Assertion (A): The F1 hybrid RrYy produces four types of gametes (RY, Ry, rY, ry), each with 25 per cent frequency. Reason (R): This 1:1:1:1 gamete production ratio demonstrates that the segregation of the R/r gene pair is independent of the segregation of the Y/y gene pair.
The four gamete types (A) are produced in equal 1/4th (25%) frequency because the segregation of the two gene pairs is independent of each other (R), as per the Law of Independent Assortment.
Question 432 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 are not possible in humans, studying the family history provides the necessary alternative data to understand inheritance patterns.
Pedigree analysis is used to trace inheritance in humans. This method is necessary because humans cannot undergo controlled crosses like pea plants, making R the reason for A.
Question 433 of 519
📘 CLASS XII
Assertion (A): Mendelian disorders are determined by alteration or mutation in a single gene, traceable by pedigree analysis. Reason (R): Chromosomal disorders are caused due to the absence or excess or abnormal arrangement of one or more whole chromosomes.
A is True: Mendelian disorders involve single gene mutations and can be traced by pedigree analysis. R is True: Chromosomal disorders involve changes in chromosome number or arrangement. Both A and R describe correct facts about the two disorder types, but R does not explain why Mendelian disorders are traceable by pedigree analysis (A).
Question 434 of 519
📘 CLASS XII
Assertion (A): Sutton and Boveri noted that the pairing and separation of a pair of chromosomes would lead to the segregation of the factors they carried. Reason (R): This parallel behavior provided the physical basis for Mendel's principles, forming the foundation of the Chromosomal Theory of Inheritance.
Sutton and Boveri observed the parallel behavior (A). They united this with Mendelian principles, calling it the chromosomal theory of inheritance, because chromosome movement paralleled gene movement, providing the missing physical proof Mendel lacked.
Question 435 of 519
📘 CLASS XII
Assertion (A): The $F_2$ phenotypic ratio 9:3:3:1 in a dihybrid cross validates the Law of Independent Assortment. Reason (R): This ratio is mathematically derived as a combination series of the 3:1 ratio for each character, showing independent segregation of traits.
The 9:3:3:1 ratio supports independent assortment. R correctly explains that this ratio is derived from multiplying two independent monohybrid ratios, which confirms the independence of the segregation process.
Question 436 of 519
📘 CLASS XII
Assertion (A): Drosophila melanogaster is highly suitable for genetic studies because it completes its life cycle in about two weeks. Reason (R): A short life cycle and the ability to produce a large number of progeny flies from a single mating allow for quick observation of successive generations and gathering of large data sets.
The short life cycle (two weeks) and large progeny count are key reasons for Drosophila suitability. R explains why these characteristics are advantageous for genetic studies.
Question 437 of 519
📘 CLASS XII
Assertion (A): A female becomes haemophilic only if she is homozygous recessive ($X^h X^h$). Reason (R): This condition is extremely rare because it requires the affected male father ($X^h Y$) to survive the late stages of life long enough to mate with a carrier female ($X^H X^h$).
Haemophilia is X-linked recessive, requiring $X^h X^h$ for expression in females (A). R explains why this necessary cross is rare (carrier mother + haemophilic father).
Question 438 of 519
📘 CLASS XII
Assertion (A): Colour blindness occurs much more frequently in males (8%) than in females (0.4%). Reason (R): The genes for colour blindness are located on the X chromosome, and males are hemizygous (having only one X), so they express the trait if they inherit a single recessive allele.
A is True: The prevalence rates are stated accurately. R is True: The X-linked nature means males express the trait with one recessive gene, while females need two, which explains the skewed prevalence.
Question 439 of 519
📘 CLASS XII
Assertion (A): Mutation is a phenomenon that leads to variation in DNA, serving as a source of heritable change. Reason (R): In addition to recombination, mutation is another major phenomenon that leads to variation in the DNA sequences of an organism.
Mutation results in the alteration of DNA sequences and consequently changes in the genotype and phenotype. R emphasizes that mutation, alongside recombination, is a major source of DNA variation, correctly positioning A within the broader context of genetic variation.
Question 440 of 519
📘 CLASS XII
Assertion (A): Genes are the units of inheritance and contain the information required to express a particular trait. Reason (R): These units, which Mendel called ‘factors’, are stably passed down, unchanged, from parent to offspring through the gametes.
Genes (Mendel's factors) are the units of inheritance that contain the required information. They are stably passed down unchanged through gametes.
