420 



SCIENCE 



[Vol. LV, No. 1425 



contents of sueli a course as I have attempted 

 to describe by briefly outlining our own ele- 

 mentary course in genetics. In doing so, I 

 have no exaggerated idea of the importance of 

 its organization or contents. In fact, we are 

 by no means satisfied with it ourselves and are 

 continually changing it from year to year. 

 Nevertheless, for our conditions, it seems to 

 work fairly well as it now stands. It consists 

 of three lectures and one laboratory period a 

 week for a term of sixteen weeks. 



Plant Breeding 1 — Genetics 



1. The methods, problems, scope and relation- 



ships of genetics. Relation to evolution, to 

 plant and animal breeding, and to eugenics. 



2. Early theories of development and heredity. 



Preformation and predelineation. — Epigen- 

 esis. — Spencer 's physiological units. — Dar- 

 win 's pangenesis. — Naegeli 's micellse. — 

 DeVries ' intracellular pangenesis. — ^Weis- 

 mann's theory of hereditj'. 



3. The pioneer plant hybridizers. 



Camerarius' demonstration of sexuality in 

 plants. — The first plant hybrid. — The first 

 extensive series of plant hybridization ex- 

 periments by Kolreuter, his results and con- 

 clusions. — Other early plant hybridizers and 

 their contributions: Thomas Knight and 

 John Goss — the ' ' splitting ' ' of hybrids ; 

 Wiegmann and Sageret — the existence of 

 characters in contrasted pairs and the fre- 

 quent suppression in the hybrid of one 

 parental form by that of the other. — Von 

 Gartner and his classification of hybrids 

 as intermediate, comingled and decided. — 

 Naudin and his principle of the segrega- 

 tion of species potentialities. 



4. Gregor Mendel — the greatest of plant hy- 



bridizers. 



Choice of material. — Methods used and 

 characters studied. — Results in first and 

 second generation hybrids with (a) one 

 character pair, (6) two character pairs. 



5. The essential features of Mendel's hypothesis. 



Independent inheritance of single charac- 

 ters. — Alternative forms of single charac- 

 ters (allelomorphism). — Dominance and re- 

 cessiveness. — Segregation and the purity of 

 the germ cells. — Recombination. 



6. Mendel's methods of testing his hypothesis. 



Behavior in subsequent generations. — 



Backcrossing the hybrids to the parental 

 forms. 



7. Definition and illustration of Mendelian terms. 



Gamete, z}'gote, homozygote, heterozygote, 

 genotj'pe, phenotype, P , F , F^, F , etc. 



8. Further illustration of Mendelian inheritance 



and the calculation of Mendelian expec- 

 tancies. 



Mono-, di- and trihybrids with and without 

 dominance. — Backcrossing heterozygotes to 

 simple, double and triple recessives. — 

 Algebraic and checkerboard methods of cal- 

 culation. 



9. The mechanism of Mendelian heredity. 



Brief evidences for the chromosome theory 

 of heredity. — Behavior of the chromosomes 

 in mitosis. — Heterotypic and homotypjc 

 divisions. — ^Parallelism of Mendelian segre- 

 gation and chromosome segregation. — 

 Chance and probabiUty in inheritance. — 

 Points at which chance is operative. 



10. Interaction of factors. 



Interaction of allelomorphie factors: heter- 

 ozygous or ' ' unfixable ' ' characters — pink 

 -Mirabilis, double carnation; homozygous 

 dominant lethal — ^the yellow mouse, yellow 

 snapdragons, diehaete Drosophila, etc. — 

 Interaction of non-allelomorphic factors: 

 appearance of new or old characters with 

 normal Mendelian ratios — comb form in 

 fowls, plant color in maize; appearance of 

 new or old characters with modified Men- 

 delian ratios such as 9:3:4, 9:7, 13:3, 

 27:9:28, 27:37, 27:9:9:3:9:7, etc.— Du- 

 plicate and triplicate genes, 15:1 and 63:1 

 ratios. 



11. Sex inheritance and sex determination. 



The chromosome theory of sex inheritance: 

 eytological evidence ; sex-linked inheritance ; 

 evidence from parthenogenesis; miscellane- 

 ous evidences; attempts at sex controL — 

 Sex inheritance in plants: mosses, ferns and 

 liverworts; dioecious forms among the 

 spermatophytes. — Sex intergrades and gy- 

 nandromorphs : Mercurialis ; gypsy moth ; 

 Drosophila, etc. 



12. The physiological basis of sex determination. 



Hormones of sex glands and their effect 

 upon the development of secondary sexual 

 characters. — Effects of castration and 

 transplation of gonads. — Effects of nutri- 

 tion. 



