OUK NATURAL INHERITANCE 71 



and brown-eyed will have weak brown pigmentation of 

 the iris, receiving the brown factor or determiner from 

 only one side of the house, a single dose or simplex 

 inheritance of brown. If a member of a simplex stock 

 marry a member of a duplex stock the children will be 

 half simplex, half duplex, all brown. If a member of a 

 simplex stock marry a member of a simplex stock, the 

 children will be in the proportion of 1 duplex brown : 

 2 simplex brown : 1 blue-eyed nulliplex. If a member 

 of a simplex stock marry a blue-eyed person half of the 

 children will be simplex and half blue-eyed (nulliplex). 

 And finally, if blue-eyed marry blue-eyed, all the chil- 

 dren will be blue-eyed. 



Increasing knowledge of Mendelian inheritance les- 

 sens the impression of almost mechanical card-shuffling 

 which the earlier work suggested. Thus we know that 

 in many cases there does not seem to be a random or 

 free assortment of genes or factors among the ripening 

 germ-cells. Many characters have been found to keep 

 together in successive generations instead of assorting 

 freely. It looks as if the genes sometimes hung to- 

 gether in blocks. This is known as linkage and it is 

 correlated with a remarkable process of crossing over, 

 wherein there is an interchange of blocks of genes 

 between a maternal and paternal pair of chromosomes 

 when closely apposed to one another. It seems clear 

 that a gene or factor in the germ-plasm may be asso- 

 ciated with more than one effect in the body ; that the 

 expression the gene finds in development depends in 

 some measure on the environmental conditions ; and 



