Tag: deviation from mendelism

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. 

Dull plumage confers a survival advantage to birds by protecting them against predators while those having brighter plumage are easily spotted. However, brighter plumage helps male birds to attract female birds for mating and confers reproductive advantage. This leads to persistence of brighter plumage population despite its more chances to get captured by a predator. Thus, the correct answer is option A.

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).