Genetics and Evolution — Previous Year Questions (Class 12 Biology)
Genetics and Evolution covers Mendelian inheritance, genetic crosses, mutations, and evolution mechanisms. NEET and CBSE test pedigree analysis, Punnett sq
TL;DR: Genetics and Evolution covers Mendelian inheritance, genetic crosses, mutations, and evolution mechanisms. NEET and CBSE test pedigree analysis, Punne…
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Genetics and Evolution covers Mendelian inheritance, genetic crosses, mutations, and evolution mechanisms. NEET and CBSE test pedigree analysis, Punnett sq
Genetics and Evolution — Previous Year Questions with Solutions
Q (2023, 2 marks): In humans, brown eyes (B) are dominant over blue eyes (b). If two heterozygous parents (Bb) have children, what is the probability of their first child having blue eyes?
Answer: Cross: Bb × Bb (both heterozygous)
Punnett Square:
B b
B BB Bb
b Bb bb
Offspring ratios:
BB (brown) = 1/4 = 25%
Bb (brown) = 2/4 = 50%
bb (blue) = 1/4 = 25%
Probability of blue eyes (bb) = 1/4 or 25%
Final Answer: 1/4 or 25%
Q (2022, 2 marks): Define and distinguish between genotype and phenotype with an example.
Answer: Genotype: The genetic makeup of an organism; the combination of alleles for a trait.
Example: Bb (heterozygous for eye color)
Phenotype: The observable physical or biochemical characteristics of an organism; how the genotype is expressed in the organism.
Example: Brown eyes (appearance)
Key difference:
Genotype is inherited DNA information; phenotype is the visible result.
Same phenotype can have different genotypes (BB and Bb both show brown eyes).
Different phenotypes result from different genotypes (BB/Bb → brown, bb → blue).
Another example:
Genotype Tt = tall pea plant (heterozygous)
Phenotype = tall (because T is dominant)
Genotype tt = dwarf pea plant
Phenotype = dwarf
Final Answer: Genotype = genetic information (DNA alleles). Phenotype = observable traits. Same phenotype can have different genotypes if one is dominant.
Q (2021, 3 marks): In a monohybrid cross between two heterozygous organisms (Aa × Aa), explain the 3:1 phenotypic ratio.
Answer: Cross: Aa × Aa (monohybrid = one trait, heterozygous parents)
Punnett Square:
A a
A AA Aa
a Aa aa
Genotypic ratio = 1 AA : 2 Aa : 1 aa
Phenotypic ratio (assuming A is dominant, a is recessive):
AA = dominant phenotype
Aa = dominant phenotype (A is expressed, a is masked)
aa = recessive phenotype
Dominant phenotype = AA + Aa = 1 + 2 = 3 parts
Recessive phenotype = aa = 1 part
Ratio = 3 : 1
This ratio always appears in monohybrid crosses between two heterozygotes because:
- 75% of offspring carry at least one dominant allele (show dominant trait).
- 25% of offspring have two recessive alleles (show recessive trait).
Final Answer: 3:1 ratio = 3 dominant : 1 recessive because 3/4 of offspring have at least one A allele.
Q (2023, 3 marks): What is a test cross? Explain its importance in genetics.
Answer: Test Cross: Crossing an organism with a dominant phenotype to a homozygous recessive organism (aa) to determine the genotype of the dominant organism.
Reasoning:
If dominant organism is AA (homozygous):
AA × aa → All Aa (all dominant phenotype, 100%)
If dominant organism is Aa (heterozygous):
Aa × aa → 1 Aa (dominant) : 1 aa (recessive), 50% : 50%
Importance:
1. Determines if an organism with dominant phenotype is homozygous (AA) or heterozygous (Aa).
2. Essential in breeding programs to identify pure lines.
3. Used in medical genetics to identify carriers of recessive diseases.
4. Helps understand inheritance patterns in families and populations.
Example in humans:
A person with normal vision (dominant) can be tested by having children with a color-blind partner (recessive aa). If all children have normal vision, the person is likely AA. If some are color-blind, the person is Aa.
Final Answer: Test cross = crossing dominant phenotype with recessive (aa) to find genotype. If all dominant offspring, parent is AA. If 1:1 ratio, parent is Aa.
Q (2022, 5 marks): Explain the theory of evolution by natural selection as proposed by Charles Darwin.
Answer: Theory of Evolution by Natural Selection:
1. Variation: Individuals within a population have different traits (heritable variations).
Example: Some finches have large beaks, others have small beaks.
2. Overproduction: Organisms produce more offspring than the environment can sustain.
Example: A plant produces thousands of seeds; only a few will grow to maturity.
3. Competition (Struggle for Existence): Resources (food, space, water) are limited.
Therefore, organisms compete for survival.
4. Survival of the Fittest (Natural Selection):
Organisms best adapted to the environment are more likely to survive and reproduce.
Less adapted organisms are less likely to survive.
Example: In drought, finches with larger beaks (can crack harder seeds) survive better.
5. Differential Reproduction: Survivors reproduce more, passing beneficial traits to offspring.
Over generations, beneficial traits become more common in the population.
6. Adaptation: Populations gradually change as advantageous traits accumulate.
Given enough time, this leads to the evolution of new species.
Key points:
- Evolution is gradual (happens over long periods).
- Driven by natural selection, not by organisms' needs.
- Results in adaptation and diversity of life.
- Supported by fossils, anatomy, genetics, and observed peppered moths and Darwin's finches.
Final Answer: Evolution via natural selection: variation + competition → survival of fittest → differential reproduction → accumulation of adaptive traits over generations → new species.
Q (2021, 3 marks): Distinguish between evolution and adaptation. Give examples.
Answer: Evolution: Change in the heritable traits of a population over time due to natural selection, mutation, and genetic drift. It is a large-scale, long-term process.
Example: Evolution of horses from small 4-toed ancestors to large 1-toed animals over millions of years.
Adaptation: A specific trait or characteristic that helps an organism survive and reproduce in its environment. Adaptations are the result of evolution.
Example: Long neck of a giraffe (adaptation) evolved over millions of years (evolution) to reach leaves in tall trees.
Key difference:
- Evolution is the PROCESS of change in populations and species.
- Adaptation is a RESULT (trait) of evolution.
Examples of adaptations:
1. Structural: Thick fur in polar bears (insulation in cold), wings in birds (flight).
2. Behavioral: Migration of birds, hibernation in bears.
3. Physiological: Ability of some plants to store water in deserts.
Adaptations are inherited (genetic) and favored by natural selection.
Final Answer: Evolution = process of change in populations over time. Adaptation = inherited trait that helps organism survive (result of evolution). Evolution produces adaptations.
Frequently Asked Questions
What is the Hardy-Weinberg principle?
States that allele frequencies in a population remain constant across generations if: no mutations, random mating, no gene flow, large population size, no natural selection. Used as a baseline to detect when evolution is occurring.
How do mutations contribute to evolution?
Mutations create new alleles (variations) in a population. Most mutations are neutral or harmful, but some are beneficial. Natural selection acts on these mutations — beneficial ones increase in frequency, driving evolution and adaptation.
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