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

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

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Case Set 1

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Riya, a Class 12 student, performed a monohybrid cross between a true-breeding tall pea plant (TT) and a true-breeding dwarf pea plant (tt). In the F1 generation, all plants were tall. She then self-pollinated the F1 plants and obtained 120 plants in the F2 generation. She noticed that some plants were tall and some were dwarf, and no plants showed an intermediate height. Her biology teacher told her that this experiment demonstrates Mendel's fundamental laws of inheritance, which were proposed after seven years of careful experimentation on garden peas. The teacher also explained that the disappearance of the dwarf trait in F1 and its reappearance in F2 is central to understanding how alleles behave during gamete formation.

Question 1: What is the expected phenotypic ratio in the F2 generation of Riya's monohybrid cross, and approximately how many plants out of 120 would be tall?

The expected phenotypic ratio in F2 is 3 tall : 1 dwarf.
Out of 120 plants, approximately 90 plants would be tall and 30 would be dwarf.

Question 2: Riya wants to know if a tall F2 plant is homozygous (TT) or heterozygous (Tt). Which cross should she perform, and what results would confirm each genotype?

Riya should perform a test cross by crossing the tall F2 plant with a homozygous recessive dwarf plant (tt).
If all offspring are tall, the F2 plant is TT; if offspring appear in 1 tall : 1 dwarf ratio, the F2 plant is Tt.

Question 3: Explain the Law of Segregation in the context of Riya's experiment. Why were no dwarf plants seen in F1, yet they reappeared in F2? State the expected genotypic ratio in F2.

The Law of Segregation states that the two alleles of a pair segregate from each other during gamete formation, and only one allele is transmitted to each gamete.
In F1, although the genotype is Tt, the dominant allele T completely masks the expression of the recessive allele t, so all F1 plants appear tall. The allele t is present but not expressed.
In F2, self-pollination of Tt plants produces TT, Tt, and tt offspring. The tt plants express the recessive dwarf trait. The expected genotypic ratio is 1 TT : 2 Tt : 1 tt.

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Case Set 2

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Anand was studying the inheritance of flower colour in Antirrhinum (snapdragon). He crossed true-breeding red-flowered plants with true-breeding white-flowered plants. To his surprise, all F1 plants had pink flowers, which was unlike what Mendel observed in peas. When he self-pollinated the F1 pink-flowered plants, the F2 generation showed red, pink, and white flowers. Anand initially thought the Law of Segregation was violated because no pure red or white flowers reappeared cleanly. His teacher corrected him, explaining that this is a different phenomenon from what Mendel described for traits like tall/dwarf in peas, and that the genotypic ratio in F2 is still the same as in a standard monohybrid cross.

Question 1: Name the phenomenon demonstrated in Anand's experiment and state the phenotypic ratio observed in the F2 generation.

The phenomenon demonstrated is Incomplete Dominance, where neither allele completely dominates the other.
The F2 phenotypic ratio is 1 Red (RR) : 2 Pink (Rr) : 1 White (rr).

Question 2: How does incomplete dominance differ from complete dominance in terms of phenotypic and genotypic ratios in the F2 generation?

In complete dominance, the F2 phenotypic ratio is 3:1 (dominant:recessive), as one allele fully masks the other. In incomplete dominance, the F2 phenotypic ratio is 1:2:1, as three distinct phenotypes appear.
In both cases the genotypic ratio in F2 remains 1:2:1 (homozygous dominant : heterozygous : homozygous recessive). So genotypic ratios are identical; only phenotypic ratios differ.

Question 3: Anand's teacher explained that dominance is not an intrinsic property of a gene but depends on the gene product and the phenotype being examined. Explain this concept with a molecular basis, and describe how the same allele pair can show incomplete dominance for one phenotype and complete dominance for another.

Dominance depends on the gene product (enzyme/protein) produced. If both alleles produce functional enzyme, phenotype is normal (equivalent alleles). If one allele produces a non-functional enzyme or no enzyme, the phenotype depends on the functional allele—making it dominant.
In the case of starch synthesis in pea seeds, the B allele produces starch efficiently and b does not. For seed shape, BB and Bb both appear round (B is dominant over b). However, for starch grain size, Bb shows intermediate-sized grains, indicating incomplete dominance. Thus, the same allele pair shows complete dominance for one trait (seed shape) and incomplete dominance for another (starch grain size).
This demonstrates that dominance is defined by the phenotype being observed, not solely by the gene itself.

