348 EXPERIMENTAL STUDY OF INHERITANCE 



For recent developments of Mendelism the student should consult 

 T. H. Morgan's Physical Basis of Heredity (1919). We cannot do 

 more than emphasise a few important propositions. 



In the history of the germ-cells there is a conjugation of homo- 

 logous chromosomes (paternal and maternal) which are subse- 

 quently assorted apart in a free or random fashion between the 

 daughter-cells, some maternal chromosomes going to one pole and 

 some to another, and similarly for the paternal chromosomes. 



But further investigation is disclosing an increasing number 

 of cases in which free assortment of genes or factors does not 

 occur, for many characters keep together in successive generations, 

 instead of assorting freety. This is called " Linkage." 



Another important fact is " Crossing Over," which means that 

 there is often an interchange of blocks of genes between homologous 

 pairs of chromosomes lying close together. 



It is practically certain that the genes are arranged in linear order 

 in the chromosomes and definitely spaced. 



In the pomace-fly, Drosophila, the number of linkage-groups 

 is the same as the number of chromosomes, and this may be a 

 general fact. 



It frequently happens that one species has twice as many 

 chromosomes as a related species, or has one particular chromosome 

 in duplicate as compared with a related species a very suggestive 

 fact. 



A particular gene may be associated with manifold effects. 

 Similar characters e.g. whiteness in poultry may be produced 

 by different genes. A particular character is often the product 

 of many genes. In short, the whole concept of a gene or hereditary 

 factor is broad and subtle. 



Impure Dominants bred with Pure Types. In the typical 

 cases discussed above, a hybrid form D(R) an impure dominant 

 is supposed to be self -fertilised or inbred. The results are accord- 

 ing to the formula I DD (pure or extracted dominants) + 2 D(R) 

 (impure dominants) -f i RR (pure or extracted recessives). 



But let us suppose the impure dominant or dominant-recessive 

 D(R) to be bred with a pure type e.g. RR (extracted recessive) 

 (in technical phrase, a heterozygote unites with a homozygote). 

 The impure dominant has, by hypothesis, equal numbers of 



