S02 ZOOLOGY 



In other words, while the body of an organism may be a hybrid, 

 the eggs and sperm are always pure dominant or recessive with 

 respect to any unit character. 



In the peas, for example, if T represents the determiner for 

 tallness, and t that for dwarfness, one-half the egg cells of the 

 first hybrid generation will carry T and one-half will carry the 

 recessive t. The same will be true of the male nuclei of the pollen 

 tube. In mating the chances are even that each kind of male 

 cell will fertilize each kind of female. The "checker-board" 

 diagram (Fig. 254) will show the probable number of offspring of 

 each possible type. 



494. The Mechanism of the Inheritance of Unit Charac- 

 ters. — These discoveries of Mendel and his successors through 

 breeding, when joined to other discoveries about the behavior 

 of chromosomes in the maturation divisions (see §§49, 50) by 

 which eggs and sperm are produced, have given us our most 

 fruitful clues to the mechanism of heredity. The uniting of 

 these two sets of discoveries is a peculiarly complex and technical 

 task, beyond the scope of the present discussion. Only a few 

 brief suggestions will be given here. The student is invited to 

 pursue the subject in some special text on heredity. 



1. The chromosomes of the gerni cells seem surely to carry 

 the genes which determine such unit characters as those dis- 

 cussed above. Furthermore the evidence appears to indicate 

 that these genes are arranged in definite linear sequence in the 

 chromosome, somewhat as beads are on a string. (We do not 

 know that the chromosomes carry all the inherited characters.) 



2. Thus in mitosis (metaphase stage), when the chromosome 

 splits, each gene would be divided also. Consequently each 

 daughter cell would have chromosomes that possess the same 

 series of genes as did the parent cell, arranged in the same order 

 in the chromosome. 



3. It will be recalled (Fig. 11) that there are 2 ic chromosomes* 

 in ordinary cells, — one half having been supplied by the egg and 

 the other by the sperm (at fertilization). It is believed that 

 these chromosomes really represent x pairs, and that each 



•The letter x in this paragraph is merely numerical, as in Figs, ii and 12. It has nothing to 

 do with the "sf-chromosome" which is connected with sex. 



