Intermediary Inheritance of the Features
The rule of the dominant and recessive inheriting of the features established by
Mendel is not confirmed in all cases of hybridization. Some times the hereditary
features in heterozygotic phenotype (Aa) are displayed in an intermediary fashion.
Such a case was observed in crosses of pure lines of the garden flower plant
snapdragon (Antirrhinum majus). One of the varieties has got red flowers and the
other — white one. Their crosses have all displayed in F₁ a pink colouring of the
flowers. In F₂ the ratio of the plants with red, pink and white flowers is 1:2:1 (Fig. 2–10).
The observed intermediary inheritance of the features shows that the applicability of
Mendel’s laws do not always account for dominance and recessiveness.
image
Figure 2–10. Intermediary inheritance of the colour of the flowers in Antirrhinum majus. One of the parents is homozygotic for red colouring (AA) and the other – for white (aa). There is no dominance, that is why the flowers of the heterozygotic plants (Aa) from F₁ have a pink colour. The offspring in F₂ yields red, pink and white colours in a ratio of 1:2:1.
Polyhybrid Crossing
The crosses of plants and animals differing in more than two hereditary
features is called trihybrid, tetrahybrid and polyhybrid. For example the
thiheterozygoticity (AaBbCc) is based on the assumption that the three allelic pairs
are situated in three homologous chromosomes. Their independent
combination leads to the formation of eight types of gametes — ABC, ABc,
AbC, aBC, Abc, aBc, abC and abc in F₁, which will yield in F₂ 64
combinations each with three hereditary features. In the case of Mendel
they are distributed according to phenotype as follows:
