Section IS — Plant Genetics and Breeding 



lines, exceeding Fi in vigour. To study the effect 

 of heterosis several features are analyzed : 

 height of plant, length and width of leaves, 

 length and number of internodes, number of 

 days from seeding to pollen shedding. From 

 the experimental data total sum of leaf surfaces 

 per plant was computed. 



To establish the differences between the par- 

 ents, Fi and the stable vigorous line No 10 

 (cross WD W9) an experiment in randomized 

 blocks was performed. The differences in all 

 features are significant and statistically proved. 

 As concerns the heterosis for earliness, it was 

 stated that in case of mutual differences in 

 earliness of parents less than 7 days, their Fi 

 exhibited pronounced heterosis for earliness. 

 If the difference surpassed 10 days, Fi was asym- 

 metrically located between the parents and clo- 

 ser to the earlier parent. A significant positive 

 correlation has also been observed between the 

 number of days from seeding to pollen shedding, 

 expressed by mean value of both parents, and the 

 plant height of their Fi (/• = 0.905 + 0.034). 



Heterosis for earliness can be explained by 

 presence of dominant genes reducing the num- 

 ber of days from seeding to pollen shedding. 

 Besides, additive genes exhibiting blending in- 

 heritance can participate in determining the 

 time of flowering. The vigorous growth is 

 probably due to interaction of complementary 

 genes, which assure more intensive growing 

 rate of longer duration in vigorous lines than 

 in parents. 



13.37. Gene Action in the Inheritance of Quantitative 

 Traits. H. F. Robinson and R. H. Moll 



(Raleigh, U.S.A.). 



be presented. These results provide a basis for 

 evaluation and development of breeding proce- 

 dures as will be illustrated in a discussion of 

 reciprocal recurrent selection. The relevance of 

 the results to the explanation of the issue of 

 heterosis will be given. 



13.38. Influence of Radiation on the Combining 

 Ability of Inbred Lines of Maize. D. L. Palen- 

 zona and R. E. Scossiroli (Pavia, Italy). 



A problem which may bring more under- 

 standing on heterosis in corn, and may have 

 important implications in practical research, 

 concerns the possibility to change under radia- 

 tion the combining ability of inbred lines of corn. 



We know the consequence of this phenomenon. 

 Some inbreds possess a general high combining 

 ability in crosses when final yield is considered, 

 while others show high combining ability only 

 with specific inbreds but low combining ability 

 with others. Is it possible to change under 

 radiation the combining ability from high to 

 low or vice versa? Is it possible using radiation 

 to improve further the combining ability of an 

 already good high combiner? 



A series of inbred lines derived from an auto- 

 diploid strain were used to establish a pilot ex- 

 periment to test if radiation applied to the origi- 

 nal stock could be effective in changing its 

 combining ability when used as pollen parent 

 in crosses with a good combiner, the well- 

 known inbred WF 9. 



The results obtained show that radiation may 

 change combining ability in crosses when yield 

 of single crosses is considered. 



Mathematical approaches to the study of gene 

 action in maize have involved development of 

 procedures and experimentally estimating the 

 additive genetic, dominance and epistatic var- 

 iances for complex characters such as yield of 

 grain. The research of the past fifteen years has 

 provided a basis for concluding that (1) ad- 

 ditive genetic variance exists in open-pollinating 

 varieties in sufficient quantity to allow for prog- 

 ress in yield improvement from selection; 

 (2) overdominance is not the predominant type of 

 action of genes concerned with yield; (3) the 

 existance of appreciable epistasis has not been 

 demonstrated and (4) genotype environmental 

 interactions are very important in quantitative 

 genetic studies and in research on breeding 

 methodology. The results of the extensive re- 

 search program on quantitative genetics will be 

 summarized and implications of the results will 



13.39. Response to Selection in Tangiiis Cotton. 



Jose A. Giles (Lima, Peru). 



A method based on phenotypic selection was 

 used in Peru for seven years to improve Tangiiis 

 variety (G. barbadense). Individual plants were 

 selected in the field and their fiber characteristics 

 analyzed. Selected plants were sown in progeny 

 rows. Progenies with best appearance were 

 chosen and put into yield trials and also propa- 

 gated in increased blocks. To start a new cycle 

 of selection, individual plants were selected from 

 progeny rows and increased blocks. 



A selection differential of the average differen- 

 ce in phenotypic value between selected progenies 

 and overall progenies was obtained for number 

 of bolls per plant, lint percent, lint index, boll 



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