94 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



action are as yet hardly profitable. In the case discussed 

 we know that the black pigment of the guinea-pig's coat 

 (melanin) is formed by the action of an enzyme (tyrosinase) 

 on a colourless precursor, and that in the albinos the 

 enzyme is lacking ; we may perhaps regard the " gene " 

 for blackness as a substance essential for the formation of 

 the enzyme, and the character for whiteness as merely the 

 absence of this substance ; then any animal which has 

 received the enzyme-forming gene, from either parent or 

 from both, will be black. 



It is demonstrable that sex is inherited on Mendelian 

 principles, and the idea has important applications. The 

 inheritance of sex is to be compared with the result of 

 crossing a mixed or DR form with a pure recessive or RR 

 form, a " back-cross." Then the germ-cells of one 

 parent are of two kinds, of the other parent all alike, and 

 the offspring will again be of two kinds, DR or RR, in 

 equal numbers. It has been demonstrated for a large 

 number of species that the cells of the female contain two 

 recognisable " X " chromosomes, while those of the male 

 contain only one X-chromosome and, corresponding to the 

 other, a small Y-chromosome : so that the males may be 

 regarded as mixed forms in this respect, and every mating 

 as a '* back-cross " between XX and XY. In other cases 

 the Y-chromosome may be altogether absent ; and in the 

 Lepidoptera and probably in birds it is the females that 

 are mixed, so that they have two kinds of eggs, male- 

 producing and female-producing, while other animals 

 have two kinds of spermatozoa. 



Mendel's second great discovery was that when parents 

 differing in two characters were mated, the offspring 

 inherited these characters independently. Thus, in 

 guinea-pigs, " rough-coat " is dominant over " smooth- 

 coat " : so that when a black smooth-coated guinea-pig 

 is crossed with a white rough-coated guinea-pig, all the 

 members of the F^ generation are black and rough-coated. 

 In the F., generation the rough-coats outnumber the smooth- 

 coats in the ratio three to one, just as the blacks, pure and 

 apparent, outnumber the whites ; but the two characters 

 are independent of each other. Thus of every four 

 black F.J guinea-pigs, three are rough-coated ; and of 



