Contents: I. Formal Genetics: 1. The fundamentals of Mendelism. 2. Modifications of the chromosome cycle: i. Chromosome cycles. ii. Apomixis. iii. The theory of polyploids. 3. The behaviour of individual chromosomes: i. Sex chromosomes. ii. Non-disjunction. 4. The linear differentiation of the chromosomes: i. Crossing-over. ii. Translocation. 5. The mechanics of the chromosomes. II. Genetics and development: 6. Genes and development. 7. The interaction of genes: the effects. 8. Gene controlled processes. 9. The genetic control of pattern. 10. Sex determination: i. Gamete sexuality. ii. Gamophase and zygophase sexuality. iii. Zygophase sexuality. iv. The theory of sexuality. III. Genetics and evolution: 11. Processes of evolution. 12. The genetic nature of taxonomic differences: i. Chromosome differences. ii. Cytoplasmic differences. 13. Evolutionary mechanisms. IV. Genetics and human affairs: 14. Animal and plant breeding. 15. Human genetics: i. The methods of human genetics. ii. The genetic structure of human populations. V. The nature of the gene: 16. The nature of the gene: i. Cytological considerations. ii. Developmental considerations. iii. General genetical considerations. iv. Gene mutation. v. The chromosome and the gene. Appendices: i. Laboratory methods for class-work on drosophila. ii. Bibliography and author-index. iii. Subject index.