72 The Source of Variability 



Most recorded cytoplasmic alleles exhibit a uniparental mode 

 of transmission. In higher plants and animals having large ova 

 (macrogametes) that contain large amounts of cytoplasm but hav- 

 ing only minute male pollen or sperm ( microgametes ) with es- 

 sentially no cytoplasm, only the macrogametes transmit the cyto- 

 plasmic alleles. This situation, called transovarian transmission, 

 results in a pattern of segregation in which all the progeny of the 

 pair have the maternal condition, but only the female progeny 

 can transmit the cytoplasmic characters to the succeeding genera- 

 tion. In the alga Chhmydomonas the same uniparental transmis- 

 sion of cytoplasmic genes occurred, even though both gametes 

 (called plus and minus) are of about the same size. This raises 

 puzzling but as yet unanswered questions concerning the mecha- 

 nism of transmission in these forms. Certain cytoplasmic factors 

 in Chlamydomonas demonstrate that this uniparental transmis- 

 sion is not changed even when the cytoplasmic allele is in company 

 with a chromosomal allele with which it interacts physiologically 

 (Sager, 1960). 



CHANGE IN GENETIC EFFECT 



The tremendous increase in diverse types of life from the primeval, 

 simple organisms of a billion years ago to the biotas of the present 

 reflects concurrent increase in character change. This change must 

 have been due to changes in the genetic system. 



These changes are considered to be of three kinds: (1) change 

 in chromosomal architecture (chromosomal mutation), (2) change 

 in the genes themselves ( genie mutation ) , and ( 3 ) changes in the 

 combinations of genetic determinants (recombination). 



1. Chromosomal Mutations 



Mutations of the chromosomes fall into two groups: changes in 

 chromosome number and changes in gross chromosome structure. 

 Mutations of the genes are also changes in chromosome structure 

 because genes are parts of the chromosomes. Yet it is possible for 

 the chromosomes to become reorganized without change in the 

 genes, and it is this type of rearrangement which is called chromo- 

 somal mutation. 



CHANGES IN CHROMOSOME NUMBER 



Chromosome sets may vary because of the addition or subtraction 



