INHERITANCE 297 



is expressed in the soma (the plant is tall), while the other, the 

 recessive (s), is not expressed. The individual is heterozygous with 

 respect to the character in question (e.g., size). 



After synapsis during the maturation of the germ cells of the 

 individual, segregation of the genes occurs with the result that each 

 gamete receives only one gene for each character. Thus the gametes 

 of homozygous individuals are all alike with respect to the gene in 

 question (e.g., all bear either S or s), while the gametes of hetero- 

 zygous individuals are of two numerically equal classes (e.g., 

 50 per cent bear S and 50 per cent bear s). 



Finally, there is an independent assortment of the genes for dif- 

 ferent characters, as evidenced by new combinations of characters 

 in the progeny of dihybrids, etc. For example, size and color are 

 independently inherited. This depends, as we shall see later, upon 

 the genes involved being in different pairs of chromosomes. 



The principles of segregation and independent assortment are 

 usually known as Mendel's laws. 



C. Alterations of Mendelian Ratios 



The immense amount of experimental breeding that has been 

 carried on since Mendel's time has accentuated the significance of 

 the principles of segregation and independent assortment, but has 

 revealed that dominance is by no means universal. A few examples 

 will bring the main facts before us. 



The seven pairs of alternative characters in Peas which Mendel 

 studied showed essentially complete dominance of one character 

 in each pair, but we now know a great many cases in which the 

 hybrid (Fi) shows a different condition from either of the parents. 

 For instance, on crossing homozygous red and white races of the 

 Four-o'clock, all the progeny in the heterozygous (Fi) generation 

 bear pink flowers, or, we may say, flowers intermediate in color 

 between the two parents. Neither red nor white is dominant : the 

 result is blending inheritance. Rut inbreeding the hybrids gives a 

 F 2 of 1 red : 2 pink : 1 white. Thus the typical Mendelian 3 : 1 ratio 

 is, so to speak, automatically resolved into the 1:2:1 ratio which, 

 when one character is dominant, is evident only on further breed- 

 ing. Segregation actually occurs as usual, because the homozygous 

 progeny of the hybrid exhibit the original parental characters 

 unmodified. (Fig. 186.) 



In certain other cases, the hybrids, instead of being true inter- 



