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ESSENTIALS OF ZOOLOGY 



with tall and dwarf peas, half of the male gametes and half of the 

 female gametes contained a gene for tallness (T) and half in each 

 case contained a gene for dwarfness (d). The outcome, therefore, 

 is shown in Fig. 74. 



The Polyhybrid Cross. — It is often desirable to know the outcome 

 of a cross in which two or more characters are considered together 

 as in the case of crossing a tall, green-podded pea (Tg) with a dwarf, 

 yellow-podded pea (dY) (Fig. 75). The determination of the pos- 

 sible kinds of gametes may be simplified by first making a small 

 checkerboard for them. Thus we find that there are four kinds of 

 male gametes and four kinds of female gametes which may be listed 

 as TY, Tg, dY, dg. Now we make a checkerboard of sixteen squares 

 and proceed as we did before. 



Ty 



Tg 



dY 



dg 



TY 



Tg 



dY 



dg 



Fig. 76. — The outcome of a dihybrid cross between two heterozygous individ- 

 uals is according to the ratio of 9 :3 :3 :1. The four kinds of male and female gam- 

 etes, tall yellow (Ty), tall green (Tg), dwarf yellow (dy) and the recessive dwarf 

 green idg) are employed as they would combine in this ratio. 



The computation of the dihybrid cross (Fig. 76) indicates that 

 nine-sixteenths of the progeny will show the two dominant traits. 

 Three-sixteenths will show the dominant trait of the first allelomorph 

 and the recessive trait of the other. Another three-sixteenths will show 

 the recessive trait of the first allelomorph and the dominant trait of 

 the second. One-sixteenth of the offspring will show both recessive 

 traits. It will be noticed, however, that the nine squares showing indi- 

 viduals that will be tall and yellow vary with each other in regard to 

 their entire content. Further examination of the squares will indicate 



