If an egg with _A_ is fertilized by a pollen with _a_ (or _vice versa_), after fertilization the chromosome constitution of the fertilized egg is _Aa_. All the other homologous chromosomes are identical and therefore need not be considered. All the nuclei of the F{ 1} generation have the chromosome constitution _Aa_. All will form eggs and pollen with nuclei of the same chromosome constitution _Aa_, but all these sex cells will go through the maturation division before they are fertilized; and this reduction division leads to the existence of two kinds of eggs in equal numbers, one containing only the _A_, the other only the _a_ chromosome; and the same happens in the pollen. When therefore the hybrids F{ 1} are mated among themselves, the following four chromosome combinations will be produced:
[Transcriber's note: Diagram illustrating the four possible pairings of chromosomal characters]
Possible combinations in fertilized eggs _AA_, _Aa_, _aa_, in the ratio 1:2:1.
Now this is exactly the ratio of Mendelian heredity in the F{ 2} generation. The plant with the chromosome constitution _AA_ will form violet flowers, those with the chromosome constitution _Aa_ will form pale violet flowers, and those with the chromosome constitution _aa_ will form white flowers.
The result would be expressed by the formula _AA:_ _Aa:_ _aa_ which is the same as that given for any character in a Mendelian case. Thus the phenomena of germ cell division and of heredity are seen to have the same essential features viz., purity of units (chromosomes, characters) and the independent transmission of the same; while as a corollary it follows in each case that each of the two antagonistic units (chromosomes, characters) is contained by exactly half the gametes produced.
It is obvious that Sutton by this idea did for heredity in general what McClung had done for sex determination or sex heredity, that is, he showed that the numerical results obtained in Mendelian heredity can be accounted for on the basis that factors for hereditary characters are carried by definite chromosomes. The cytological basis of sex determination becomes only a special case of the cytological basis of Mendelian heredity. In the examples quoted the plants giving rise to violet and to white flowers are homozygous for the colour of flower having the chromosome constitution _AA_ and _aa_ respectively; while the plants with pale violet flowers are heterozygous, having the chromosome constitution _Aa_ in their nuclei. The former give rise to identical sex cells _A_ and _A_ or _a_ and _a_; while the heterozygous plants give rise to different sex cells _A_ and _a_.