580 



COLLEGE ZOOLOGY 



Figure 4U7. Garden of the Augustinian monastery at Brno, Czechoslu\akia, ineabunng only 

 20 feet by 120 feet, in which Mendel experimented with the garden pea. After 8 years of study, 

 he stated his conclusions in what are now known as Mendel's laws. This photograph was made 

 by Professor Hugo litis many years ago when the garden was essentially the same as it was at 

 the time of Mendel's investigations. (Courtesy of Mendel Museum, Fredericksburg, Va.) 



cent tall plants, [33 V3 per cent homozygous 

 (SS) and 66^3 per cent heterozygous (Ss)], 

 and 25 per cent homozygous short plants 

 (ss). Therefore, 2 phenotypes appear, tall 

 and short, but 3 genotypes are present (SS, 

 Ss, and ss). When a monohybrid cross is 

 made, as described above, 1 phenotype ap- 

 pears in the Fi generation and all the or- 

 ganisms are like the dominant plant. There 

 is also but a single genotype (Ss). In the 

 F2 generation, however, there are 2 pheno- 

 types, 75 per cent of the offspring (SS and 

 Ss) on the average being tall, and 25 per 

 cent (ss) short; but there are 3 genotypes, 

 25 per cent w^ith 2 genes for tallness (SS), 

 50 per cent with 1 gene for tallness and 1 

 for shortness (Ss), and 25 per cent with 2 

 genes for shortness (ss). Mendel actually 

 obtained in the F2 generation, 787 dominant 

 tall plants and 277 recessive short plants. 



The headpiece at the beginning of this 

 chapter shows that when black cocker 



spaniels, both of which are heterozygous for 

 black and brown coat color, are mated, a 

 ratio of 3 black to 1 brown is obtained. Thus 

 it appears that black is dominant to brown 

 in these dogs, 



Dihybrids or the inheritance 

 of two independent genes 



A dihybrid organism is the result of a 

 cross between parents differing in 2 genes 

 (Fig. 409). For example, round shape of 

 peas is dominant over wrinkled shape, and 

 yellow color is dominant over green. If 

 homozygous yellow-round peas are crossed 

 with green-wrinkled peas, the Fi generation 

 of pea plants will bear only yellow-round 

 peas. If the F/s are crossed, the traits ap- 

 pear to be inherited independently. During 

 the reduction division, 4 types of female and 

 4 types of male gametes will be produced. 

 How these will recombine when fertilization 



