Genetics 303 



THE SECOND MENDELIAN LAW 



Mendel's elucidation of his first law is basic to an understanding of 

 all genetics. Usually, however, characters are studied in combination. 

 From a practical viewpoint, animal and plant breeders must deal with 

 several characters at a time; thus an understanding of the behavior of 

 several factors in a single cross is important. 



The Study of Two Characters. — Mendel, in his logical fashion, 

 undertook this study by using two characters which he had previously 

 independently studied in monohybrid crosses. One of these crosses in- 

 volved pure-breeding lines of yellow round peas and wrinkled green peas. 

 He noted that in the Fi generation, all the peas were yellow and round. 

 A cross between two Fi individuals produced progeny of four different 

 types which were in definite ratios to one another. From the F2 indi- 

 viduals, it was apparent that each pair of characters or allelomorphs 

 was inherited independently of the other. From this, he formulated 

 his second important law : in a cross involving more than one pair of 

 characters, the inheritance of each pair is independent of the others. 



Again the truth of this law is verified by cytological studies. It 

 will be recalled that during meiosis, the chromosomes of different homol- 

 ogous pairs moved independently of one another. The genes located 

 on the different pairs of chromosomes likewise would be carried with 

 the chromosomes and distributed in a random fashion. 



A Dihybrid Cross. — This second law was formulated from ob- 

 servations of crosses such as that described above. Inasmuch as two 

 characters are involved, the cross is known as a dihybrid cross. 



PARENTS FT R 



YELLOW 

 ROUND 



YYRR 

 YR 



GREEN 

 WRINKLED 



yyrr 



y 



YELLOW 

 ROUND 



YyRr 

 YR Yr yR yr 



Fig. 93. — A dihybrid cross in peas. Y = a gene for yellow, y = a gene for 

 green; R = a gene for round; and r = a gene for wrinkled. Yellow and round are 

 dominant over green and wrinkled. 



