MENDELIAN INHERITANCE AND CHROMOSOMES 785 



Ripe ovum Much cytoplasm Single set of chromosomes No centrosome 



Spermatozoon Trace of cytoplasm Single set of chromosomes One centrosome 



Fertilized ovum Much cytoplasm Double set of chromosomes One centrosome 



We see that virtually all the cytoplasm of the beginning embryo comes from 

 the mother, and represents her variety. The minute centrosome comes from 

 the spermatozoon. But the nucleus represents the two stocks half and half, 

 and is the only part of the germ cell that fulfills the conditions for equal inheri- 

 tance from the two parental stocks. The conclusion has been drawn, that 

 the transmission of such differences as we find between different breeds of the 

 same species is due to peculiarities in the nuclei. 



Mendelian Inheritance and Chromosomes. Two types of inheritance 

 have long been recognized, the blending, and the alternative or Mendelian 

 forms. In the former, the offsprings are intermediate between the stocks of 

 their parents. In the latter, a certain characteristic from one stock is mani- 

 fested by the child to the exclusion of the alternative trait shown in the other 

 stock. Many supposed cases of blending are now believed to be complex 

 instances of alternative inheritance. Albinism, or the lack of pigment in hair, 

 skin, and eyes, is a good example of an alternatively heritable trait. 



When two pure bred animals that carry two opposite and alternative 

 traits are crossed with each other, one type "dominates," or shows in full 

 force in all the young. Thus in most mammals color dominates over albin- 

 ism, and all the young from the cross look like thoroughbred colored animals. 

 We conclude that if either egg or sperm carries color, the offspring is always 

 colored in appearance. But if these animals are now crossed with the white 

 stock, half of the offspring will be white and the others colored. This indi- 

 cates that the dark colored hybrids are producing equal numbers of two kinds 

 of germ cells, one of which possesses, and the other lacks the color-producing 

 factor. These last colored young are really hybrids producing two kinds of 

 germ cells, like their hybrid parent. But their white brothers have received 

 white from both their parents, and are essentially a pure stock once more. 



Thus in simple alternative (Mendelian) inheritance, the hybrid looks like 

 a pure-bred representative of the "dominant" form, the other, or "recessive" 

 character not coming to view. And its germ cells are not hybrid, but half 

 of them are pure for one form, and the other half pure for the other form. 

 The character of the offspring depends upon the particular germ cells that 

 produced it. If they are both alike, the young is "pure," no matter how 

 hybrid its parents. 



These facts lead to certain statistical relations known as the Mendelian 

 ratios, in the proportionate numbers of pure dominant, mixed, and pure 

 recessive young that will be found in each kind of cross. When two pairs of 

 Mendelian characters are crossed at once, as for example, two colors of hair 

 and two contrasting lengths of hair, each pair of characters usually follows 

 these Mendelian rules quite independent of the other pair. From one such 



