C. H. Waddington 
An Introduction to Modern Genetics [PDF ebook] 

First published in 1939 (second impression in 1950), this book provides an account of the changes in, and main principles of, genetics at that time. These are illustrated by references to the most authoritative and then recent investigations. Special attention is paid to the way in which genetics overlaps with other fields of inquiry, since it is often in these border-line subjects that the most important advances are to be expected. The book is particularly arranged to suit the convenience of students whose previous knowledge of genetics is small, and contains annotated bibliographies of suggestions for further reading.