Incomplete dominance - class-X

In complete dominance, the dominant allele fully masks the recessive allele. Therefore, the phenotype of the heterozygote is identical to the homozygous dominant, making the phenotypic and genotypic ratios identical in a monohybrid cross (3:1 vs 1:2:1 is not the case here, but rather the expression of traits).

Incomplete dominance: It is a type of intragenic (or interallellic) interaction where both the alleles of a given trait express as a blend (mixture) as against a normal Mendelian pattern where one allele is dominant over the other. As a resulting of this blending, an intermediate character is expressed. This situation occurs due to the fact that the dominant gene is not in a position to completely suppress the expression of recessive gene. With the result, the heterozygous offspring will be phenotypically and genotypically different from either of the homozygous parent.

Hybrid - Rr pink flower and white flower - rr
Genotypes: Rr            x      rr
Gametes:     R, r                 r
Offspring:      Rr, rr
The ratio of pink flower: white flower = 1:1

Incomplete dominance refers to a genetic situation in which one allele does not completely dominate another allele and, therefore, results in a new phenotype. It shows that the dominant allele is incompletely dominant.
Examples of incomplete dominance are:

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.

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. 

• Gregor Mendel was the founder of modern genetics. He performed experiments of on pea plant and established many rules of heredity, now referred to as the laws of Mendelian inheritance.
  
• Hugo de Vries gave the concept of genes, rediscovered laws of heredity, introduced the term mutation and gave the mutation theory of evolution.
  
• Carl Correns is known for his independent discovery of the principles of heredity and also discovered non-mendelian inheritance like cytoplasmic inheritance or cytoplasmic inheritance.
  
• Tschermark developed several new disease-resistant crops. Along with Vries and Correns, he discovered Gregor's work.
  

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’.

Gregor Johann Mendel, an Austrian Monk, discovered the principles of heredity through the experiments on the pea plant. Mendel conducted hybridization experiments on garden peas for seven years and proposed the laws of inheritance in living organisms. They are Law of dominance, Law of segregation, Law of Independent assortment. These laws are exceptional in some cases like incomplete dominance, co-dominance, etc.

So,
the correct option is ‘Mirabilis’.

A.Codominance – Phenotypic and genotypic ratio is 1:2:1

B.Dihybrid cross – Phenotypic ratio is 9:3:3:1 and genotypic ratio is 1:2:2:4:1:2:1:2:1

C.Monohybrid cross with complete dominance – phenotypic ratio is 3:1 and genotypic ratio is 1:2:1

D.Monohybrid cross with incomplete dominance - Phenotypic and genotypic ratio is 1:2:1

So,
the correct option is ‘Codominance & Monohybrid cross with incomplete dominance’.

The inheritance of flower colour in the Antirrhinum majus (snapdragon or dog flower) is an example of incomplete or partial dominance. Incompleted dominance is the phenomenon in which neither of the two alleles of a gene is completely dominant over the other. In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the $F _1$ plants obtained were pink (Rr) coloured. When the $F _1$ plants were self-pollinated, the $F _2$ generation resulted in the ratio, 1 (RR) Red :2 (Rr) Pink: 1 (rr) White. The phenotypic ratios had changed from the normal 3 : 1 dominant : recessive ratio to 1: 2: 1. R was not completely dominant over rand  this made it possible to distinguish Rr (pink) from RR (red) and rr (white). 

So, the correct answer is '1:2:1'.

Incomplete dominance is the phenomenon of neither of the two alleles being dominant  so that expression in the hybrid is intermediate between the expressions of the two alleles in homozygous state. $F _2$ phenotypic ratio is : 2 : 1, similar to genotypic ratio.

• Incomplete dominance is the condition where a dominant trait fails to show dominance completely in the heterozygous condition. For example, in snapdragon homozygous dominant results in the red color of the flower while homozygous recessive results in white flower. While heterozygous condition results in the pink color of the flower.
• Hence, in each case of incomplete domination, the heterozygous condition will result in a new phenotype.
• Following Punnet's square is representing dihybrid cross for genes A and B showing incomplete domination - 

  	AB	Ab	aB	ab
  AB	AABB	AABb	AaBB	AaBb
  Ab	AABb	AAbb	AaBb	Aabb
  aB	AaBB	AaBb	aaBB	aaBb
  ab	AaBb	Aabb	aaBb	aabb
• As we can see by counting phenotype ratio is same as genotype ratio here i.e. 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.
• So, the correct option is '1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1'. 

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. In this case, the F2 generation of a cross between dihybrids involving both the genes with incomplete dominance would result in developing eight different classes of phenotypes as evident from the calculation and phenotype variation in the Punnett square. 

• The starch synthesis in pea plants is controlled by a single gene which has two alleles B and b.
  
