Sex Determination

Since the female and male sexes occur in nearly the same ratios in both plant and animal kingdom (1:1), the thought that the determination of the sexes is also caused by hereditary factors that are inherited according to the Mendelian laws was not far-fetched. A 1:1 ratio results, as has been shown in the primula example, if continuous reciprocal crossings between homo- and heterozygous forms occur. In other words: the cross of an individual that produces two types of germ cells in equal numbers with another individual that has just one type of gamete results in a 1:1 ratio. C. CORRENS examined this statement in a series of cross-experiments with a range of different plant species and proved with the crossings of monoecious with dioecious plant species that the inheritance of the sexes follows indeed the same scheme as a back-cross does.

The best-known example is his analysis of the segregation ratios in Bryonia alba and Bryonia dioica. Bryonia alba is monoecious, i.e. in every flower exist both anthers and styles. Each plant is thus male and female at the same time. Bryonia diocia is dioecious. A female has only flowers with styles, male plants have only stamina. The crossings resulted in:

1. Bryonia alba female (male) x Bryonia alba male (female)
   F₁: 100% Bryonia alba (monoecious)
2. Bryonia dioica female x Bryonia alba male (female)
   F₁: 100% hybrids, female
3. Bryonia alba female (male) x Bryonia dioica male
   F₁: 50% hybrids female + 50% hybrids, male
   
4. Bryonia dioica female x Bryonia dioica male
   F₁: 50% Bryonia dioica female + 50% Bryonia dioica male

CORRENS concluded the following: Both crossings of a female plant with a male of the same species (1 and 4) represented the natural conditions and could thus serve as control crossings. In cross 1 (Bryonia alba), only monoecious Bryonia alba-offspring develops. Cross 4 (Bryonia dioica) results in male and female offspring in the ratio 1:1. Especially remarkable are the results of the reciprocal crossings between both species. 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 was subsequently extended on numerous plant species and nearly all animals. We will show later that the factor that determines the gender is not just a single gene, but a group of genes, whose existence is coupled to the existence of sex chromosomes (X,Y) . In general, the female sex is characterized by XX and the male by XY. Exceptions do exist, the situation with birds for example is the exact opposite. Sometimes is the Y-chromosome missing. The genotype would then be X0 (0=zero). 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, usually no discrete sex chromosomes exist. Many plants that are usually monoecious exist also as dioecious mutants and within many genera exist monoecious and closely related dioecious species.