HEREDITY 271 



From this it can be seen that of the four possible combinations, 

 there is one chance for a pure or homozygous yellow, one for a pure 

 or homozygous green and two for a mixed yellow and green or 

 heterozygous combination. A homozygote or homozygous individ- 

 ual is one that has received from its parents like genes for a 

 given character, and a heterozygote one that has received unlike 

 genes for a character. The gametes produced by an individual 

 that is homozygous for a given character will be alike with regard 

 to the genes for the character. On the other hand, an individual 

 that is heterozygous for a given character will produce two 

 numerically equal classes of gametes with regard to the genes. 



Independent Assortment of Genes. — It seems clear, then, that 

 the results of crossing a single pair of allelomorphic conditions 

 find a rational explanation in the assumption that the two kinds 

 of genes of the F h and of similar hybrids, are segregated in differ- 

 ent gametes, according to what Mendel called the law of segrega- 

 tion. Mendel determined further that when more than two 

 pairs of allelomorphic conditions are hybridized simultaneously, 

 each allelomorphic pair follows the law of segregation, each 

 allelomorphic pair of genes being assorted independently of other 

 pairs. If a plant bearing peas that are both yellow in color and 

 smooth in contour is crossed with one whose seeds are green and 

 wrinkled, the seeds of the Fi plants are yellow and smooth. 

 Yellow, as before is dominant to green, and smooth is dominant 

 to wrinkled. If these plants are inbred, the F 2 from them are 

 produced in a ratio of 9 yellow smooth : 3 green smooth : 3 yellow 

 wrinkled: 1 green wrinkled. All the yellows taken together are 

 to the greens as 3:1, all the smooths taken together are to the 

 wrinkled as 3 :1 ; but some of the yellows are wrinkled and some of 

 the greens are smooth. In other words, while the results for 

 each pair of allelomorphic characters are in accord with the law 

 of segregation, a recombination of characters has been brought 

 about as a result of an independent assortment of the genes in the 

 formation of the gametes of the Fi. That the actual composition 

 of the F 2 in this experiment is in accord with the principles of 

 segregation and independent assortment is shown in the accom- 

 panying diagram (Fig. 163). If one arranges the Fi gametes in 

 a horizontal and vertical series, along two sides of a square, the 

 points of intersection of imaginary lines from any pair of gametes 

 will give the gene composition of the resulting zygote. 



