Chapter 4: Principles of Inheritance and Variation Assertion-Reason Questions | biology Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): In Mendel's monohybrid cross, all F1 plants were tall when tall (TT) and dwarf (tt) plants were crossed. Reason (R): The allele for tallness (T) is dominant over the allele for dwarfness (t) and masks its expression in the heterozygous condition.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 2

Logic Check

Assertion (A): In Antirrhinum (snapdragon), the F1 offspring of a cross between red and white flowered plants show pink flowers. Reason (R): This is because the allele for red colour is completely dominant over the allele for white colour.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Individuals with blood group AB show co-dominance. Reason (R): Both IA and IB alleles are expressed simultaneously in the heterozygote (IAIB), producing both A and B antigens on red blood cells.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): Mendel's Law of Independent Assortment holds true for all pairs of genes. Reason (R): Genes located on the same chromosome (linked genes) do not assort independently because they tend to be inherited together.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): In humans, the sex of the offspring is determined by the father. Reason (R): Females produce only one type of ovum (X-bearing), while males produce two types of sperm—X-bearing and Y-bearing.

Reasoning Options

ByteNotes

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 6

Logic Check

Assertion (A): In birds, the female is the heterogametic sex. Reason (R): Female birds have ZW sex chromosomes and produce two types of gametes (Z-bearing and W-bearing eggs).

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): In honey bees, male drones are haploid and develop from unfertilised eggs by parthenogenesis. Reason (R): The sex of honey bees is determined by the XY chromosomal mechanism, similar to humans.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 8

Logic Check

Assertion (A): Sickle-cell anaemia is caused by a single base change in the beta globin gene. Reason (R): The mutation changes codon GAG to GUG, replacing glutamic acid with valine at the sixth position of the beta globin chain.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): Haemophilic females are extremely rare in the human population. Reason (R): For a female to be haemophilic, she must receive the haemophilia allele from both a carrier mother and a haemophilic father, which is a very rare combination.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 10

Logic Check

Assertion (A): Down's syndrome is caused by trisomy of chromosome 21, resulting in 47 chromosomes. Reason (R): The extra chromosome arises due to non-disjunction (failure of segregation) of chromosome 21 during meiosis.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 11

Logic Check

Assertion (A): Polyploidy involves an increase in the number of individual chromosomes. Reason (R): Polyploidy results from failure of cytokinesis after telophase, leading to an increase in a whole set of chromosomes.

Reasoning Options

ByteNotes

Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 12

Logic Check

Assertion (A): Thalassemia is a qualitative disorder of haemoglobin synthesis. Reason (R): In thalassemia, there is a reduced rate of synthesis of one of the globin chains, leading to abnormal haemoglobin and anaemia.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): Phenylketonuria is an example of a pleiotropic disorder. Reason (R): The single gene mutation causing PKU leads to both mental retardation and reduction in hair and skin pigmentation.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): Turner's syndrome females are sterile. Reason (R): Turner's syndrome females have the karyotype 47, XXX and do not develop functional ovaries.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 15

Logic Check

Assertion (A): In Mendel's dihybrid cross, the F2 phenotypic ratio is 9:3:3:1. Reason (R): This ratio results from the independent assortment of two pairs of genes located on different chromosomes.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): A test cross is used to determine whether a dominant phenotype organism is homozygous or heterozygous. Reason (R): In a test cross, the organism with dominant phenotype is crossed with a homozygous dominant (TT) parent.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): Mendel's work remained unrecognised until 1900. Reason (R): One major reason was that Mendel's concept of discrete, non-blending factors was not accepted by his contemporaries who observed apparent continuous variation in nature.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): In polygenic inheritance, the effect of each allele is additive. Reason (R): In polygenic traits like human skin colour, each dominant allele contributes equally to the total phenotypic expression, resulting in a continuous range of phenotypes.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): Chromosomal aberrations are commonly observed in cancer cells. Reason (R): Loss (deletions) or gain (insertions/duplications) of DNA segments can result in alteration of chromosomes and disruption of normal gene function.

Reasoning Options

ByteNotes

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 20

Logic Check

Assertion (A): In a true-breeding line, the trait expression remains stable and uniform over several generations of self-pollination. Reason (R): True-breeding lines are homozygous for the trait, so self-pollination always produces identical offspring.

