174 FUNDAMENTALS OF CYTOLOGY 



large numbers of gene pairs, but relatively few chromosome pairs; hence, 

 each chromosome must carry many genes. The inheritance of two char- 

 acter pairs dependent upon differential genes in the same chromosome 

 pair would be unlike that described above, for random assortment of 

 chromosomes would play no part in determining their combinations. 

 This is illustrated in the right-hand part of Fig. 123. If the two gene 

 pairs in the Fi plant were located in one chromosome pair as shown, 

 there would be formed only two kinds of spores and gametes instead of 

 four, and the F^ generation would show only the two parental combina- 

 tions, tall-green and dwarf-yellow. The two characters contributed 

 together by each grandparent are still associated. This phenomenon 

 is known as linkage. As would be expected, there are as many groups 

 of linked genes as there are chromosome pairs; there are, for example, 4 in 

 Drosophila and 10 in maize. Independent inheritance is exhibited only 

 by characters having their differential genes in different chromosome 

 pairs. From a mathematical point of view it is remarkable that Mendel 

 happened to select for special study seven independent character pairs, 

 for the garden pea has since been found to have just seven pairs of 

 chromosomes. 



Assignment of Genes to a Chromosome. — How is it ascertained in 

 what chromosome of a genome a given gene and those linked with it are 

 located? In plants one of the convenient methods involves the use of 

 individuals occasionally appearing in the breeding plot with one extra 

 chromosome. This condition arises as a result of nondisjunction, 

 commonly at sporogenesis. Two members of a pair that should disjoin 

 in meiosis fail to do so, a spore and later a gamete therefore arising 

 with one member of its genome in duplicate. Union of this gamete with 

 a normal one yields a plant with one of its chromosomes in triplicate, all 

 the other chromosomes being in duplicate as usual. At meiosis the three 

 members of the "trisome" usually disjoin two from one, so that some of 

 the spores and gametes carry an extra chromosome. It can readily be 

 calculated that Mendeiian ratios in plants obtained when this trisomic 

 plant is testcrossed will not be the same for characters dependent upon 

 genes in the trisome as for characters due to genes in the other chromo- 

 somes, provided the trisomic plant is heterozygous for genes in the 

 trisome. For example, when a plant carries the genes ^ ^ a in the 

 trisome and the factors B b in & normal chromosome pair, the population 

 obtained after a testcross to a plant with a a h h is expected to show a 

 phenotypic ratio of 5 : 1 for a character due to A and the normal 1 : 1 

 ratio for a character due to B. Hence, by observing what characters 

 appear in trisomic ratios,, and by examining the plants cytologically to 

 see which chromosome of the genome is present in triplicate, the con- 

 clusion can be drawn that the genes responsible for the abnormal ratios 



