Genetics and Plant Breeding

Incomplete dominance is the only option which does not obey Mendel's principles. Mendel postulated law of segregation, independent assortment, dominance, purity of gametes, etc. According to Mendel's theories, the alleles constituting the genes will separate and segregate during meiosis for the formation of gametes. It produces the expression of either dominant or recessive - dominant expresses in both homo and heterozygous conditions, recessive expresses only in homozygous condition. But incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype in which the expresses physical trait is a combination of the dominant and recessive phenotypes.

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.

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 means that one allele for a trait is not completely dominant over the other one and thus the heterozygous genotype shows a combination of dominant and recessive phenotypes. Mirabilis plant shows incomplete dominance of flower colour. Multiple allelism is a condition when more than two alleles of a gene govern the trait. Human blood group inheritance is the example of multiple alleles. It is governed by three alleles namely I$^{A}$, I$^{B}$ and I$^{A}$ and I$^{B}$ show codominance while allele i is recessive to both "I$^{A}$" and "I$^{B}$". This gives total six genotypes and four phenotypes- A (I$^{A}$ I$^{A}$ and I$^{A}$i), B (I$^{B}$ I$^{B}$ and I$^{B}$i), AB (I$^{A}$ I$^{B}$) and O (ii). 

Mendel's laws of inheritance were based on complete dominance. Phenomena like incomplete dominance, where the heterozygote shows an intermediate phenotype, were discovered later and deviate from his original findings.

Incomplete dominance is the condition when none of the factors of a gene is dominant; the phenotype of a heterozygous dominant individual is a blend of dominant and recessive traits. A monohybrid cross between two pure varieties of Mirabilis jalapa with red (RR) and white (rr) flowers gives red, pink and white colored flowers in 1:2:1 phenotypic ratio in $F _2$ generation. This is because the red color exhibits incomplete dominance over white color and the heterozygous dominant genotype (Rr) produces different phenotype from homozygous dominant (RR).  Flower color in Mirabilis jalapa is governed by a single gene. Thus, the correct answer is option B.

Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in a phenotype that is a blend or intermediate between the two parents. Thus, it is often called intermediate inheritance.

A.Law of dominance – When two contrasting alleles for a character come together in an organism, only one is expressed and another one is not expressed. Expressed character is called Dominant character and the character which is not expressed is called Recessive character

B.Incomplete dominance – Expression of new phenotypes in the progeny, other than the parents is called Incomplete dominance

C.Segregation – This law states both parental alleles of F1 generation separate and are expressed phenotypically in F2 generation

D.Independent assortment – When a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independence of the inheritance of another character

So, the correct answer is ‘Incomplete dominance’

Appearance of new phenotypes or intermediate phenotypes, other than parental phenotypes in progeny  is incomplete dominance. In this, In the F2 generation, 1 red flowered plants(RR), two pink flowered plants(Rr), 1 white flowered plants(rr) are formed. So, the phenotypic and genotypic ratio is 1:2:1.

So, the correct option is ‘1:2:1 and 1:2:1’.

Appearance of new phenotypes or intermediate phenotypes, other than parental phenotypes in progeny  is incomplete dominance. A plant having  24cm long internodes is crossed with plant having 12cm long internodes. In the hybrids 18cm long internodes appear. It is due to blending of characters. So, it is called Incomplete dominance or blending inheritance.

So, the correct option is ‘Incomplete dominance’.