accomplished through the gametes four combinations are possible — 1 (AA),
2 (Aa), 1 (aa). Since the dominant gene for yellow colouring (A) participates in
three combinations it follows that in the offspring of F2 the ratio of the yellow
seeds to the green ones in phenotype is 3:1.
Not always the phenotype (totality of the displayed features
and properties of the organism) is a reflection of the genotype
(the totality of all hereditary features in the organism).
Individuals of an identical phenotype can be differing in the
genotype. In order to understand the genotype of a given
organism after a definite feature, a number of crosses are
required to be carried out in the course of several generations
until the so-called pure (inbred) lines are obtained. Phenotype
and genotype can be homozygotic when the father’s
and mother’s allelic couples are identical in the determination of
a given feature (AA or aa), or heterozygotic — when they differ (Aa).
The emergence of the recessive features in F2 shows
that the recessive genes in the hybrid individuals do not
disappear and are transmitted in
the progeny. Besides, dominant and recessive genes are transmitted in the
posterity independently of one another. This regularity became known as
the first law of Mendel.
image
Figure 2–8. Illustrative presentation of the first law of Mendel on the uniformity of individuals in F1 hybrids and the segregation of features in the progeny which determines the 3:1 ratio of the dominant to the recessive in the second generation (F₂).
Dihybrid Crossing
Mendel has carried out experiments for crosses between varieties differing in
two features as well. One of them was characterized with round yellow seeds
and the other one — with wrinkled green seeds. Since the genes determining
the yellow colour and the round shape have been dominant then the whole
posterity in F₁ was with round yellow seeds. When crosses in F₁ among
themselves were performed, then in F₂ except the initial phenotypes (round
yellow and wrinkled green seeds) two new (recombinant) phenotypes have