HEREDITY AND VARIATION 219 



the plants are crimson (homozygous), one-half i)ink (heterozy- 

 gous), and one-fourth white (homozygous), the one-to-two-to-one 

 ratio which we have just mentioned above. It is evident that 

 pink is here not a true Mendelian character at all, in the sense 

 that it is inherited and will segregate, but that it is merely the 

 expression of two factors in a heterozygous condition. 



Independent Assortment. — When Mendel studied the inheri- 

 tance of two or more factors simultaneously he discovered the 

 further important fact that segregation Avhich takes place 

 between the members of any one factor-pair is quite independent 

 of that which takes place in any other, so that in the second 

 generation from the cross all sorts of recombinations, many 

 of them quite unlike those found in the original parents, may occur. 

 Let us consider a plant which is homozygous for purple flowers 

 and also for smooth seeds and which we may therefore represent 

 by the formula PP SS, to be crossed with a plant homozygous 

 for white flowers and also for rough seeds, WW RR . The for- 

 mula of the Fi hybrid offspring would of course be PW SR, and as 

 smooth seed coat is dominant over rough, this plant would look 

 like the purple-flowered, smooth-seeded parent. When gametes 

 are formed by this plant, half of them carry the factor P 

 and half the factor W. But it is clear that every sexual cell 

 must carry within itself not only the factors for flower color but 

 also those for seed surface and for all other plant characters, 

 as well, and half of the gametes thus must carry the factor 

 S and half the factor R. Now we find that in any given sexual 

 cell, it is purely a matter of chance as to whether the factor 

 for purple flowers is associated with that for smooth seeds or 

 with that for rough seeds. The particular combination of 

 factors which enters the Fi plant from each parent (purple with 

 smooth and white with rough, in this case) has.no effect whatever 

 upon the way in which they are associated in the gametes 

 produced by this plant. Their assortment is independent. Such 

 a plant as the i^i hybrid in this example will therefore produce 

 four kinds of gametes in equal numbers: P S; P R; W S, and 

 W R. If two such plants are crossed, there will be sixteen 

 possible combinations among their sexual cells, for there will 

 be four kinds of pollen grains and four kinds of egg cells and 

 union is quite at random. Any one of these combinations is as 

 likely to occur as any other, and the sixteen types will thus 

 tend to be equally numerous. Since two of these characters arc 



