684 



HEREDITY AND CELL DIVISION 



which was imported from Spain in about 1850, was for long a 

 trouble to breeders, because many of the chicks came black or 

 a peculiar white and grey colour ; we now know that the blue type 

 is a heterozygote, and it can be produced by mating the black 

 and white forms (Fig. 532). 



-i'i 



Ma 



Fig. 532. — To illustrate the results of crossing two pure-bred (homozygote) 

 races of Andalusian fowls. — From Morgan. 



Pj, The parents, one splashed-white and the other black ; Fj, the first hybrid generation, all individuals 

 of which are alike, of a bluish-black shade due to the blending of the colours of the parents ; Fg, the 

 second hybrid generation, bred by mating Fj individuals together. One quarter resembles each of the 

 parents and the remaining half are Uke Fj. 



It has long been customary to state the essential theory of 

 Mendelism in two parts : 



1. Segregation. Every gamete produced by a heterozygote 

 contains unchanged either one or the other of any two 

 factors determining alternative unit characters in respect 

 of which its parental gametes differed. 



2. Recombination. The factors determining different unit 

 characters are recombined at random in the gametes of an 

 individual heterozygous in respect to these factors. 



These are ' Mendel's Laws ' of the textbooks, but they have in 



