Chapter 4: Principles of Inheritance and Variation Long Questions | biology Class 12 CBSE

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Chapter 4: Principles of Inheritance and Variation Long Questions | biology Class 12 CBSE | ByteNotes.in

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Question 1

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Analyse how Mendel's experimental design ensured the validity and reliability of his results, and explain the three laws he proposed based on his findings.

Mendel used true-breeding pea plants as parents, ensuring homozygosity and eliminating pre-existing genetic variation. His large sample sizes gave statistical credibility, and confirmation across successive generations proved his results reflected general rules rather than chance.
Law of Dominance: In a pair of dissimilar alleles, one (dominant) is expressed in F1 while the other (recessive) is masked. This explains why all F1 plants in a monohybrid cross resemble one parent.
Law of Segregation: Alleles of a gene pair segregate randomly during gamete formation so that each gamete carries only one allele. This explains the 3:1 phenotypic ratio and the 1:2:1 genotypic ratio at F2.
Law of Independent Assortment: When two or more gene pairs are crossed, the segregation of one pair is independent of the other. This produces a 9:3:3:1 phenotypic ratio in F2 of a dihybrid cross.
Mendel's use of mathematical logic to explain biological data and his analysis across multiple generations transformed genetics into a quantitative science, establishing the conceptual foundation for all subsequent developments in heredity.

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Question 2

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Compare complete dominance, incomplete dominance, and co-dominance with examples and explain the phenotypic and genotypic ratios at F1 and F2.

Complete dominance: F1 resembles one parent (dominant). In pea plants (TT x tt), all F1 are tall (Tt). F2 phenotypic ratio is 3:1 (tall:dwarf); genotypic ratio is 1:2:1.
Incomplete dominance: Neither allele dominates fully; F1 shows an intermediate phenotype. In Antirrhinum (RR x rr), F1 is pink (Rr). F2 ratio is 1 red : 2 pink : 1 white — genotypic and phenotypic ratios are both 1:2:1.
Co-dominance: Both alleles are fully expressed simultaneously in heterozygotes. In ABO blood groups, IAIB genotype produces blood group AB — both A and B antigens are present on RBC surfaces.
Dominance depends on the gene product and which phenotypic trait is examined. If a modified allele produces a non-functional enzyme, the unmodified allele's product determines the trait — that allele is dominant.
A single gene (like B/b controlling starch in pea seeds) can appear as dominant for one phenotype (seed shape — round dominant) but show incomplete dominance for another phenotype (starch grain size — intermediate in Bb).

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Question 3

Long Form

Explain the Chromosomal Theory of Inheritance and how Morgan's experiments on Drosophila supported and extended it through linkage and recombination.

Sutton and Boveri (1902) proposed that genes are located on chromosomes, noting parallel behaviour: both occur in pairs, segregate during gamete formation, and different pairs assort independently.
Morgan verified this using Drosophila melanogaster — an ideal model organism with a two-week life cycle, large progeny numbers, clear sexual dimorphism, and many hereditary variations visible under low-power microscopy.
Morgan discovered that genes on the same chromosome do not assort independently — they show linkage. Dihybrid crosses with sex-linked genes (yellow body, white eye) produced F2 ratios deviating significantly from 9:3:3:1.
Tightly linked genes (e.g., white and yellow in Drosophila, 1.3% recombination) stay together more frequently, while loosely linked genes (e.g., white and miniature wing, 37.2% recombination) separate more often due to crossing over.
Alfred Sturtevant used recombination frequencies as a measure of distance between genes and constructed the first genetic (linkage) maps, foundational tools later used in human genome sequencing.

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Question 4

Long Form

Explain different types of sex determination mechanisms with suitable examples and state the significance of the Henking observation in the discovery of sex chromosomes.

