Mendel’s Experiments and Laws – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Why did Mendel choose pea plants for his experiments? Explain how the choice helped him to discover the basic laws of inheritance.
Answer:
Mendel chose pea plants (Pisum sativum) because they had clear, contrasting traits like tall/short height or round/wrinkled seeds.
These plants could both self-pollinate and cross-pollinate, allowing Mendel to control the parent plants.
Pea plants also produced many seeds quickly, giving Mendel a large sample size for accuracy.
The traits were easy to observe and track through generations.
This helped Mendel notice patterns easily, like how certain traits disappeared in one generation but reappeared in the next.
The choice of pea plants made it possible for Mendel to form the foundation of genetics.
Q2. Explain the results of Mendel’s monohybrid cross using the example of plant height. What conclusion did Mendel draw from this experiment?
Answer:
In his monohybrid cross, Mendel crossed a pure tall (TT) plant with a pure dwarf (tt) plant.
All offspring of the first (F₁) generation were tall (Tt), showing only the tall trait.
When these F₁ tall plants were self-pollinated, the second (F₂) generation had both tall and dwarf plants in a ratio of 3:1.
Mendel concluded that each plant has two units (alleles) for each trait.
One allele can mask the other, which led him to the idea of dominant and recessive traits.
He also realized that the hidden (recessive) trait can reappear in later generations.
Q3. Describe Mendel’s Law of Dominance with a suitable example. Why does the recessive trait not appear in the F₁ generation?
Answer:
Law of Dominance states that one trait (dominant) covers up the effect of another (recessive) in a hybrid.
For example, in a cross between a tall (TT) and a dwarf (tt) pea plant, all F₁ plants are tall (Tt).
The Tall (T) gene dominates and hides the effect of the dwarf (t) gene.
The recessive trait (dwarf) doesn’t appear in the F₁ generation because its effect is covered by the dominant trait.
Only if two recessive alleles come together (tt), as seen in the F₂ generation, the recessive trait is visible.
This law helped explain why certain traits seem to “skip” a generation.
Q4. What is the Law of Segregation? Use Mendel’s experiments to illustrate its importance in inheritance.
Answer:
Law of Segregation says each organism has two alleles for every trait, but only one is passed to each gamete.
In a tall pea plant with genotype Tt, there are both T (tall) and t (dwarf) alleles.
During reproduction, alleles separate or segregate so the gamete gets only one allele – either T or t.
When F₁ plants (Tt) are self-pollinated, their gametes combine in four ways: TT, Tt, tT, and tt.
This produces a 3 tall: 1 dwarf ratio in the offspring.
This law explains how even hidden traits (like dwarfness) can appear in future generations.
Q5. What is a dihybrid cross? Describe Mendel’s findings from his dihybrid cross experiment and state the phenotypic ratio obtained.
Answer:
A dihybrid cross involves studying the inheritance of two traits at the same time.
Mendel crossed pea plants with yellow, round seeds (YYRR) and green, wrinkled seeds (yyrr).
All F₁ offspring were yellow and round (YyRr) showing dominance of yellow and round traits.
When F₁ plants were self-pollinated, F₂ generation showed 4 types:
Yellow round, yellow wrinkled, green round, and green wrinkled.
The observed ratio was 9:3:3:1 (9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled).
This proved that different traits are inherited independently from each other.
High Complexity (Analysis & Scenario-Based)
Q6. Suppose a tall pea plant with genotype Tt is crossed with a dwarf plant with genotype tt. Predict the possible offspring and explain the expected ratio.
Answer:
The tall plant (Tt) can make gametes with T or t, while the dwarf plant (tt) can only make gametes with t.
When they cross:
Tt × tt → possible combinations are Tt (tall) and tt (dwarf).
So, half the offspring will be Tt (tall) and half will be tt (dwarf).
The phenotypic ratio is 1 Tall : 1 Dwarf.
This shows that the trait for tallness is not always dominant if one parent is homozygous recessive.
It highlights the working of Mendel’s Law of Segregation in real crosses.
Q7. Analyze how the Law of Independent Assortment would apply to a cross between plants with the genotypes YyRr and Yyrr. What types of offspring can you expect?
Answer:
Y = yellow, y = green; R = round, r = wrinkled.
Cross is between YyRr × Yyrr.
The first plant can produce four types of gametes: YR, Yr, yR, yr. The second can produce Y r and y r.
Combining them, the offspring could be: YyRr (yellow, round), Yyrr (yellow, wrinkled), yyRr (green, round), and yyrr (green, wrinkled).
The traits for color and shape show recombination, not always inherited as they were in the parents.
This proves that color and shape genes assort independently, supporting Mendel’s law.
Q8. Explain, with reasoning, why the law of independent assortment could not be observed in a monohybrid cross.
Answer:
A monohybrid cross deals with only one contrasting trait, such as height or color.
The Law of Independent Assortment involves two or more traits inherited together.
If there’s only one trait, there’s no second trait to show independent inheritance.
So, only the Law of Segregation and Law of Dominance can be observed in a monohybrid cross.
Independent assortment needs at least two genes located on different chromosomes.
That’s why Mendel could prove this law only with his dihybrid cross.
Q9. A student crosses two pea plants: one with genotype RrYy and another with genotype rryy. List the gametes and offspring combinations and explain the phenotypic ratio.
Answer:
R = round, r = wrinkled; Y = yellow, y = green.
Parent 1 (RrYy) forms gametes: RY, Ry, rY, ry. Parent 2 (rryy) forms only ry.
Crossing gametes:
RY × ry = RrYy (round, yellow)
Ry × ry = Rryy (round, green)
rY × ry = rrYy (wrinkled, yellow)
ry × ry = rryy (wrinkled, green)
Offspring phenotypes:
1 round yellow : 1 round green : 1 wrinkled yellow : 1 wrinkled green
The ratio is 1:1:1:1, showing all combinations due to independent assortment of alleles.
Q10. How did Mendel’s experiments disprove the blending theory of inheritance? Can you give an example from his crosses to support your answer?
Answer:
The blending theory said offspring are a “blend” or mix of parental traits.
For example, a tall and short plant should always give medium-height offspring, according to blending.
Mendel found that crossing tall (TT) and dwarf (tt) plants made all tall (Tt) plants in the F₁ generation, not medium.
In the F₂ generation, some plants were tall and some were dwarf, and the dwarf trait reappeared unchanged.
This showed traits are inherited as separate units (genes/alleles), not simply mixed or blended.
Thus, Mendel’s laws replaced the blending theory with the particulate theory of inheritance.
