INBREEDING AND CROSSBREEDING 



The Effect of Inbreeding and Crossbreeding in a Wild Plant of the Sunflower Family^ 



J. L. Collins 

 University of California 



EVEN in very remote times the 

 mating of closely related individ- 

 uals in both the human race and in 

 animals was considered to give rise 

 either sooner or later to harmful or 

 even disastrous results. Although such 

 ideas contained much that was myth 

 and superstition they were certainly 

 founded on facts of such a nature that 

 they could not be overlooked. Without 

 this nucleus of fact the idea of evil 

 resulting from inbreeding could not 

 have persisted as it has through cen- 

 turies. 



It is only in the more recent of mod- 

 ern times that a logical explanation of 

 the causes of such results has been 

 made. A consistent explanation was 

 possible only after a more thorough 

 understanding of some hereditary phe- 

 nomena in general had been worked 

 out. 



INBREEDING AND HEREDITY 



In the cells of which a plant or an 

 animal is composed there are a definite 

 number of small particles of proto- 

 plasm called chromosomes. These 

 chromosomes are supposed to carry 

 the determiners called genes of factors 

 for the characters which are hereditary. 

 If the gene for a particular character 

 is not present in the chromosome, that 

 character will not be produced in the 

 mature individual. Much of our 

 knowledge relating to the causes of 

 inbreeding and crossbreeding effects 

 have come from experiments on inheri- 

 tance in maize. 



It has been shown that inbreeding 

 itself produces no evil results, but that 

 the characters which appear do produce 

 an evil result because the hereditary 

 gene or factor by which the char- 

 acter is conditioned is already present 

 in the germplasm. If no such genes 

 are present in the germinal material 

 then no amount of inbreeding can 



produce them. In fact, inl^reeding is 

 now considered as a method of testing 

 the hereditary material to determine if 

 it carries the genes for any hidden or 

 recessive characters. The fact then 

 that inbreeding produces abnormal 

 forms and a reduction of vigor in some 

 species and not in others is due to the 

 presence or absence of the genes in the 

 germplasm. For example, no such 

 results attend inbreeding in such self- 

 fertilized crops as barley or beans 

 because in them self-fertilization is the 

 normal method of reproduction and 

 these plants are homozygous for all 

 their genes, all the abnormal and weak 

 plants having long ago segregated out 

 of the race and perished in the struggle 

 against their more hardy sister plants, 

 without leaving any progeny to per- 

 petuate their kind. 



CROSSED AND SELF-FERTILIZED PLANTS 



Maize, on the other hand is a natur- 

 ally cross-fertilized plant and heter- 

 ozygosity is the general condition of 

 the germinal material instead of 

 homozygosity as in the case of the 

 self-fertilized species. In this hetero- 

 zygous condition the genes for these 

 recessive harmful or abnormal charac- 

 ters may be carried along in the chro- 

 mosome under the protection of the 

 hardy, useful, dominant characters and 

 manifest their presence only in the 

 absence of the latter. Inbreeding in a 

 naturally cross-fertilized species fur- 

 nishes conditions favorable for the 

 meeting of these recessive genes in the 

 zygotes and hence their appearance in 

 some of the individuals. 



The increase in size and vigor ob- 

 served in the progeny when two inbred 

 strains or an inbred and unrelated 

 non-inbred strains are crossed is due to 

 the establishment of a heterozygous 

 germplasm containing more dominant 

 genes influencing size and vigor than 



^ By invitation. 



89 



