3. Overdominance. This is a type of dominance in which the heterozygote exhibits a measurable character that is significantly in excess of the characters carried by the two alleles making up the heterozygous genotype. This is an intraallelic interaction in which the two alleles in the same gene pair interact to outperform both phenotypic measurements carried by either parental alleles.
Considering the parents of a cross which possess homozygous genotypes (s1s1 and s2s2), it is the dominance relation in which the heterozygote (s1s2) exhibits a phenotypic measurement that outyields or outperforms the two parental characters. It is therefore valuable in many breeding programs both in plant and in animal breeding. For example, the breeding program may include the selection of inbreds which, when crossed, produce F1 hybrids which exhibit quantitative traits which significantly exceed those of both parental inbreds.
In this dominance relation our hypothetical heterozygote (s1s2) will have a stem length (very tall) that is so much in excess of those of the s1s1 and s2s2 stems.
Similar to incomplete dominance and codominance, the genotypes s1s1, s1s2, and s2s2 will exhibit distinct phenotypes. Therefore, the F2 progeny will consist of three phenotypes with a ratio that is identical to the genotypic ratio, that is, 1:2:1.
Incidentally, when Mendel crossed garden peas with 1 ft and 6 ft in length, hybrids (F1) were produced which varied in length between 6 ft and 7½ ft. However, he explained that the marked increase might only be due to improved vigor of the hybrids (Mendel 1865).
The effects of overdominance on the phenotypes of the F1 and F2 progeny are further demonstrated below with reference to Mendel’s experiment on stem length of garden pea.
• F1 genotype: s1s2
• F1 phenotype: very tall (long stem)
• F2 genotypes and ratio: 1:2:1 (1s1s1:2s1s2:1s2s2)
• F2 phenotypes and ratio: 1:2:1 short, very tall, and tall (1s1s1:2s1s2:1s2s2)
(Ben G. Bareja, Nov. 15, 2013)