Henking (1891) traced a specific nuclear structure (X body) during spermatogenesis in insects and noted only 50% of sperm received it. Later identified as the X chromosome, this observation laid the foundation for chromosomal sex determination.
XO type (e.g., grasshopper): Males have one X chromosome plus autosomes; females have two X chromosomes. Males are heterogametic. Females have more chromosomes than males.
XY type (e.g., humans, Drosophila): Females are XX, males are XY. Males produce X-bearing and Y-bearing sperm; the type of sperm that fertilises the egg determines offspring sex. In humans, 22 pairs are autosomes and the 23rd pair are sex chromosomes.
ZW type (e.g., birds): Males are ZZ (homogametic) and females are ZW (heterogametic). Females produce two types of eggs — this is female heterogamety.
Haplodiploid (honey bee): Fertilised eggs become diploid females (32 chromosomes); unfertilised eggs become haploid males (16 chromosomes) by parthenogenesis. Males produce sperm by mitosis and have no fathers.

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Question 5

Long Form

Describe the molecular basis of sickle-cell anaemia, its mode of inheritance, and explain how it differs from thalassemia at the molecular level.

Sickle-cell anaemia arises from a single base substitution — GAG to GUG — at the sixth codon of the beta-globin gene. This replaces glutamic acid with valine in the HbS haemoglobin molecule.
HbS haemoglobin polymerises under low oxygen tension, distorting red blood cells from a biconcave disc to a sickle shape, reducing oxygen delivery and causing vessel blockage.
It is autosomal recessive, controlled by alleles HbA and HbS. Only homozygous HbSHbS individuals show the disease; heterozygotes (HbAHbS) are carriers with a 50% probability of passing the mutant allele.
Thalassemia is a quantitative defect — too few globin chains are synthesised — caused by mutations or deletions in alpha-globin genes (HBA1, HBA2 on chromosome 16) or beta-globin gene (HBB on chromosome 11).
Sickle-cell anaemia is a qualitative problem (incorrectly functioning haemoglobin), while thalassemia is a quantitative problem (insufficient haemoglobin synthesis); both are autosomal recessive disorders transmitted by carrier parents.

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Question 6

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Explain the Punnett square method for a dihybrid cross and derive the expected phenotypic ratio at F2. What does this ratio reveal about the relationship between gene pairs?

In a dihybrid cross (RRYY x rryy), F1 plants are all RrYy (round, yellow). The F1 produces four gamete types — RY, Ry, rY, ry — each with a frequency of 25%, due to independent assortment.
Placing these four gametes on both axes of a 4x4 Punnett square produces 16 possible zygote combinations representing all possible genotypes in F2.
Phenotypic ratio in F2 is 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green (9:3:3:1), which can be derived as (3 round : 1 wrinkled) x (3 yellow : 1 green).
This ratio demonstrates that the two gene pairs (R/r and Y/y) segregate independently — the inheritance of seed colour is not influenced by seed shape and vice versa.
Genotypically, F2 produces 9 different genotypes. The 9:3:3:1 ratio is only obtained when the two gene loci are on different chromosomes (unlinked); linked genes deviate from this ratio.

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Question 7

Long Form

Explain pedigree analysis with reference to the tools it uses and how it helps differentiate between dominant, recessive, autosomal, and sex-linked inheritance patterns.

Pedigree analysis is the representation and study of trait inheritance across multiple generations in a family tree, using standardised symbols (squares for males, circles for females, shaded for affected).
Controlled crosses cannot be done in humans; hence pedigree analysis is the primary tool in human genetics to trace the transmission of traits, abnormalities, and diseases.
Dominant traits: if a trait skips a generation, it is likely recessive; if every affected individual has an affected parent, the trait is dominant. Recessive traits can appear in children of unaffected carrier parents.
Sex-linked inheritance (e.g., haemophilia, colour blindness): more males are affected than females; an unaffected carrier mother passes the allele to sons; daughters of affected fathers are obligate carriers.
Autosomal inheritance affects both sexes equally; the trait appears regardless of sex. Pedigree analysis helps determine the probability of a disorder in future offspring, aiding genetic counselling.

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Question 8

Long Form

Describe the chromosomal basis and clinical features of Down's syndrome, Klinefelter's syndrome, and Turner's syndrome, explaining how each arises.

