PART I. INTRODUCTORY 5

Mendel's law of segregation 5 Linkage and chromosomes 5

Crossing-over 7

The Y chromosome and non-disjunction 8

Mutation in _Drosophila ampelophila_ 10 Multiple allelomorphs 11 Sex-linked lethals and the sex ratio 14 Influence of the environment on the realization of two sex-linked characters 16 Sexual polymorphism 17 Fertility and sterility in the mutants 18 Balanced inviability 19 How the factors are located in the chromosomes 20 The sex-linked factors of _Drosophila_ 21 Map of chromosome X 22 Nomenclature 24