HEREDITY AND VARIATION 9 



races of this plant ; almost every one ol the known shad< olonx arises 



through the interaction of two or more of these geni 



Two further instances of interaction require brief notice. In the 



Chinese Primula (Primula sinensis) the normal (palmatifid Of " palm " 

 shape depends upon the presence of at least seven principal ii ting 



" dominant " genes. The " recessive " genes which have been identified by 

 corresponding characteristic leaf-shapes are ' fern," ' tongue," ' oak," 

 " maple," " claw " and two different " crimping " genes. All th< 

 discovered in the first instance through their effects upon leaf -form, arc known 

 to influence the characters of other parts of the plant as well, in parti nlar 

 the shape of the corolla. A parallel case is that of the Japanese Morning 

 Glory (Pharbitis Nil). Here the normal leaf-form depends upon eleven 

 (ten " dominant " and one " recessive "). The gene termed " willow " not 

 only produces the leaf-shape indicated by the name, but also causes the 

 cotyledons and corolla-segments to be narrow and makes the flower function- 

 ally unisexual (male) ; the " maple " gene, besides affecting the form of the 

 leaf, produces a polypetalous condition in the flower. These two cases ha 

 been mentioned for two reasons. First, the phenotypic characters involved 

 are more clearly of taxonomic and of biological significance than the minor 

 differences in flower-colour, which are not obviously important biologically, 

 while they are regarded as trivial by taxonomists. Secondly, these ca 

 illustrate the relation of certain genes to several distinct phenotypical char- 

 acters which appear to have no direct morphological or physiological connec- 

 tion with one another ; it is probable that most if not all genes are of this 

 pleiotropic nature. 



Linkage. 

 So far, Mendelian inheritance has been considered purely in relation to 

 genes which segregate independently of one another. In terms of the chromo- 

 some interpretation of inheritance, independently segregating genes are tl. 

 which are located in different chromosomes. As a corollary, it would seem at 

 first sight to follow that genes located in the same chromosome should always 

 be transmitted in association with one another. If this association v. 

 absolute, then clearly all the known genes of a particular plant should 

 capable of classification into a number of groups corresponding to the nun 

 of chromosomes, the members of each group being always transmitted to- 

 gether, but segregating independently of the members of any other group. 

 ' The actual state of affairs is not so simple as this. When any plant Of animal 

 is subjected to intensive gene-analysis, the genes are found to fall into a number 

 of linkage-groups, corresponding to, or at any rate aot exceeding, the char- 

 acteristic number of chromosomes. All the km m o genes - 4 Make, 6 ff example 

 (over 300), can be arranged in ten linkage-groups—corresponding to the 

 (haploid) chromosome number. The members of a links up show 



varying degrees of association in inheritance— linka< i1 is called with 



each other. In Mendelian experiments linkage reveals itself by the regular 

 occurrence of marked departures from the theoretical numerical ratio 

 segregation, certain characters appearing in association with one another 

 either more or less frequently than accords with normal expectation. 

 first case of such an aberrant ratio to be detected occurred in the early work ol 



