[Chap. XXXVIII HYBRID SEGREGATION 459 



somes and heredity, such as the dupHcation of chromosomes and genes 

 in ordinary cell division and pairing of synaptic mates in reduction 

 division, are much too definite to be regarded as the results of chance. 



The basic facts of segregation and recombination of hereditary factors 

 as distinct units in the sperms and eggs, and the consequent ratios of 

 hybrid progeny were discovered by Mendel before chromosomes had 

 been clearly recognized. The subsequent discovery of chromosomes and 

 of their definite behavior, however, furnished a basis of fact that enabled 

 biologists of this century to explain the principles of hybrid segregation 

 formulated by Mendel, and to extend and clarify our knowledge of 

 hvbridization. It was difficult, for instance, to understand why certain 

 factors seemed to be "linked together" until it was discovered that they 

 were in the same chromosome, and would be present wherever that 

 particular chromosome was present. Two or more factors that always 

 occur together are referred to today as linkage groups. All the factors in 

 a single chromosome constitute one linkage group. Thus through a slow 

 accumulation of facts and the discovery of their dependent relations, 

 the subject of hvbridization has been removed from the realm of mystery 

 and speculation and has become an understandable science. 



Mendel's experiments. Mendel investigated the results of cross-fertili- 

 zation in several kinds of plants, and also in mice and bees. From several 

 seedsmen he obtained seeds of more than thirty varieties of peas, mostly 

 of one species ( Pisum sativum ) , and planted them in a small garden at 

 his monastery. He noted that certain distinctive characters of each va- 

 riety remained constant from year to year. Since some of these characters 

 were easy to detect, he decided that peas would be admirable plants 

 with which to test experimentally the various ideas about hybrids that 

 were current at the time. His success in discovering fundamental laws 

 of heredity when all others had failed depended in part on his choice of 

 suitable experimental material, and in part on his persistence in keeping 

 exact records of all the progeny of his plants through a series of several 

 generations. 



To appreciate Mendel's experiments one should imagine him at work 

 in his garden among peas some of which were slightly taller than he was; 

 others were dwarfs less than knee-high. Some were white flowered; 

 others were violet-red flowered. On some plants the flowers and pods 

 developed only in terminal clusters at the top of the stem; on others 

 they appeared in the axils of the leaves along the stem. The pods on 

 some plants were yellow; on others they were green. Some pods were 



