, the whites, blacks, reds, and buffs have been studied. Bateson and Punnett were the first to show that the white of the White Leghorn is dominant. They also showed that the white of the White Rose Comb bantams is recessive. Another white, that of the White Silky, is also recessive, but due to a different factor from the white factor of the Rose Comb bantams; for, when these two whites are bred together they give colored birds in the first generation. Hurst showed later that the white of the Leghorn is dominant over the black of the Hamburg and the buff of the Cochin. The dominance is often not complete, since tints of black or of buff or even patches of these colors may occur. The latter may be confined to the head, neck, and breast. The black plumage of the Hamburg is dominant over the buff of the Cochins, but incompletely so, as the black background may be marked and shaded with brown. Whether we are dealing here with one pair of factors, or two pairs, could only be determined by an F₂ ratio; whether it is 3:1 or 9:3:3:1.

The blue color of the Andalusian is known not to be a simple color, but to be a fine mosaic of splashed white and black. The color is produced in birds that are heterozygous for splashed white and black, or at least for certain kinds of white and black. This relation was first demonstrated by Bateson and Punnett (1902 and 1905) and later Saunders (1906). It appears also from certain crosses made by Davenport that some of the whites (such as that of the Leghorn) and black (such as that of the Minorca) may at times also give some blue birds when crossed. Whether there are also other races with dominant white color different from that of the Andalusian white (and the same holds for black races also) or whether a special (recessive) white was present in this cross when the blue appeared, was not made out by Davenport.