Tag: classical genetics

Homozygous dominant red flowered plant- RR
Homozygous recessive white flowered plant- rr
Genotypes:  RR     x     rr
Gametes:     R             r
F$ _{1}$:   Rr (pink-coloured flower)
This is an example of incomplete dominance.

Red flower plant fuses with white flower plant.

Genotype        :      Red (RR)   x    White (rr)                            

Gametes         :        R                        r

In the plant Mirabilis jalapa, commonly called as four o'clock plant, the inheritance of flower colour is an example for incomplete dominance. The plant produces two types of flowers red coloured and white coloured. This condition is an example for a pair of contrasting characters. When a plant which is homozygous for red flowers (AA) is crossed with a plant which is homozygous for white flowers (aa), the plants of the F$ _{1}$ generation produce pink flowers which is a blend of red and white condition. This result clearly indicates that neither red flowered condition nor white flowered condition is dominant. However, when two hybrid plants with pink flowers (Aa) are crossed, the F$ _{2}$ generation plants show red flowered, pink flowered and white flowered condition in the ratio 1:2:1. This ratio is very much in accordance with the law of segregation.
This example very clearly indicates
1. The phenomenon of incomplete dominance
2. That the genes responsible for red and white flowers do not actually mix, since both the pure characters reappear in the F$ _{2}$ generation
3. That there is no specific gene responsible for producing pink flowers
4. That the homozygous white flowered plants have genes aa which is unable to produce     the colouring pigment
5. That the heterozygous pink flowered plants have genes Aa and hence can produce only half the amount of colouring pigment that is normally produced in a red flowered plant (AA).

 The F1-hybrid plants have a different phenotype (pink flowers) than either of the true-breeding parents. This is an example of incomplete dominance. When the F1-hybrid plants are self-fertilized, both parental phenotypes (red flowered plants and white flowered plants) reappear in the F2 generation.

Even though Mendel did not try to explain traits that did blend, other scientists eventually did. The discovery of incomplete dominance is usually credited to German Botanist Carl Correns who studied four o'clock plants. Instead of having only two colors of petals on the flowers, there was an intermediate colour that would show up that corresponded to Mendel's genotype ratio of 1:2:1 instead of his pheontype ratio of 3:1. This showed that each genotype corresponded to its own unique phenotype and that the heterozygotes were showing a blend of both alleles instead of one being completely dominant over the other.
Incomplete dominance was actually seen and recorded long before Carl Correns published his works and even before Gregor Mendel worked with his pea plants and published his findings. Since there was no discipline known as Genetics at that time, however, it was not fully explored. Scientists dating back to ancient times discussed the blending of traits in their writings which could be attributed to incomplete dominance. However, it wasn't until after Mendel and Correns that the term "incomplete dominance" came into common usage and the mechanism for how it worked was known.

Incomplete Dominance is the blending of alleles to create a phenotype that is a combination of both traits the alleles code for. Incomplete dominance is a type of non-Mendelian genetics.

As we know that trait is represented by two contrasting factors of a gene in a heterozygous individual; the allele/factor that can express itself in the heterozygous individual is called as the dominant trait. The other factor, whose effect is masked by the presence of dominant factor, is called recessive factor. In the case of complete dominance, the heterozygous individual shows dominant phenotype. Law of incomplete dominance tells that where none of the factors of a gene is dominant, the phenotype of the heterozygous dominant individual is the blend of dominant and recessive traits. For example, flower colour in Mirabilis jalapa and sweet peas (Lathyrus odoratus) shows incomplete dominance and the monohybrid cross between two pure varieties gives 1:2:1 phenotypic ratio in F$ _2$ generation which is 3:1 in dominant traits. Monohybrid cross in Oenothera lamarckiana does not give typical 3:1 ratio, hence does not follow complete dominance. Pisum sativum was the experimental plant of Mendel and show complete dominance. 

Partial dominance is also known as incomplete dominance. It occurs when the dominant allele is not able to mask the effect of the recessive allele completely. It is commonly seen in Snapdragon and Mirabilis jalapa. In F$ _2$ generation, the red allele cannot completely mask the effect of white allele resulting in a new phenotype i.e. pink.

Incomplete dominance is the condition when none of the factors of a gene is dominant and the phenotype of a heterozygous dominant individual is a blend of dominant and recessive traits. For example, flower colour in Antirrhinum majus shows incomplete dominance and the monohybrid cross between two pure varieties gives 1 red : 2 pink : 1 white phenotypic ratio in F$ _2$ generation. Here, the heterozygous genotypes show the blending of dominant and recessive phenotypes. Epistasis is the non-allelic gene interaction where the presence of one gene masks the expression of another gene; it is an attribute of polygenic trait while flower colour in Antirrhinum majus is controlled by a single gene; option B is wrong. As we know that, a trait is represented by two contrasting factors of a gene in a heterozygous individual; the allele/factor that can express itself in a heterozygous individual is called as a dominant trait. The other factor whose effect is masked by the presence of dominant factor is called as the recessive factor. A cross between pure breeding dominant and recessive varieties produces all dominant varieties which are not the case in flower colour in Antirrhinum majus; option A is incorrect. When both recessive and dominant traits are expressed in a heterozygous genotype; it is codominance; but in Antirrhinum majus, a new blending trait is expressed in heterozygous genotype, option C is incorrect. 

Incomplete dominance is the condition when the dominant allele is not able to mask the effect of recessive allele completely. This results in an organism showing a physical appearance with a blend of both the alleles. It is also called partial dominance.