CHAPTER XVII 

 LINKAGE 



In Chapter XV attention was directed to the remarkable parallelism 

 which exists between the distribution of the Mendelian characters and 

 that of the chromosomes. A vast number of breeding experiments with 

 both plants and animals have shown that new combinations of characters 

 are formed at the time of fertilization, when two parental sets of chromo- 

 somes are brought together, and that a segregation of characters occurs 

 at the time of reduction, when the chromosomes are sorted out into two 

 groups. Moreover, the distribution of a single allelomorphic pair of 

 Mendelian characters parallels precisely that of a single homologous pair 

 of chromosomes. These facts indicate clearly that chromosomes and 

 characters are in some manner causally related. This conclusion is 

 strongly supported by the cytological aspects of sex inheritance, maleness 

 and femaleness in a large number of reported cases being definitely corre- 

 lated with the activity of certain distinguishable chromosomes. 



It has also been pointed out that the hypothesis upon which these 

 phenomena are generally interpreted is that the characters are repre- 

 sented in the chromosomes by material factors, or genes, which in some 

 way control the development of characters in the individual. Since, now, 

 an organism usually has many more Mendelian character pairs than it has 

 chromosome pairs, one pair of chromosomes must as a rule carry genes 

 for more than one pair of characters. Furthermore, the different pairs 

 of chromosomes are entirely independent of one another in distribution. 

 It would therefore follow that if two allelomorphic character pairs have 

 their genes located in different chromosome pairs, they will be quite 

 independent of each other in their inheritance through a series of genera- 

 tions; whereas, if their genes are located in the same pair of chromo- 

 somes, they will be inherited together. The latter condition — the 

 persistent association of characters belonging to different allelomorphic 

 pairs through a series of generations — actually exists and is known as 

 linkage. 



The phenomenon of linkage was discovered in 1906 by Bateson and 

 Punnett in the sweet pea. They found flower color to be linked with the 

 shape of the pollen grain: purple flowers nearly always had long grains, 

 while red flowers had round grains. The possible relationship between 

 linkage and the chromosome hypothesis was pointed out by Lock in the 

 same year. Linkage relations have since been worked out in a consider- 



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