i I . / ^ 



^! X Y ' Composition of parents i W Z ! y Z Z 



o! ; 9'K-^..y ^1 



(X CX') iX| |Yl Gametes produced (W) fZ) iZl [Z 



X X ) ^1 X Y I OHspring f W Z j ^; Z Z 



TWO TYPES OF SEX DETERMINATION 



In many species of mammals, insects and plants, all eggs have an X-chromosome, but 

 half the sperm cells have an X-chromosome and half a Y-chromosome. Fertilization 

 by an X-bearing sperm results in a female. In many birds and butterflies the eggs 

 are of two kinds — one with a Z and one with a W chromosome. The combining of 

 two Z-chromosomes results in a male 



ceeding generations sometimes become separated. Assuming that there is a 

 real connection between these two sets of facts, Morgan and his associates 

 developed their famous chromosome "map" oi Drosophila. In this map hun- 

 dreds of spots on the chromosomes are assigned to the various genes that are 

 supposed to determine particular characteristics. Relative positions of genes 

 are based on the relative consistency with which two or more traits remain 

 linked in successive hybrid generations. Fragmentary chromosome maps on 

 the same plan have been made for various species of plants and animals, 

 including man. 



Multiple Factors — Multiple Action After being for centuries the source 

 of endless confusion, superstition, and fruitless speculation, the problems of 

 "heredity" began to clear up almost suddenly when scientists attacked them 

 experimentally around the turn of the century. 



We have learned to think oi genes as particular objects — perhaps particular 

 kinds of molecules — because this idea has helped us analyze (1) the behavior 

 of the chromosomes during cell-division, during the formation of eggs and 

 sperms, and during fertilization, and (2) the distribution of characteristics in 

 particular species of plants and of animals. 



We now know pretty definitely that the inheritance of characteristics and 

 the chromosome behavior are closely related. But we have learned also that 

 no one gene does actually bring about a particular characteristic. On the con- 

 trary, all the findings point to the probability that (1) each gene, or "deter- 

 miner", produces a multitude of effects and not merely the one which happens 

 to catch our attention as a basis for experimenting; and (2) each "character" 



492 



