If Bryonia alba is female and Bryonia dioica is male, as in cross 2, then only female hybrids develop. This result can be explained by the assumption that both the female Bryonia diocia and the monoecious Bryonia alba produce only one type of gamete. The reciprocal crossing 3 however results in a segregation, because the male Bryonia alba behave, concerning the sexes, like heterozygous hybrids and produce two different types of gametes. This means that the male sex of Bryonia alba is heterozygous, while the female is homozygous. This statement could subsequently be extended on numerous plant species and nearly all animals.
Exceptions do exist, the situation with birds for example is the exact opposite.
Sometimes is the Y-chromosome missing. Sex chromosomes of animals can easily be distinguished, a situation that is completely different in higher plants. The sex determination described here is valid for dioecious plants, but most species are monoecious, i.e. gynoecium and androecium are present on the same plant. In such a situation can usually no discrete sex chromosomes be found. Many plants that are usually monoecious exist also in dioecious mutants and within many genera exist monoecious and closely related dioecious species.