HEREDITY AND VARIATION 



Heritable Variations 



The genetic data so far presented indicate two general reasons why offspring 

 are not exactly like their parents. In the first place, the environment may 

 influence the developing young and produce a fluctuation, or somatic varia- 

 tion. As was pointed out in connection with Johannsen's studies on beans 

 (p. 174), such variations are not inherited; they do not afi^ect the germ cells. 

 In the second place, the results of hybridization experiments show that new 

 combinations of genes give rise to individuals differing from their parents. 

 Disjunction and independent assortment of the pairs of chromosomes contain- 

 ing the linked genes of the several groups give rise to gametes different in 

 their genie content. The random combination of gametes to form zygotes can 

 produce a great number of phenotypes and more genotypes in cases where 

 dominance occurs. The possibilities of new combinations of linkage groups 

 are limited, however, by chance, and the same variations are produced many 

 times. This sort of thing is sometimes said to be like dealing hands of cards. 

 Many combinations can be dealt, but the cards themselves, which would be 

 comparable to the chromosomes carrying groups of genes, remain unchanged 

 and occur in the same numbers and kinds. 



New combinations of genes may also arise as a result of changes brought 

 about in any linkage group by the process of crossing over. Crossing over 

 happens when comparable regions of homologous chromosomes become ex- 

 changed (Fig. 6.23C and D) and gives rise to unexpected classes of off- 

 spring. If a male Drosophila with white eyes and a yellow body is mated to 

 a female with red eyes and a gray body, all the F^ offspring, both males and 

 females, will have red eyes and gray bodies. The genes for these characters 

 are known to be located on the X-chromosome, so that the ordinary expecta- 

 tion of ^2 can be easily ascertained by referring to Figure 6.17. If one of 

 these heterozygous females with red eyes and gray bodies is then mated to a 

 male with white eyes and a yellow body, 99 per cent of the offspring are of 

 the expected kinds: equal numbers of males and females with red eyes and 

 gray bodies and with white eyes and yellow bodies (Fig. 6.24). The other 

 1 per cent is made up of equal numbers of males and females with red eyes 

 and yellow bodies and with white eyes and gray bodies. These individuals 

 arise from zygotes containing chromosomes in which crossing over has 

 occurred. Crossing over has been extensively studied, and the percentage of 

 cross-over types to be expected in given crosses is known; the amount of 

 crossing over differs among different genes. The concept that genes are 

 arranged in a given linear order was deduced from crossing over, and chromo- 

 some maps showing the distances between the loci occupied by certain genes 

 have been compiled from the data collected on crossing over (Fig. 6.16). 



The changes in combinations of entire linkage groups that occur from 

 generation to generation as a result of disjunction of homologous chromosomes 

 and subsequent combinations of gametes, as well as the changes in gene 



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