THE BEGINNING OF DEVELOPMENT 33 



two similar chromosomes, each with a gene for 

 short fingers, and each of their germ-cells contains 

 one chromosome with the short-fingered gene. When 

 such a sperm fertihzes a normal egg containing a 

 gene for ordinary fingers, the children have one of 

 each kind of gene. In this particular case it is the 

 short-fingered gene which affects the development: 

 it is therefore said to be dominant over the ordinary 



PARENTS Ss Ss 



GERM-CELLS 



CHILDREN ISS ISs ISs 



Fig. 5. — Diagram of the inheritance of short fingers. S is the factor 

 for short fingers and s that for ordinary fingers. The lines show 

 the ways in which the factors may come together in fertilization. 



gene, which is recessive to it. When the germ-cells 

 are formed in the children of such a marriage, the 

 two genes, lying in the two similar chromosomes, 

 are separated at the reduction division, and the 

 germ-cells have half of them one normal gene and 

 half of them one short-finger gene. If two such 

 children marry (Fig. 5), it is pure chance which 

 genes come together in the fertilized eggs, so that 

 in half the eggs a normal gene will meet a short- 

 finger, giving short-fingered adults, in a quarter of 

 the eggs there will be two short-finger genes, giving 

 more short-fingered adults, and in the last quarter 



