Incomplete dominance (1:2:1), Biology

INCOMPLETE DOMINANCE (1:2:1)

Sometimes two genes of allelomorphic pair do not show dominant-recessive relationship, but when present simultaneously (or come together), they show intermediate condition or blend together, which is called incomplete dominance. It is due to the fact that the dominant character or gene is not in a position to completely suppress the recessive one. With the result, the heterozygote has a different phenotype (as well as different genotype) from homozygous for either allele.

  • First case of incomplete dominance was reported in Mirabilis jalapa (4 o'clock plant) by Carl Correns (1903).

Incomplete dominance is found in both plants and animals. Good examples are seen in Mirabilis jalapa, Antirrhinum majus, (snapdragon) and Andalusian fowl.

Example :- 1

In Mirabilis jalapa (4 o'clock plant) and Antirrhinum majus (Snapdragon or Dog flower) there are two types of flower colour in pure state, red and white.

A cross between a plant pure for red flowers and a plant pure for white flowers yield hybrid plants with pink flowers in F1 generation. Neither red nor white is completely dominant, so that both colour appear in the hybrids as a blend which is pink. This is obviously a contradiction of Mendel's assumption that no blending of characters takes place. When two of the hybrid plants with pink flowers are crossed, the F2 generation includes plants with red, pink and white flowers in the usual Mendelian ratio of 1 : 2 : 1.

1584_incomplete dominance.png

Incomplete dominance.

This cross shows :-

(i)         Incomplete dominance.

(ii)         The genes for red and white colour do not actually mix in the F pink hybrids as both the pure characters (red and white) reappear in the F1 plants.

(iii)        There is no specific gene for pink flowers.

(iv)        Quantitative effect of genes. The homozygous plants rr are unable to produce the flower pigment. The heterozygous plants Rr can produce only half the amount of flower pigment that is produced by the homozygous plant RR. In the F2 generation of the above cross :-

(i)         The genotypic ratio is the same as Mendelian ratio, being 1 : 2 : 1 (1 RR : 2 Rr : 1 rr).

(ii)         The phenotypic ratio differs from the Mendelian ratio, also being 1 : 2 : 1 (red : pink : white) instead of 3 : 1 (red : white).

(iii)        Half of the F2 generation show F1 phenotype instead of 3/4. 

The phenotypic Fratio of 1 : 2 : 1 is characteristics of incomplete dominance.

Example :- 2

Incomplete dominance is also found in Andalusian fowl. The Andalusian fowl is found in three colours : Black, white and blue. Pure forms are black (BB) and white (bb). If these two forms are crossed, F1 individuals appear blue (Bb) coloured. The blue hybrids on crossing with each other (Bb, Bb) give rise to one black (BB), two blue (Bb) and one white (bb) in 1 : 2 : 1 ratio. The blue appearance of the hybrids is due to very fine alternating white and black strips on the feathers.

Posted Date: 10/10/2012 4:18:07 AM | Location : United States







Related Discussions:- Incomplete dominance (1:2:1), Assignment Help, Ask Question on Incomplete dominance (1:2:1), Get Answer, Expert's Help, Incomplete dominance (1:2:1) Discussions

Write discussion on Incomplete dominance (1:2:1)
Your posts are moderated
Related Questions
Adverse Effects of amantadine  Amantadine may cause anorexia, nausea, peripheral edema and, particularly in the elderly, minor CNS effects such as nervousness, anxiety, insomn

how do viruses reproduce

VAS A DIFFRENTIA - Originate from wollfian duct . It comes out fo cauda epdidymis enters into abdominal cavity through inguinal canal, forms loop round the uretor and j

Q. What do you mean by Monographs? A monograph is defined as "the complete account as can be made at a given time of any one family, tribe or genus, nothing being neglected of

Define Nutritional Requirements of Fats and Oils? As with the other macronutrients, we do compute requirements of visible fats too for all age groups and give guidelines for se

Science as a human endeavour: Science is a human endeavour. Human beings, from prehistoric times, attempted to control nature for their own welfare. For this, they had to ob

Dosage and  administration  Menactra is given as a single intramuscular injection. The need for and timing of booster doses is unknown. An unpublished study summarized in the p

What are the infructescences, pseudofruits and parthenocarpic fruits? The Infructescences are aggregated fruits formed from inflorescences, aggregated flowers. Grape clusters a

A bee visits several flowers in succession on a single willow herb plant. In doing so, the bee transfers pollen from the younger flowers, near the top of the inflorescence (group o

Different Feeding Habits Feeding habits of animals depend on the nature of the food consumed. Herbivorous : When plants are eaten. Examples - Horse, cow, rabbit, apes.