Reasoning Options

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Chapter sections

Chapter 4: Principles of Inheritance and Variation | Assertion-Reason

Assertion (A): In Mendel's monohybrid cross, all F1 plants were tall when tall (TT) and dwarf (tt) plants were crossed. Reason (R): The allele for tallness (T) is dominant over the allele for dwarfness (t) and masks its expression in the heterozygous condition.

Assertion (A): In Antirrhinum (snapdragon), the F1 offspring of a cross between red and white flowered plants show pink flowers. Reason (R): This is because the allele for red colour is completely dominant over the allele for white colour.

Assertion (A): Individuals with blood group AB show co-dominance. Reason (R): Both IA and IB alleles are expressed simultaneously in the heterozygote (IAIB), producing both A and B antigens on red blood cells.

Assertion (A): Mendel's Law of Independent Assortment holds true for all pairs of genes. Reason (R): Genes located on the same chromosome (linked genes) do not assort independently because they tend to be inherited together.

Assertion (A): In humans, the sex of the offspring is determined by the father. Reason (R): Females produce only one type of ovum (X-bearing), while males produce two types of sperm—X-bearing and Y-bearing.

Assertion (A): In birds, the female is the heterogametic sex. Reason (R): Female birds have ZW sex chromosomes and produce two types of gametes (Z-bearing and W-bearing eggs).

Assertion (A): In honey bees, male drones are haploid and develop from unfertilised eggs by parthenogenesis. Reason (R): The sex of honey bees is determined by the XY chromosomal mechanism, similar to humans.

Assertion (A): Sickle-cell anaemia is caused by a single base change in the beta globin gene. Reason (R): The mutation changes codon GAG to GUG, replacing glutamic acid with valine at the sixth position of the beta globin chain.

Assertion (A): Haemophilic females are extremely rare in the human population. Reason (R): For a female to be haemophilic, she must receive the haemophilia allele from both a carrier mother and a haemophilic father, which is a very rare combination.

Assertion (A): Down's syndrome is caused by trisomy of chromosome 21, resulting in 47 chromosomes. Reason (R): The extra chromosome arises due to non-disjunction (failure of segregation) of chromosome 21 during meiosis.

Assertion (A): Polyploidy involves an increase in the number of individual chromosomes. Reason (R): Polyploidy results from failure of cytokinesis after telophase, leading to an increase in a whole set of chromosomes.

Assertion (A): Thalassemia is a qualitative disorder of haemoglobin synthesis. Reason (R): In thalassemia, there is a reduced rate of synthesis of one of the globin chains, leading to abnormal haemoglobin and anaemia.

Assertion (A): Phenylketonuria is an example of a pleiotropic disorder. Reason (R): The single gene mutation causing PKU leads to both mental retardation and reduction in hair and skin pigmentation.

Assertion (A): Turner's syndrome females are sterile. Reason (R): Turner's syndrome females have the karyotype 47, XXX and do not develop functional ovaries.

Assertion (A): In Mendel's dihybrid cross, the F2 phenotypic ratio is 9:3:3:1. Reason (R): This ratio results from the independent assortment of two pairs of genes located on different chromosomes.

Assertion (A): A test cross is used to determine whether a dominant phenotype organism is homozygous or heterozygous. Reason (R): In a test cross, the organism with dominant phenotype is crossed with a homozygous dominant (TT) parent.

Assertion (A): Mendel's work remained unrecognised until 1900. Reason (R): One major reason was that Mendel's concept of discrete, non-blending factors was not accepted by his contemporaries who observed apparent continuous variation in nature.

Assertion (A): In polygenic inheritance, the effect of each allele is additive. Reason (R): In polygenic traits like human skin colour, each dominant allele contributes equally to the total phenotypic expression, resulting in a continuous range of phenotypes.

Assertion (A): Chromosomal aberrations are commonly observed in cancer cells. Reason (R): Loss (deletions) or gain (insertions/duplications) of DNA segments can result in alteration of chromosomes and disruption of normal gene function.

Assertion (A): In a true-breeding line, the trait expression remains stable and uniform over several generations of self-pollination. Reason (R): True-breeding lines are homozygous for the trait, so self-pollination always produces identical offspring.

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