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519 questions in this collection · Page 50 of 52
Question 491 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): Females affected by Turner’s Syndrome are sterile. Reason (R): Turner’s Syndrome is caused by the absence of one of the X chromosomes (45, X0), leading to rudimentary ovaries and lack of secondary sexual characters.
Sterility (A) is a key feature of Turner’s Syndrome. R provides the underlying chromosomal cause (45, X0) which results in rudimentary ovaries and lack of secondary sexual characteristics, directly explaining the sterility.
Question 492 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): Deletions and insertions of base pairs of DNA cause frame-shift mutations. Reason (R): Frame-shift mutations, alongside point mutations and recombination, are phenomena that lead to variation in DNA.
A is True: Deletions/insertions cause frame-shift mutations. R is True: Frame-shift mutations lead to DNA variation. R correctly categorizes A but does not specifically explain why deletions/insertions *cause* the frame shift (A).
Question 493 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): In polygenic inheritance, human skin colour exhibits a whole range of possible phenotypes. Reason (R): The phenotype reflects the additive contribution of each allele, where the number of each type of allele in the genotype determines the darkness or lightness of the skin.
A describes the observable outcome of polygenic inheritance (a gradient of phenotypes). R explains the genetic basis of this gradient: the effect of each allele is additive, determining the degree of the trait expressed.
Question 494 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): In sickle-cell anaemia, the substitution of Glutamic acid by Valine occurs at the sixth position of the beta globin chain. Reason (R): This amino acid substitution results due to a single base substitution at the sixth codon of the beta globin gene from GAG to GUG.
A is the result (amino acid change). R is the cause (codon change). The codon change (R) leads directly to the amino acid change (A).
Question 495 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): In gamete formation, the alleles of a parental pair separate or segregate from each other such that a gamete receives only one of the two factors. Reason (R): Since the segregation of alleles is a random process, there is a 50 per cent chance of a gamete containing either allele.
A is the Law of Segregation. R explains the random, measurable consequence of this principle (50% probability for either allele).
Question 496 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): Henking named the nuclear structure he observed during insect spermatogenesis the ‘X body’. Reason (R): He could not explain the significance of this structure at the time of its discovery.
Henking named the structure the X body (A) because he could not explain its significance (R).
Question 497 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): In birds, the female is the heterogametic sex (ZW). Reason (R): The male is homogametic (ZZ) and produces only one type of gamete (Z), while the female produces two different types of gametes (Z and W).
A states that the female is heterogametic. R explains this by detailing that the female produces two different gamete types (Z and W).
Question 498 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): Chromosomal aberrations are commonly observed in cancer cells. Reason (R): Chromosomal aberrations are caused by the loss (deletions) or gain (insertion/duplication) of a segment of DNA.
A is True: Chromosomal aberrations are seen in cancer cells. R is True: R defines the mechanism of chromosomal aberrations. While R underlies A, R does not specifically explain why this type of abnormality is linked to cancer (A).
Question 499 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): The recessive character is only expressed in the homozygous condition (tt). Reason (R): The Law of Dominance states that in a heterozygous condition (Tt), the dominant factor (T) dominates and masks the expression of the recessive factor (t).
The recessive trait requires homozygosity for expression (A). R explains that in the heterozygous condition, the dominant factor suppresses the recessive factor, making the homozygous condition necessary for the recessive trait to appear.
Question 500 of 519
📘 CLASS XII
Principles Of Inheritance And Variation
Assertion (A): The descriptive terms tall and dwarf are the phenotype of a pea plant. Reason (R): The allelic pair of genes, such as TT or tt, is referred to as the genotype of the plant.
A is True: Phenotype refers to the descriptive appearance. R is True: Genotype refers to the allelic composition. R defines genotype, and A defines phenotype; they are separate definitions and R does not explain A.