Question

The 1:2:1 ratio with the pink flower in $$F_2$$ generation indicate the phenomenon of
A. Dominance
B. Codominance
C. Incomplete dominance
D. Segregation

Answer 1

Hint: Alleles are variations of a gene occupying the same position on a chromosome. Alleles are always present in pairs. One allele out of the pair, can be dominant or recessive. However, it has been proved by experiments that alleles can be codominant or one allele can incompletely dominate the other.

Complete answer:
Option (a) is incorrect. Dominance is the phenomenon when one allele out the two of a pair, is capable of completely masking the expression of the other allele. Complete dominance can be seen in crosses between a true breeding tall plant with genotype (TT) and a true breeding short plant of genotype (tt). The F1 generation yields all heterozygous tall plants with the genotype (Tt). If the $F_1$ individuals are crossed, the $F_2$ generation yields 1 homozygous (TT) tall plant, 2 heterozygous (Tt) tall plants and one homozygous (tt) short plant. That is the genotypic ratio is 1:2:1. Here, the allele for tall plants completely dominates the allele for the short plant and masks its expression.
Option (b) is incorrect. Codominance is the phenomenon that results when both of the alleles in a pair are expressed. That is to say, neither allele is able to mask the expression of the other. For example, in the human blood grouping system, parents with blood group A and B will always give rise to an offspring with blood group AB. In this case, the alleles for blood groups A and B are codominant.
Option (c) is correct. Incomplete dominance is the phenomenon when an allele is unable to completely dominate the other allele in a pair. This results in an intermediate phenotype. For example, a true breeding red flower with (RR) genotype is crossed with a true breeding white flower with (rr) genotype. It results in the production of heterozygous pink flowers with genotype Rr in $F_1$ generation. When the Rr flower are self-pollinated, the $F_2$ generation yields 1 (RR) red flower, 2 pink (Rr) flowers and 1 white (rr) flower, that is a genotypic ratio of 1:2:1. The pink colour of the flower is due to the incomplete dominance of the allele R over r.
Option (d) is incorrect. Segregation is the process of passing alleles to the gametes in a random manner. To elaborate, a parent organism contains a pair of alleles for a trait. While gametes form in the parents, these alleles segregate from the pair and only one allele is passed on to the gametes. This process of allele segregation is random.

Hence, the correct answer is option (C).

Note: Characters or traits are influenced by alleles, which are different forms of a gene. Alleles are generally present in pairs. In the pair, one allele can dominate, codominant or incompletely dominate the other allele. Incomplete dominance gives rise to a new phenotype.