HEREDITY AND VARIATION 



which can be shown to be made up of two genotypes. On the other hand, the 

 dwarf peas, or any individual that appears recessive, will always breed true 

 for the recessive characteristic; the recessive phenotype is identical with 

 the genotype. 



If any dominant characteristic be represented by D and any recessive char- 

 acteristic by d, the following summary of the possible crosses can be made. 



(Parents) DD x DD 

 (Parents) dd X dd 



(Parents) DD x dd 



(Parents) Dd x Dd 



(Parents) DD x Dd 



(Parents) Dd X dd 



100 per cent DD (offspring) 



100 per cent dd (offspring) 



1 00 per cent Dd (offspring) 

 25 per cent DD plus 50 per cent Dd plus 25 per cent 



dd (offspring) 

 50 per cent DD plus 50 per cent Dd (offspring) 

 50 per cent Dd plus 50 per cent dd (offspring) 



It must be understood that large numbers of specific cases were studied in 

 order to obtain the percentages that have been indicated as characteristic 

 of these crosses. Figure 6.3 gives some of the actual numbers of individuals 

 in the Fg generations in Mendel's original monohybridization experiments. 



Extension of Mendelian Theory. Mendel published the results of his 

 studies in 1866, but they remained unknown until 1900, when his paper was 

 discovered by three scientists who had independently reached the conclusions 

 that he had stated so clearly. Since that time, his results have been con- 

 firmed by experiments with many plants and animals. For example, when a 

 gray mouse is crossed with a white mouse, all the offspring are gray. This 

 result indicates that gray coat color is dominant to white coat color in mice 

 (Fig. 6.4). In the Fg generation gray and white mice occur in the ratio of 

 3:1. Of the gray mice which constitute three-fourths of this Fg generation, 



Fig. 6.3. Data from Mendel's original experiments, from which was derived the 3 : 1 ratio 

 characteristic of the F2 generation in monohybridization. (From H. E. Walter, Genetics, 

 copyright 1922 by The Macmillan Co., reprinted by permission.) 



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