376 



THE STUDY OF ANIMAL LIFE CHAP. 



It may be of interest to refer to cases where the parents 

 differ in two pairs of contrasted characters (or allelo- 

 morphs), eg, when a tall yellow-seeded pea (both characters 

 dominant) is crossed with a dwarf green-seeded pea (both 

 characters recessive). The hybrid offspring (Fj) are all 

 tall yellow r s, but what will their offspring be ? If we sup- 

 pose that there are 16 of them, three-fourths (12) w r ill be 

 tall and one-fourth (4) dwarf. But of the 12 tails, three- 

 fourths (9) will be yellow-seeded and one-fourth (3) green- 

 seeded. And of the 4 dwarfs likewise, three-fourths (3) 

 will be yellow-seeded, and one-fourth (1) green-seeded. 

 So the formula for " dihybridism ' runs, in this case : 

 9 tall yellow -f- 3 tall green + 3 dwarf yellow + 1 dwarf 

 green. This may be conveniently registered on a scheme :- 



A very instructive illustration of Mendelian inheritance 

 is given by Professor Punnett, that of the Blue Anda- 

 lusian fowl (see fig. 118). The Blue Andalusians are 

 known not to breed true ; they yield, besides blues, 

 two sorts of " wasters," namely blacks and whites marked 

 with some black splashes. The blacks breed true, the 

 whites breed true, but the blues will not. The reason 

 is plainly that Blue Andalusians are impure dominants, 

 D(R), the blacks and the white being the pure types. 

 The student should linger over this case, for it illustrates 

 what very frequently occurs, that the Fj generation does 

 not, as regards the character in question, resemble one 

 parent exclusively, but is incomplete in its expression of 

 dominance. It might be readily mistaken for a blend, 

 but the subsequent history, as the diagram shows, corrects 

 the misinterpretation. 



Let us now in reference to the Andalusian fowl illus- 



