Question

1:2:1 phenotypic and genotypic ratios occur in case of
A. Blending/Intermediate inheritance
B. Pseudoalleles
C. Multiple alleles
D. Complementary genes

Answer 1

Hint: Phenotypic ratio is showing physical expression while the genotypic ration shows genetic makeup. In incomplete dominance, the progeny shows the intermediate character of both the parents.

Step by step answer:Blending or Intermediate inheritance – It is also known as Incomplete Dominance. It is observed in some organisms that the phenotype is neither purely dominant nor purely recessive. It is rather the intermediate between the two conditions. This means that F1 progeny resembles neither of the parents. This condition is known as Incomplete Dominance.
Example – Flower color of Antirrhinum majus or Snapdragon flower or Dog flower. When the true-breeding red flower (RR) was crossed with the true-breeding white flower (rr), then the F1 progeny had pink flowers (Rr). On crossing the F1 generation, we obtain the phenotypic and genotypic ratios as follows –
Phenotypic ratio – 1:2:1
Genotypic ratio – 1:2:1
Pseudoallele – It is also known as Linkage. The genes that are present on the same chromosome are known as the linked genes. These linked genes tend to remain together or inherit together. This tendency of the two linked genes or pseudoalleles to remain and inherit together is known as Pseudoallele or Linkage. Linkage is the opposite of crossing over because in crossing the genes segregate and in linkage they stay together. Example – Eye color of Drosophila. On crossing the F1 generation, we obtain the phenotypic and genotypic ratios as follows –
Phenotypic ratio – 3:1
Genotypic ratio – 2:2
Multiple alleles – Mendel had said that each gene has two contrasting forms or alleles. However, some such genes are found which have more than two alleles. The presence of such genes or the presence of more than two alleles for a gene is known as multiple alleles. Multiple alleles are only detected in a population and not in an individual.
Example – The gene I that determines the blood group has three different alleles namely, \({I^A}\), \({I^B}\), and \(i/{I^O}\). The combination of these alleles makes the different blood groups. A person can inherit only two alleles at the most.
The phenotypic and genotypic ratios for \(\left( {{I^A}{I^A}} \right),\left( {{I^A}{I^O}} \right),\left( {{I^B}{I^B}} \right),\left( {{I^B}{I^O}} \right),\left( {{I^A}{I^B}} \right),\left( {{I^O}{I^O}} \right)\)are as follows –
Phenotypic ratio – 2:2:1:1
Genotypic ratio – 1:1:1:1:1:1
Complementary genes – The complementary genes are the two genes that are present on separate loci. They interact together to produce a dominant phenotypic character. If any of these two genes are present alone, they cannot express themselves. Hence, both these genes are complementary to each other. Example – In Lathyrus odoratus or Sweet pea, the purple color of the flower is due to two genes. If any one gene out of these two is missing then the color of the flower will be white. On crossing the F1 generation, we obtain the phenotypic and genotypic ratios as follows –
Phenotypic ratio – 9:3
Genotypic ratio – 1:2:2:4:1:2:1:2:1
Thus, 1:2:1 phenotypic and genotypic ratios occur in the case of Blending/Intermediate inheritance.

Hence, option (A) is correct.

Note: The terms phenotypic ratio and genotypic ratio differ from each other.
Phenotypic ratio – The phenotype is defined as the externally observable character. The ratio of the phenotypes of all the offspring after a cross is known as the phenotypic ratio.
Genotypic ratio – The genotype is defined as the genetic constituent of any character. The ratio of the genotypes of all the offspring after a cross is known as the genotypic ratio.