a hybrid from the first filial generation is crossed with the dominant
homozygous parent it is known as an out cross. The offsprings from such a
cross will show the dominant trait but their genotype will be in the ratio of
1 homozygous dominant : 1 heterozygous dominant.
Mendel's Trihybrid Experiment
In this experiment, Mendel crossed two parents differing from each other in
three sets of contrasting characters. For example, in one of his experiments,
he crossed pure breeding plants which had tall stems, round and yellow seeds
with pure plants with dwarf stems, wrinkled and green seeds.
In certain plants such as Mirabilis jalapa, when pure line with
red petals is crossed with a pure line with white petals, it is observed that
in the F1 generation the flowers have pink petals instead of red
petals. In this cross, the red phenotype and its determining allele R is
incompletely dominant over the white phenotype and its allele r.
In overdominance, the phenotype of the heterozygous individual is more
extreme than that of either parent. For example, the amount of florescent eye
pigment in heterozygous, white-eyed drosophila is much more than what is
found in either of its parent.
In Co-dominance, the heterozygous individual shows the phenotype of both the
parents. For example, in AB blood group of man, both the alleles IA
and IB are equally expressed.
Two independent non-allelic genes affecting the same character of an
individual interact in such a way that one masks the expression of the other.
The gene which prevents the expression of the other non-allelic gene is
called as epistatic and the suppressed gene is called hypostatic.
They are three or more alternative forms of a gene (alleles) that can
occupy the same locus. However, only two of the alleles can be present in a
single organism. For example, the ABO system of blood groups is controlled by
three alleles, only two of which are present in an individual.