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Case Set 3

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In a genetics class, students were asked to study ABO blood group inheritance. A child with blood group O was born to parents where the father has blood group A and the mother has blood group B. The teacher explained that ABO blood groups are controlled by a single gene I with three alleles: IA, IB, and i. The alleles IA and IB are both dominant over i, but when both IA and IB are present in the same individual, both are expressed. This serves as an excellent example of multiple allelism and co-dominance. Students were then asked to determine the possible genotypes of the parents and predict the possible blood groups of their other offspring.

Question 1: What are the genotypes of the father (blood group A) and mother (blood group B) in this case? Give a reason.

The father's genotype must be IAi (heterozygous for blood group A), because the child has blood group O (genotype ii) and must have received one 'i' allele from each parent.
The mother's genotype must be IBi (heterozygous for blood group B), for the same reason—she contributed the second 'i' allele to the child with blood group O.

Question 2: What are all the possible blood groups and their expected proportions in the offspring of these two parents (IAi × IBi)?

The cross IAi × IBi produces offspring: IAIB (blood group AB), IAi (blood group A), IBi (blood group B), and ii (blood group O) in a ratio of 1:1:1:1.
So the four possible blood groups are A, B, AB, and O, each with an expected frequency of 25%.

Question 3: How does co-dominance differ from incomplete dominance? Using the ABO blood group system as an example, explain why IAIB represents co-dominance and not incomplete dominance.

In incomplete dominance, neither allele is fully dominant, and the heterozygote shows an intermediate phenotype (blending of parental traits). In co-dominance, both alleles are fully expressed simultaneously in the heterozygote, with no blending.
In the ABO system, individuals with genotype IAIB have blood group AB. Both IA and IB produce their specific sugar molecules (Type A and Type B antigens) independently on the surface of red blood cells.
This is co-dominance because both alleles are fully and independently expressed—the result is not an intermediate between A and B but the simultaneous presence of both A and B antigens, which is a distinct blood type AB.

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Case Set 4

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Priya is studying the dihybrid cross conducted by Mendel involving two characters: seed colour (Yellow Y dominant over green y) and seed shape (Round R dominant over wrinkled r). The parental plants had genotypes RRYY and rryy. Priya obtained F1 seeds all of which were round and yellow (RrYy). On self-pollinating the F1, she obtained 640 seeds in the F2 generation. She recorded the phenotypes and found four different phenotypic classes. Her teacher explained that the results support a key Mendelian law that states the inheritance of one character is independent of the inheritance of another character, as long as the genes are on different chromosomes.

Question 1: What are the four phenotypic classes expected in F2 and what is their ratio? How many seeds of each class would Priya expect from 640 seeds?

The four phenotypic classes in F2 and their ratio are: Round Yellow (9) : Wrinkled Yellow (3) : Round Green (3) : Wrinkled Green (1), i.e., a 9:3:3:1 ratio.
From 640 seeds: Round Yellow = 360, Wrinkled Yellow = 120, Round Green = 120, Wrinkled Green = 40.

Question 2: State the Law of Independent Assortment. How does a Punnett square for the dihybrid cross RrYy × RrYy support this law?

The Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair.
The Punnett square for RrYy × RrYy has 16 boxes, representing 16 possible combinations of 4 types of gametes (RY, Ry, rY, ry) from each parent, each at 25% frequency. The equal representation of all gamete types shows that R/r and Y/y segregate independently, confirming the law.

Question 3: Priya is told that Morgan's work on Drosophila showed that the Law of Independent Assortment does not always hold. Explain why linked genes deviate from the expected 9:3:3:1 ratio in a dihybrid cross, and distinguish between tightly linked and loosely linked genes.

Linked genes are located on the same chromosome and tend to be inherited together rather than independently. When two genes are linked, they do not assort independently during meiosis, so the parental gene combinations appear in much higher proportions than non-parental combinations. This causes the F2 ratio to deviate significantly from the expected 9:3:3:1.
Tightly linked genes are located very close together on the same chromosome and show very low recombination frequency (e.g., white and yellow genes in Drosophila showed only 1.3% recombination). They almost always segregate together.
Loosely linked genes are farther apart on the same chromosome, and crossing over (recombination) between them occurs more frequently, leading to higher recombination percentages (e.g., white and miniature wing genes showed 37.2% recombination). Alfred Sturtevant used recombination frequency to map the distances between genes on chromosomes.

