THE GENERAL ASPECTS OF HEREDITY 19 



strain contains numbers of such recessive characters, 

 which may be transmitted for generations without 

 appearing, until union with an individual carrying 

 the same recessive character may ultimately bring 

 it out in some (25 per cent.) of the offspring. The 

 presence of similar, undesirable recessive characters 

 in the germ plasm is thus the chief danger from 

 inbreeding or intermarriage of cousins. The main 

 features of a recessive character are, then, that it 

 disappears in the first generation of a cross between 

 a pure dominant and a pure recessive, while it re- 

 appears in about one-quarter of the offspring of two 



Germ cells X 



XX XY 



Fig. 7. — Diagram to show the History of the Sex 



Chromosomes. 



individuals heterozygous for the character. It will 

 appear in about half the offspring of matings between 

 a heterozygous normal and a (pure) recessive. 



Another type of Mendelian character, which in its 

 inheritance follows exactly the distribution of the sex 

 chromosomes, is known as sex-linked inheritance. 

 Such characters evidently depend for their origin 

 upon mutations occurring in the sex chromosomes. 

 As an example of this in man we may consider colour- 

 blindness. The nature of this inheritance-mechanism 

 will be clear from the following diagram (Fig. 7), 

 showing the history of the sex chromosomes as they 

 appear to behave in man. 



