152 GENETICS IN RELATION TO AGRICULTURE 



the quantitative relations obtaining in the determination of dominance. 

 Apparently the relations are about the same as those shown in the case 

 of bar eye in Drosophila, for conceivably, if such a thing could be obtained, 

 an endosperm arising from an Ff cell might show the same variation 

 between flinty and floury that is shown in the bar-eye character of flies 

 of the genetic constitution (B'X] (b'X). 



Mosaic Expression of the Hybrid Character. Another type of hybrid 

 condition is that in which the Aa individuals are a mosaic of the char- 

 acters of the two parents. This condition is very strikingly illustrated 

 in Blue Andalusian fowls. Andalusian fowls are of three types: 

 black, splashed white, and the so-called blue. Of these types the black 

 and splashed white breed true, but the blue is a hybrid and constantly 

 segregates in the ratio 1 black : 2 blue : 1 splashed white. When 

 black and splashed white are mated, the progeny are all blue. The 

 Blue Andalusian fowl of the Poultry Standard of Perfection is, there- 

 fore, a heterozygous form and for that reason all attempts to establish 

 it as a pure breeding race have failed. The case, however, is of interest 

 here because the Blue Andalusian is a peculiar mosaic of the characters 

 exhibited by the black and splashed white. Its "blue" color is simply 

 due to a fine but uneven sprinkling of black pigment through the 

 feathers; and on some portions as for instance the feathers of the breast, 

 the black is present as a distinct edging or lacing of the feathers. 



Similar mosiac hybrids which represent a simple heterozygous con- 

 dition have been reported by Nabours in grouse locusts of the genus 

 Parattetix. Nabours found nine distinct races which bred true for 

 particular color patterns. Hybrids, however, between any two of these 

 species display the entire color pattern of both parents, the color patterns 

 being merely superimposed one upon another and in such a manner that 

 the entire pigmentation of both parents is present in the hybrid and is 

 distributed in the same fashion. If then two races of Parattetix A and 

 B be crossed, the hybrid AB will be a mosiac of the two parents, and it is 

 possible by simple inspection of such a hybrid form to determine what 

 races entered into it. Such a hybrid will give a population consisting 

 of IA:2AB:IB, thus demonstrating that the case rests on a simple 

 factor basis and that the mosaic pattern is simply an expression of a 

 heterozygous condition in which both A and a, if we designate them thus, 

 work out their full possibility in the development of the hybrid. In 

 certain cases which did not appear to conform to this simple interpreta- 

 tion, a microscopic examination was resorted to. This examination dem- 

 onstrated that the lack of agreement was apparent rather than real. 

 Thus in Fig. 72 the superficial characters of the hybrid (BI) between P. 

 leuconotus (BK) and P. nigronotatus (II) are for the most part those of P. 

 leuconotus except for the broad black band across the pronotum which is 



