Assertion (A): In pea seeds, the heterozygous genotype (Bb) results in a Round seed shape. Reason (R): If starch grain size is the phenotype examined, the same genotype (Bb) results in intermediate size starch grains, showing incomplete dominance.
Both A and R are true observations from the pea starch synthesis gene study. A describes dominance for shape, and R describes incomplete dominance for grain size. R does not explain why the seed shape (A) is round.
Question 472 of 519
📘 CLASS XII
Assertion (A): The phenotypic ratio 9:3:3:1 observed in the F2 generation of a dihybrid cross confirms the Law of Independent Assortment. Reason (R): The Law of Independent Assortment states that segregation of one pair of characters is independent of the other pair of characters.
The 9:3:3:1 ratio (A) is the observation that led Mendel to propose the Law (R), which states the independent segregation of two pairs of traits.
Question 473 of 519
📘 CLASS XII
Assertion (A): Mendel's work remained unrecognised until 1900. Reason (R): His concept of genes (factors) as stable and discrete units was not accepted by contemporaries due to the apparently continuous variation seen in nature.
Mendel's work was unrecognized (A) for several reasons, including the fact that his discrete, non-blending factor concept (R) contradicted the prevailing view of continuous variation.
Question 474 of 519
📘 CLASS XII
Assertion (A): Chromosomes and genes (factors) both occur in pairs. Reason (R): Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of the factors they carried.
Both occur in pairs (A). R describes the conclusion of the Chromosomal Theory of Inheritance, where the physical behavior of chromosomes (pairing/separation) explains the behaviour of genes (segregation).
Question 475 of 519
📘 CLASS XII
Assertion (A): Mendelu2019s Law of Independent Assortment does not hold true for genes that are closely located on the same chromosome. Reason (R): These genes are called 'linked genes' and exhibit high proportions of parental combinations because of their physical association.
The Law of Independent Assortment fails for linked genes (A). R explains this failure by noting that linked genes show physical association, leading to higher parental combinations rather than independent assortment.
Question 476 of 519
📘 CLASS XII
Assertion (A): The genes for yellow body and white eyes in Drosophila are very tightly linked. Reason (R): These genes showed a very low recombination frequency of 1.3 per cent.
Tightly linked genes (A) are defined by very low recombination rates (R). R provides the specific experimental evidence (1.3%) demonstrating tight linkage.
Question 477 of 519
📘 CLASS XII
Assertion (A): Human skin colour is a classic example of a polygenic trait. Reason (R): In polygenic inheritance, the phenotype reflects the additive contribution of each allele.
Human skin colour is polygenic (A). This means it is controlled by multiple genes where the effect of each allele is additive (R), leading to a range of phenotypes.
Question 478 of 519
📘 CLASS XII
Assertion (A): Phenylketonuria exhibits pleiotropy, manifesting as mental retardation and reduced hair/skin pigmentation. Reason (R): The underlying mechanism of pleiotropy is the effect of a single gene (mutation in phenyl alanine hydroxylase gene) on multiple metabolic pathways.
Phenylketonuria is pleiotropic (A). R explains this phenomenon by stating that a single gene affects metabolic pathways, leading to multiple phenotypic outcomes.
Question 479 of 519
📘 CLASS XII
Assertion (A): In human sex determination (XY type), the female is homogametic. Reason (R): Females produce only one type of ovum carrying an X-chromosome.
The female is homogametic (A). R explains this by confirming that females produce only one type of gamete (ova carrying X).
Question 480 of 519
📘 CLASS XII
Assertion (A): Pedigree analysis is a crucial tool in human genetics to trace the inheritance of specific traits or diseases. Reason (R): Since control crosses that can be performed in plants are not possible in case of human beings, studying the family history provides an alternative approach to understand inheritance patterns.
Pedigree analysis is used to trace human inheritance (A). R explains that this method is necessary because control crosses are not possible in humans.
