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
The next step in the nitrogen cycle is the assimilation of inorganic nitrogen in the type of ammonia into organic nitrogen-having compounds. Total organisms assimilate  ammonia  by

SCLEROTI C - White of eye. Outer thick, made up of fibres connective tissue. Protective layer. Give definite shape. Its anterior 1/6 part is thin, transparent known as

Fusion of Egg and Sperm Pro-Nuclei The fusion of the sperm and egg which is the beginning of the fertilization process is completed with the fusion of the male and female pro

Haemodynamics :  The type of valves chosen should have excellent haemodynamics. In studies at rest and on exercise the valve should produce only acceptable diastolic gradients in

Describe the importance of Neuropsychological tests Neuropsychological tests are generally thought of as assessment instrument for a variety of cognitive, perceptual, and motor

How would you design, in principle, an experiment to test the hypothesis that a certain variety of lettuce seed needed daylight in order to germinate?  You would need as large

Explain about the Cheese Making - methods of food processing? Cheese is a way of preserving milk for long periods of time. In this process, the milk in cheese becomes something

What are protein hydrolysates? Proteins that have been treated with enzymes to break them down into amino acids or shorter peptides are referred to as protein hydrolysates.

What are mutagenic agents? The Mutagenic agents, or mutagens, are physical, chemical or biological factors that can cause alteration in DNA molecules. Instances of believed

What the difference is between type I diabetes mellitus and type II diabetes mellitus? Type I diabetes, also called as juvenile diabetes, or insulin-dependent diabetes (this na