Assertion (A): In the dog flower (*Antirrhinum sp.*), the F1 hybrid from a cross between red (RR) and white (rr) parents is pink. Reason (R): The phenotype of the F1 generation is intermediate because the allele R is not completely dominant over the allele r, demonstrating incomplete dominance.
The F1 (Rr) in Snapdragon is pink. This is because R was not completely dominant over r, resulting in an intermediate phenotype, which is the definition of incomplete dominance.
Question 382 of 519
📘 CLASS XII
Assertion (A): Human ABO blood grouping is a good example of co-dominance. Reason (R): When the alleles $I^A$ and $I^B$ are present together in a person, they both express their own types of sugars simultaneously.
ABO blood grouping is an example of co-dominance. When $I^A$ and $I^B$ are together, they both express their sugars, which defines co-dominance.
Question 383 of 519
📘 CLASS XII
Assertion (A): The phenotype of a polygenic trait reflects the additive contribution of each allele. Reason (R): Polygenic inheritance is generally controlled by three or more genes and takes into account the influence of the environment.
In a polygenic trait, the phenotype reflects the additive contribution of each allele. R is also true, as polygenic inheritance involves multiple genes and environmental influence. However, R describes the general nature of polygenic inheritance, not why the contribution is additive (A).
Question 384 of 519
📘 CLASS XII
Assertion (A): Phenylketonuria is a pleiotropic disease resulting in mental retardation and reduction in hair and skin pigmentation. Reason (R): The underlying mechanism of pleiotropy is the effect of a single gene on multiple metabolic pathways which contribute towards different phenotypes.
Phenylketonuria is a pleiotropic disease with multiple phenotypic expressions (mental retardation, reduced pigmentation). Pleiotropy arises because a single gene affects multiple metabolic pathways.
Question 385 of 519
📘 CLASS XII
Assertion (A): The X body discovered by Henking in insects was later identified as the X-chromosome. Reason (R): The X-chromosome is involved in the determination of sex, and 50 per cent of the sperm produced in XO type insects receive this structure.
Henking discovered the X body (1891), which was later identified as the X-chromosome. It was designated the sex chromosome due to its role in sex determination, and in XO insects, 50% of sperm receive it.
Question 386 of 519
📘 CLASS XII
Assertion (A): T.H. Morgan observed a significant deviation from the expected 9:3:3:1 ratio in dihybrid crosses of *Drosophila* involving sex-linked genes. Reason (R): Genes located on the same chromosome showed physical association, described as linkage, which prevented independent segregation.
Morgan observed that the F2 ratio deviated significantly from 9:3:3:1 because the genes were located on the same chromosome. This physical association is called linkage.
Question 387 of 519
📘 CLASS XII
Assertion (A): The distance between two genes on the same chromosome can be measured by the frequency of recombination between them. Reason (R): Genes that are very tightly linked show a very low recombination percentage, while loosely linked genes show higher recombination.
Alfred Sturtevant used the frequency of recombination as a measure of the distance between genes. Tightly linked genes show very low recombination (e.g., 1.3%), while loosely linked genes show higher recombination (e.g., 37.2%).
Question 388 of 519
📘 CLASS XII
Assertion (A): Thalassemia is considered a qualitative problem, resulting in the synthesis of an incorrectly functioning globin molecule. Reason (R): Sickle-cell anaemia is a quantitative problem because it results from the reduced rate of synthesis of globin chains.
A is False: Thalassemia is a quantitative problem (too few globin molecules), while sickle-cell anaemia is a qualitative problem (incorrectly functioning globin). R is True: Thalassemia is due to reduced rate of synthesis of globin chains (quantitative problem), and sickle-cell anaemia is a qualitative problem.
Question 389 of 519
📘 CLASS XII
Assertion (A): The sex of a human child is determined by the genetic makeup of the sperm that fertilizes the ovum. Reason (R): Females produce only X-carrying ova, while males produce 50 per cent X-carrying sperm and 50 per cent Y-carrying sperm.
The genetic makeup of the sperm determines the sex of the child. This is because females produce only X ova, whereas males produce X or Y sperm, leading to XX (female) or XY (male) offspring.
Question 390 of 519
📘 CLASS XII
Assertion (A): In honey bees, male drones are haploid, having 16 chromosomes. Reason (R): Males develop from unfertilised eggs by means of parthenogenesis and thus do not have a father.
Male drones are haploid (16 chromosomes). They develop from unfertilised eggs via parthenogenesis, which is why they do not have a father.
