INHERITANCE IN ORGANISMS 561 



homozygous and dominant and the two heterozygous individuals, would 

 show the dominant character, while the other, which is homozygous and 

 recessive, would exhibit the recessive character. 



602. Multiple Hybrids. — So far we have considered individuals 

 heterozygous for only a single pair of characters. Such individuals are 

 termed monohybrids. Since every individual is a combination of large 

 numbers of characters, however, it is clear that an individual is likely 

 to be a hybrid not for one characteristic but for more. If hybrid for two, 

 the individual may be termed a dihyhrid; if for three, a trihybrid; and if 

 for more, it is usually called a midtiple hybrid. In these cases the variety 

 of sex cells produced would be greater and the number of possible com- 

 binations in the offspring would be very much greater. A checkerboard 

 diagram for a dihybrid cross in which the characters are represented by 

 R, r, B, and b^ior rough and smooth, black and albino — would appear 

 as in the diagram (Fig. 386). 



There would be four types of sperm cells and four types of egg cells. 

 The phenotypic ratio would be 9 : 3 : 3 : 1 , in which 9 represents individuals 

 that show both dominant characters; 3 are dominant for the first pair 

 of characters but recessive for the second; 3 are recessive for the first 

 pair but dominant for the second ; and 1 is recessive for both pairs. The 

 genotypic ratio would include nine types of individuals as indicated by 

 the diagram. 



A corresponding phenotypic ratio for a trihybrid cross in which 

 there are eight types of sperm cells and eight of egg cells would be 

 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 . In this case 27 would show all three dominant char- 

 acters and only 1 would show all three recessive characters, the others 

 being varying combinations of dominants and recessives. There are 

 here 27 different genotypes. 



603. Actual Cases. — Certain cases may be given to illustrate the 

 actual results of crossings. It should be observed that since these 

 ratios are based on chance they may not hold good in individual cases 

 where small numbers are involved. The larger the numbers the closer 

 they are likely to be approached. Mendel crossed tall pea plants with 

 short pea plants, where tallness is a dominant character and shortness a 

 recessive one. The hybrids were all tall. He found that in breeding 

 these hybrids three-fourths of the offspring were tall and one-fourth short 

 but that of the three-fourths which were tall one-third were pure for this 

 character and two-thirds were hybrid. Mendel actually obtained in the 

 second filial generation from original parents, one of which was tall and 

 the other short, 787 tall plants and 277 short ones, when, if the results 

 had been mathematically exact, he should have had 798 tall and 266 short 

 ones. Other experiments have resulted in ratios more nearly mathemati- 

 cally exact, and the departure has not been considered sufficient to invali- 

 date the principle involved. 



