i6 HEREDITY AND EUGENICS 



hair. From this it follows, as shown in the diagram 

 in Fig. I (p. lo), that three-quarters of the individuals 

 in the next generation (Eg) will have short hair, the 

 remaining quarter having Angora hair. This is because 

 when the eggs and sperms unite in fertilisation, half 

 the eggs and half the sperms will contain the chromo- 

 some determining short hair, while the other half 

 carry its mate with the determiner for long hair. 

 The fertilised eggs will then be of three types : ( i ) con- 

 taining a pair of chromosomes, both of which carry 

 the determiner for short coat. When these eggs 

 develop into organisms which can be bred together, 

 they can obviously give only short-coated offspring. 

 They are homozygous dominants. (2) These ferti- 

 lised eggs will contain a pair of chromosomes with 

 determiners respect ivety for long and short coat, and, 

 according to the laws of chance combination, they will 

 be twice as numerous as the last. They are the 

 heteroz3^gous dominants, their bodies indistinguish- 

 able from the pure dominants (when dominance is 

 complete), but their germ plasm as well as all their body 

 cells containing an '' unequal " pair of chromosomes 

 which will separate as in the E^ to produce the next 

 generation. These two classes of F2 animals, together 

 making up three-quarters of the offspring, are visibly 

 short-coated. (3) In the third class of fertilised eggs 

 both chromosomes of this pair will contain the 

 determiner for long coat. They will develop into 

 long-coated animals, their body cells and germ cells 

 will all contain the descendants of this pair of chromo- 

 somes, and they wdll give long-coated offspring w^hen 

 bred together. They are the homozygous recessives, 

 and because they result from chromosome recom- 

 binations taking place according to chance, they 

 are as numerous as the first class, the homoz^^gous 

 dominants. 



The history of the chromosomes in organisms was 



