Explain with examples that Mendel’s dominant factor fails to govern phenomenon of inheritance in many cases.


By incomplete dominance Mendel fails to govern the phenomenon of inheritance. In case of flower colour, Mendel found that red colour of flowers was dominant over white colour of flowers. The white coloured flowers reappeared in F2.

Later a deviation to this rule was observed in plant species commonly called four o clock (Mirabilis jalapa). The plant produces flowers with red and white colours. When pure breeding red (R) flowered plants were crossed with pure breeding white (r) flowered plants, the F1 plants were pink (Rr) flowered. Appearance of pink flowers, an intermediate shade between red and white, provided relief to those who believe in blending inheritance when F2 hybrids were crossed (Rr × Rr), the F2 generation showed a phenotypic ratio of 1 red : 2 pink : 1 white, instead of typical 3: 1 ratio. This ratio contradicted Mendel’s principles. Appearance of red and white factors (genes) in F2 indicates that these have not been altered or blended while present together in the pink flowered individuals. F2 reds and whites are same as the parental red and white.

This was declared as case of incomplete dominance. In this case neither gene is dominant to the other. Each expresses itself in the presence of its allele to produce an intermediate effect.

Another important aspect of incomplete dominance in that phenotypic ratio (1: 2: 1) is the same as genotypic ratio (1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive) resulting from cross when two monohybrids are crossed. When dominance is incomplete, a cross of two monohybrids (Rr × Rr) gives a phenotypic ratio of 1: 2: 1 which is identical to the genotypic ratio. The heterozygous individual shows incomplete dominance.



Incomplete dominance in four o clock.

CODOMINANCE

In case of blood types in human, both genes (A & B) produce an effect in a heterozygous individual. This is called codominance. The genes which govern A and B blood types are alleles. Each control the formation of a different red blood cell Protein or antigen. Antigen a in case of person having blood group A and antigen b in individuals with blood group B. Neither gene is dominant to the other. The heterozygous individuals with blood group AB contains both antigen a and b. Both proteins are detected in equal amounts in the red cells.
The case of incomplete dominance and codominance suggest that the dominance is not universal and absolute.

Comments

Post a Comment

Popular posts from this blog

What is the Dynamic Theory of Profit

Image
Clark's Dynamic Theory of Profit And Its Criticism The Dynamic Theory of Profit is associated with the name of an American Economist, J. B. Clark. In the world of reality, according to J. B. Clark, profit arises only in a dynamic economy. An economy is said to be dynamic when there is a change in population growth or change in the method of production or a change in the consumer’s wants, etc. A society which is without these changes is called a static society. In a static society, only monopoly profits continue to exist. All other economic profits are gradually eliminated by competition. In a dynamic society, an entrepreneur is always confronted with continuous unpredictable changes in demand for his product. The variation in demand may take place due to changes in fashions, tastes, the standard of living, distribution of income, population, new inventions, international repercussions, technological advances, etc. A prudent entrepreneur will always keep an eye on future demand
Read more

Compare the anatomy of Bifacial and Isobilateral leaves

BIFACIAL LEAF: It is also called dorsiventral leaf. This leaf is more strongly illuminated on upper surface than on lower surface. This unequal illumination induces a difference in the internal structure between upper and lower sides. Its internal structure shows: (1)        Upper Epidermis This is single layer of cells with thick cuticle which checks excessive evaporation of water from the surface. It does not contains chloroplasts, stomata are also absent. (2)        Lower Epidermis This is also single layer but with a thin cuticle. It bears many stomata, two guard cells of which contain some chloroplast, none are present in the epidermal cells. Internal to each stomata is large cavity called respiratory cavity. Lower epidermis of leaf is meant for exchange of gases (oxygen and carbon dioxide) between the atmosphere and the plant body. Excess water also evaporates from lower epidermis. (3)        Mesophyll The ground tissue lying between upper epidermis and th
Read more

osmoregulation in terrestrial and aquatic animals

Mechanism of osmoregulation is very important for all groups of animals whether inhibting land or water. Unlike plant cells, animal cells when placed in hypotonic solution burst due to the continuous absorption of water. On the contrary they would shrink and die if constantly placed in hypertonic solution. Normally uptake and loss of water are in balance for proper survival of cell. (1)        Osmoregulation in terrestrial animals: Terrestrial animals are more likely to loose water by evaporation through their permeable surfaces exposed to amsophere. Among various animal groups only Arthropods and vertebrates became the most successful land dwellers. They have developed number of strategies to maintain osmoregulation of their body fluid. (a) Water proof external coverings: To prevent water loss through external surfaces, vertebrates like reptiles, birds and mammals have water proof keratinized epidermis. Similarly the insects have developed external water proof layer called
Read more