• BB (homozygote) seeds were round whereas, bb was wrinkled. 
• The ratio obtained is 1 (long grain): 2 (intermediate grain): 1 (small grain), therefore, out of 2000 progenies obtained 1000 progenies have intermediate size starch grain.

Exception to Mendel's laws can be explained in Mirabilis jalapa with incomplete dominance. Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes. Incomplete genetic dominance is similar to, but different from co-dominance. In co-dominance, an additional phenotype is produced, however both alleles are expressed completely. Co-dominance is exemplified in AB blood type inheritance. Example: Incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. The allele that produces the red color (R) is not completely expressed over the recessive allele that produces the white color (r). The resulting offspring are pink. The genotypes are: (RR) Red, (rr) White, and (Rr) Pink.

When the phenotypic ratio matches the genotypic ratio (e.g., 1:2:1), it indicates that the heterozygote has a distinct phenotype from both homozygotes, which is the hallmark of incomplete dominance.

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes.
Incomplete genetic dominance is similar to, but different from co-dominance. In co-dominance, an additional phenotype is produced, however both alleles are expressed completely. Co-dominance is observed in AB blood type inheritance.
Example: Incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. The allele that produces the red color (R) is not completely expressed over the recessive allele that produces the white color (r). The resulting offspring are pink. The genotypes are: (RR) Red, (rr) White, and (Rr) Pink.
In case of incomplete dominance, F$ _{2}$ generation has genotypic ratio equal to phenotypic ratio.

It is a type of intragenic (or interallellic) interaction, where both the alleles of a given trait express as a blend (mixture) as against a normal Mendelian pattern, where one allele is dominant over the other. As a resulting of this blending, an intermediate character is expressed. This situation occurs due to the fact that the dominant gene is not in a position to completely suppress the expression of recessive gene. With the result, the heterozygous offspring will be phenotypically and genotypically different from either of the homozygous parent. Following are the two familiar examples of incomplete dominance. 

Pink roses are often the result of incomplete dominance. When red roses, which contain the dominant red allele, are mated with white roses (which is recessive), the offspring will be heterozygotes, and will express a pink phenotype.
Examples are:
A snapdragon flower that is pink as a result of cross-pollination between a red flower and a white flower when neither the white or the red alleles are dominant.
A brown fur coat on a rabbit as a result of one rabbit's red allele and one rabbit's white allele not dominating.
A child with wavy hair as a result of one parent's curly hair and the other's straight hair.
An Andalusian fowl produced from a black and a white parent is blue.
A carnation that is pink that is a result of cross-pollination between a red carnation and a white carnation.
A black sheep and a white sheep mate and have a grey sheep.
A black dog and a brown dog mate and the result is a dog with a brindle coat.
A white cat and a brown cat mate and the result is an orange cat.
A tall horse and short horse mate and the offspring is off medium stature.
A big American bulldog and a small American bulldog mate and their offspring is medium-sized.
A long tailed dog and a short tailed dog mate and the offspring has a medium lengthened tail.
A blue bird and a red bird mate and the resulting offspring have mixed colors.

Incomplete dominance is a form of intermediate inheritance, in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes.
Incomplete genetic dominance is similar to, but different from co-dominance. In co-dominance, an additional phenotype is produced, however, both alleles are expressed completely.
Co-dominance is exemplified in AB blood type inheritance.
Example: Incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. The allele that produces the red color (R) is not completely expressed over the recessive allele that produces the white color (r). The resulting offspring are pink. The genotypes are: (RR) Red, (rr) White, and (Rr) Pink.

Crossing between F$ _1$ generation and the recessive parent is called a test cross. 

Inheritance of AB blood group in human follows co-dominance while inheritance of flower colour in Mirabilis jalapa follows incomplete dominance. 

A pea plant with round seeds having large starch grains is crossed with another pea plant with wrinkled seeds having small starch grains. The F1 heterozygotes formed self-pollinated. The phenotypic ratio of plants with round seeds and intermediate starch grains to plants with wrinkled seeds and larger starch grains in F2 generation are 6:1.

So, the correct option is ‘6:1’

In the dog flower Antirrhinum there are two types of flower color in pure state: red and white. When the two types of plants are crossed, the hybrid or plants of generation have pink flowers. This is due to a phenomenon called the incomplete dominance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles ( in this case the red and white color and incompletely dominant and hence the third color pink is observed when they mix).

Assume that the broad shape of leaves is governed by alleles B,b where 'B' represents dominant allele for broad leaves. Similarly, 'R' and 'r' are alleles for the colour of the flower. 'RR' progenies show red colour,' rr' progenies show white colour and 'Rr' progenies show pink colour due to incomplete domination.