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Case Set 5

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During a genetics lecture, the teacher discussed the Chromosomal Theory of Inheritance. The teacher explained that Walter Sutton and Theodore Boveri independently proposed that genes are located on chromosomes, based on the parallel behaviour of chromosomes and Mendelian factors during meiosis. The confirmation of this theory came later through the work of Thomas Hunt Morgan, who worked with Drosophila melanogaster. Morgan chose this organism for his experiments because of its many advantages as an experimental organism. He showed that genes are physically located on chromosomes and that genes on the same chromosome are linked. He also introduced the concepts of linkage and recombination.

Question 1: List any two features of Drosophila melanogaster that make it a suitable organism for genetic experiments.

Drosophila melanogaster completes its life cycle in about two weeks, allowing rapid generation of results.
A single mating produces a large number of progeny, allowing statistically significant data collection. Additionally, male and female flies are easily distinguishable, making genetic crosses straightforward.

Question 2: How did Sutton and Boveri draw a parallel between chromosome behaviour and Mendel's laws to propose the Chromosomal Theory of Inheritance?

Sutton and Boveri observed that chromosomes, like Mendelian factors (genes), occur in pairs. Chromosomes segregate during meiosis so only one of each pair is transmitted to a gamete, paralleling the Law of Segregation.
Different chromosome pairs segregate independently of each other during meiosis, paralleling the Law of Independent Assortment. They also noted that alleles of a gene are located at homologous positions on homologous chromosomes.

Question 3: Morgan found that genes on the same chromosome were linked but not always inherited together. How did Morgan and Sturtevant use recombination frequency to construct genetic maps? What is the significance of these maps?

Morgan observed that genes on the same chromosome showed linkage (parental combinations were more frequent than non-parental types). However, crossing over during meiosis could separate linked genes, producing recombinant offspring. The frequency of such recombinants depended on the physical distance between genes.
Alfred Sturtevant, Morgan's student, realised that the frequency of recombination between two gene pairs on the same chromosome was proportional to the distance between them. He used recombination percentage as a measure of distance (1% recombination = 1 map unit or centimorgan) and mapped gene positions on the chromosome.
Genetic (linkage) maps are significant because they show the linear arrangement and relative distances of genes on a chromosome. They serve as a crucial starting point in sequencing whole genomes (e.g., Human Genome Project) and are used in identifying genes associated with inherited diseases.

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Chapter 4: Principles of Inheritance and Variation | Case Study Questions

Passage

Question 1: What is the expected phenotypic ratio in the F2 generation of Riya's monohybrid cross, and approximately how many plants out of 120 would be tall?

Question 2: Riya wants to know if a tall F2 plant is homozygous (TT) or heterozygous (Tt). Which cross should she perform, and what results would confirm each genotype?

Question 3: Explain the Law of Segregation in the context of Riya's experiment. Why were no dwarf plants seen in F1, yet they reappeared in F2? State the expected genotypic ratio in F2.

Passage

Question 1: Name the phenomenon demonstrated in Anand's experiment and state the phenotypic ratio observed in the F2 generation.

Question 2: How does incomplete dominance differ from complete dominance in terms of phenotypic and genotypic ratios in the F2 generation?

Passage

Question 1: What are the genotypes of the father (blood group A) and mother (blood group B) in this case? Give a reason.

Question 2: What are all the possible blood groups and their expected proportions in the offspring of these two parents (IAi × IBi)?

Question 3: How does co-dominance differ from incomplete dominance? Using the ABO blood group system as an example, explain why IAIB represents co-dominance and not incomplete dominance.

Passage

Question 1: What are the four phenotypic classes expected in F2 and what is their ratio? How many seeds of each class would Priya expect from 640 seeds?

Question 2: State the Law of Independent Assortment. How does a Punnett square for the dihybrid cross RrYy × RrYy support this law?

Question 3: Priya is told that Morgan's work on Drosophila showed that the Law of Independent Assortment does not always hold. Explain why linked genes deviate from the expected 9:3:3:1 ratio in a dihybrid cross, and distinguish between tightly linked and loosely linked genes.

Passage

Question 1: List any two features of Drosophila melanogaster that make it a suitable organism for genetic experiments.

Question 2: How did Sutton and Boveri draw a parallel between chromosome behaviour and Mendel's laws to propose the Chromosomal Theory of Inheritance?

Question 3: Morgan found that genes on the same chromosome were linked but not always inherited together. How did Morgan and Sturtevant use recombination frequency to construct genetic maps? What is the significance of these maps?

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