These are chromosomal disorders caused by aneuploidy — gain or loss of individual chromosomes resulting from non-disjunction during cell division.
Down's syndrome: trisomy of chromosome 21 (47 chromosomes). Features include short stature, small round head, furrowed tongue, partially open mouth, flat face, broad palm, congenital heart disease, and mental retardation.
Klinefelter's syndrome: karyotype 47 XXY (extra X chromosome). Affected individuals show overall masculine development but also gynecomastia (breast development) and are sterile.
Turner's syndrome: karyotype 45 XO (one X chromosome absent). Affected females are sterile with rudimentary ovaries, short stature, and lack of secondary sexual characters.
All three are detectable by karyotyping. Down's syndrome is autosomal, while Turner's and Klinefelter's involve sex chromosome aneuploidy. Non-disjunction can occur during meiosis I or II in either parent.

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Question 9

Long Form

Explain the concept of mutation, its types, and discuss how various mutagenic factors contribute to genetic variation and human disorders.

Mutation is a heritable change in the DNA sequence that alters the genotype and consequently the phenotype. Along with recombination, it is a primary source of genetic variation.
Point mutation: change in a single base pair (e.g., GAG to GUG in sickle-cell anaemia). Frame-shift mutations arise from insertion or deletion of base pairs, altering the reading frame.
Chromosomal mutations: deletions, insertions, or duplications of DNA segments cause chromosomal aberrations, commonly observed in cancer cells. Loss or gain of chromosome segments alters gene dosage.
Aneuploidy (change in number of individual chromosomes) and polyploidy (increase in whole chromosome sets) are numerical mutations. Polyploidy is exploited in plant breeding; aneuploidy underlies disorders like Down's syndrome.
Mutagens are physical or chemical agents that induce mutations. UV radiation is a well-known physical mutagen. Understanding mutagens is essential for cancer biology, radiation safety, and drug development.

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Question 10

Long Form

Using the example of ABO blood groups, explain the concepts of co-dominance, multiple alleles, and the relationship between genotype and blood group phenotype.

The ABO blood group system is controlled by gene I with three alleles: IA, IB, and i. The plasma membrane of RBCs carries sugar polymers whose type is controlled by these alleles.
IA and IB are co-dominant — when both are present, both A-type and B-type sugars are expressed, resulting in blood group AB. Neither allele masks the other.
IA and IB are each dominant over i — IA causes blood group A even in the presence of i (IAi = group A); IB causes blood group B even with i (IBi = group B); ii = group O (no sugar produced).
Three alleles produce six genotypes (IAIA, IAi, IBIB, IBi, IAIB, ii) and four phenotypes (A, B, AB, O), demonstrating multiple allelism — more than two alleles at a single locus in the population.
Multiple alleles can only be detected at the population level; each individual still carries only two alleles. This system also illustrates that dominance is not a fixed property but depends on the gene product and phenotype being analysed.

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Chapter 4: Principles of Inheritance and Variation | Long Questions

Analyse how Mendel's experimental design ensured the validity and reliability of his results, and explain the three laws he proposed based on his findings.

Compare complete dominance, incomplete dominance, and co-dominance with examples and explain the phenotypic and genotypic ratios at F1 and F2.

Explain the Chromosomal Theory of Inheritance and how Morgan's experiments on Drosophila supported and extended it through linkage and recombination.

Explain different types of sex determination mechanisms with suitable examples and state the significance of the Henking observation in the discovery of sex chromosomes.

Describe the molecular basis of sickle-cell anaemia, its mode of inheritance, and explain how it differs from thalassemia at the molecular level.

Explain the Punnett square method for a dihybrid cross and derive the expected phenotypic ratio at F2. What does this ratio reveal about the relationship between gene pairs?

Explain pedigree analysis with reference to the tools it uses and how it helps differentiate between dominant, recessive, autosomal, and sex-linked inheritance patterns.

Describe the chromosomal basis and clinical features of Down's syndrome, Klinefelter's syndrome, and Turner's syndrome, explaining how each arises.

Explain the concept of mutation, its types, and discuss how various mutagenic factors contribute to genetic variation and human disorders.

Using the example of ABO blood groups, explain the concepts of co-dominance, multiple alleles, and the relationship between genotype and blood group phenotype.

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