Incomplete Dominance

Not all characters follow Mendel’s Law of dominance.

Mendel's work leads to the discovery that how characters are inherited in organisms.

According to Mendel, there are two copies of each gene for each character and these copies are called as Alleles.

When two dissimilar alleles are present, then if one of the alleles expresses itself (known as Dominant allele) while another one (Recessive allele) remains nonfunctional.

For example, in pea plant with genotype [Tt], allele expresses itself and plant is tall while allele for short height remains unexpressed.

Hence based on Mendelism, here, allele 'T’ is dominant while allele 't’ is recessive.

But, there are several exceptions to Mendelism as genes are not inherited always in a similar manner.

Let us learn in detail about Incomplete Dominance.

Incomplete Dominance is such an exception of Mendelism.

In incomplete dominance, genes are not related as dominant and recessive but express themselves when presenting together in hybrid.

Hence, hybrid exhibit character intermediate to the effect of two parental alleles.

Hence, here a third character is also seen in addition to two characters that are present in the presence of homozygous conditions or non-hybrid conditions of alleles.

Let us discuss Incomplete Dominance with an Example

Inheritance of flower color in Four'o clock (Mirabilis jalapa) and Snapdragon (Dog flower or Antirrhinum) are a good example of Incomplete dominance.

In these plants, a cross between true breeding red-flowered plants (RR) and true breeding white flowers (rr) produce offspring with pink colour.

Hence, here an intermediate character (pink) is seen which is a hybrid of both red and white colour.

When Pink flowered plants are self-pollinated or crossed among themselves, plants with red (RR), pink (Rr) and white (rr) appeared in ratio 1:2:1.

Here, Phenotypic ratio is same as the genotypic ratio (1:2:1) of a monohybrid cross, but this ratio is changed from Mendelian 3 (dominant): 1 (recessive) ratio.

In , ‘R’ ( red color) was not completely dominant over ‘r’ (white color), so 'Rr’ appeared as pink which is an intermediate expression of two alleles.

There are also many other examples of incomplete dominance, apart from flower colour described previously.

Let us know about the other examples also

Colour in Andalusian fowls and Starch grain size in pea plant are such examples.

Andalusian fowl is a breed of domestic chicken and they occur in three colors: black, white and blue.

A cross between pure black (BB) and pure white (bb) produces blue hybrids.

When these F1 hybrids are crossed, they produce black, blue and white fowls in the ratio of 1:2:1.

In , gene controls the size of starch grains as starch is stored in seeds of these plants.

A homozygous BB plant produces large starch grains and homozygous bb produces small starch grains.

While heterozygous Bb produces starch grain of intermediate size, hence for the synthesis of starch grains, alleles B and b show Incomplete dominance.

Revision

Not all characters are inherited and show dominance like Mendelian characters, there are many exceptions.

Incomplete dominance is such an exception in which when hybrid alleles are present, they produce a new intermediate character.

Flower color in Snapdragon and Four O’ clock is a classic example of Incomplete dominance.

In these plants, homozygous conditions produce red (RR) and white (rr) flower, while hybrid (Rr) produces a pink color.

Colors in Andalusian fowls and starch grain size in Pisum sativum are other examples of Incomplete dominance.

The End.