GENETICS 237 



other two kinds of eggs (bv and BV). The Fi female is represented as 

 mated to a black vestigial male, whose spermatozoa are necessarily hv. 

 These spermatozoa fertilize the four kinds of eggs and produce four kinds 

 of offspring which should be in the same proportion as the kinds of eggs. 

 The first two kinds (gray vestigial and black long) together make up 

 about 83 per cent of the family just as the Bv and hV eggs made 83 per 

 cent of the eggs. The other two classes (black vestigial and gray long), 

 coming from eggs containing broken and recombined chromosomes, con- 

 stitute about 17 per cent. If these two pair of genes had been in different 

 pairs of chromosomes and so had been independent of one another, the 

 last generation would have exhibited a 1:1:1:1 ratio, each kind making 

 about one-fourth of the total, as in the two pairs of characters in Fig. 200. 

 The distorted ratio is the evidence that the genes are all in one pair of 

 chromosomes. 



Mendel's Law; Mendelian Heredity. — Gregor Mendel never stated 

 his discoveries in the form of a concise principle, but this has been done 

 by others since. Heredity as Mendel conceived it differed in two impor- 

 tant respects from heredity as understood by his predecessors. A state- 

 ment of these two differences is commonly spoken of as Mendel's law. 

 Using present terminology, one might state this law as follows. The 

 genes of any pair separate from each other in the production of the germ cells, 

 so that each germ cell receives only one of them; and the distribution of each 

 pair of genes to the germ cells is independent of the distribution of other pairs. 

 The separation of genes of the same pair is effected by the reduction 

 division in maturation. Independence of the genes of different pairs 

 exists when the pairs of genes are in different pairs of chromosomes, 

 since these pairs of chromosomes are independently placed on the spindle 

 of the reduction division. As is indicated in the preceding section, this 

 latter condition is not always met. Many pairs of genes are in the same 

 pair of chromosomes. Autosomal linkage, which results from this 

 association, is very common. Such linkage is a violation of the second 

 part of Mendel's law. Apparently Mendel never witnessed this relation 

 between any two pairs of genes. 



Despite the fact that Mendel's law as stated does not provide for 

 linkage, all the phenomena so far described are still regarded as belonging 

 to Mendelian heredity. The concept of Mendelism has been widened to 

 include them. Any heredity is now considered Mendelian if it is depend- 

 ent on chromosomes. Most heredity is so dependent. Yet in some plants 

 the plastids go over directly from one generation to the next, and what- 

 ever color characters these plastids determine are independent of chromo- 

 somal genes. Heredity of plastid colors in such plants is not Mendelian. 

 Possibly, even probably, there are some other structural units which are 

 transmitted directly like plastids. 



