138 GENERAL PRINCIPLES OF ZOOLOGY 



If a determinant be so strong that its fellow cannot come to expression it is 

 spoken of as dominant, the one that succumbs is recessive. 



Mendel's Law. Long before the phenomena of fertilization were known, 

 Mendel had shown by experiments on plants, that if two individuals, easily 

 distinguished by some peculiarity like color, be crossed, the cross so produced 

 was, in many cases, not a blend of the two, but resembled exclusively one of the 

 two parents. On crossing red and white flowered peas, no matter in which 

 direction, the crosses were invariably red. The determinant for red was so 

 dominant that the recessive determinant for white is powerless. But that the 

 white determinant was present in the cross (according to the morphological con- 

 clusions regarding fertilization this must be so) was shown when the red-flowered 

 crosses were self fertilized (fig. 99). Then one fourth of the offspring were 



o 



0=O 3=0 3 O 



} white, J red, red, 



breeding true, capable of splitting. breeding true. 



FIG. 99. Diagram of Mendelian inheritance. Black and white representing red and 



white flowering kinds. 



white and bred true; that is the white color, the recessive character, persisted 

 in all the succeeding generations. Of the remaining three fourths, one fourth 

 also bred true, but red, a sign that the recessive character was lost. Such pure 

 breeding animals are called homozygous because the sygote, the fertilized cell 

 from which they arise, contains, with reference to the peculiarity under investi- 

 gation, only similar determinants. The remaining two fourths are heteroz\<gotes; 

 that is they possess determinants of two kinds and are thus like the first crossed 

 generation, the red color alone appearing, owing to the dominance of the red 

 determinant over the recessive white. If these be 'split' by breeding, the next 

 generation will consist of one fourth red, one fourth white,'each breeding true, 

 and two fourths red with recessive white. If the process be continued, the 

 percentage of homozygotes (pure-breeding forms) rapidly increases at the ex- 

 pense of the heterozygote forms. 



Chromosome Reduction. The results of Mendel's experiments may now 

 be brought into harmony with the phenomena of maturation and fertilization, 

 transferring for this purpose the role of the chromosomes to the hypothetical 

 determinants. It has been found in numerous cases that the material of the 

 egg and sperm nuclei the two assortments of chromosomes do not fuse with 

 each other but remain distinct in all divisions of the somatic cells (p. 130), a 

 phenomenon apparently applicable to all organisms. Hence the similar 

 determinants, the 'pairlings,' or as they have been called, the allelomorphs (in 

 the example quoted, the determinants for red and white) remain separate from 

 each other in the cleavage cell and in all of its descendants. They persist in the 

 cells of the flower, where, according to their dominance, they determine the 

 color of the blossom; they are also continued in the sex-cells, the oocytes and 

 spermacytes, of the hybrid generation. Now the sex-cells, in the so-called 

 growth period preceding the maturation divisions possess only half the number of 

 chromosomes characteristic of the tissue cells. 



