Question

From the following select state true (T) and false (F):
A. The genotypic ratio of flower colour in snapdragon is exactly similar to genotypic ratio of Mendelian monohybrid cross.
B. In a diploid organism, there are four copies of each gene.
C. If we want to determine the genotype of violet flowered pea plant, then it is crossed with recessive parent.

• A. (A – T), (B – F), (C – T)
• B. (A – T), (B – T), (C – T)
• C. (A – F), (B – F), (C – T)
• D. (A – F), (B – T), (C – T)

Answer 1

Answer: (A)

Snapdragon (Antirrhinum majus) shows incomplete dominance for its flower color. True-breeding red-flowered plant (RR) was crossed with a true-breeding white-flowered plant (rr). F1 offspring (Rr) had pink flowers. Here, one allele is incompletely dominant over others so that the intermediate phenotype is produced by F1 hybrid with respect to the parents. If F1 is selfed, the plants of F2 generation are offsprings of three types, red (RR), pink (Rr), and white-flowered (rr) in the ratio of 1:2:1. In heterozygous conditions (Rr), the phenotypic effect of one allele is more pronounced than that of others, and then the mixing of both the color (red and white) occurs that in the development of pink color. The incomplete dominance is a deviation from Mendel’s Law of Dominance. The incomplete dominance showed deviation from the phenotypic ratio of the monohybrid cross of 3:1, however the genotypic ratio is the same as expected i.e. 1:2:1 (RR : Rr : rr) that is same as Mendel's monohybrid cross phenotypic ratio.
Any diploid organism has two sets of each chromosome (homologous chromosomes) and hence there will be 2 copies of each gene in the same locus of the homologous chromosomes.
By simply looking at the phenotype of a dominant trait, it is not possible to know the genotypic composition. Thus, a test cross is performed to find out the genetic makeup of an organism for a character. A cross between an individual with an unknown genotype and a homozygous recessive individual is called a test cross.