Fission Rule (Second Mendelian Rule)

Definition and example - easily explained

The fission rule states that in the case of a heterozygous parental generation (both flowers with wR), the branch generation1 splits into different phenotypes.

* In dominant-recessive heredity, 3/4 of the flowers show the phenotype of the dominant gene (R). Because every flower that carries at least one dominant (R) gene will have a red appearance. This applies to the one pureblood (RR), as well as the two mixed (Rw) flowers.

In contrast, 1/4 of the flowers characterize the recessive gene phenotype (w). When crossing two heterozygous flowers, the recessive genes (w) and (w) also combine to form white flowers (ww).

While the phenotype develops in the ratio 3 (red) to 1 (white), the genotype has a ratio of 1 (homozygous red) to 2 (heterozygous red) to 1 (homozygous white).

* Intermediate inheritance differ in the phenotypic feature expression in this respect only of the dominant-recessive inheritance, as that heterozygous flowers receive the mixed color. This gives a ratio of 1 (homozygous red) to 2 (heterozygous pink) to 1 (homozygous white).

Fission rule in dominant-recessive inheritance

Fission rule in intermediate inheritance

Summary

The branch generation1 of a heterozygous parental generation with identical feature expression (same color) splits into different phenotypes