Question

Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance.
(i) When Mendel crossed pea plants with pure tall and pure short characteristics to produce F₁ progeny, which two observations were made by him in F₁ plants?
(ii) Write one difference between dominant and recessive trait.
(iii) (A) In a cross with two pairs of contrasting characters
                   RRYY × rryy
      (Round Yellow)   (Wrinkled Green)
Mendel observed 4 types of combinations in F₂ generation. By which method did he obtain F₂ generation? Write the ratio of the parental combinations obtained and what conclusions were drawn from this experiment.

OR

(iii) (B) Justify the statement :
“It is possible that a trait is inherited but may not be expressed.”

Answer 1

Step 1: Understanding Mendel's Experiments:
Mendel investigated heredity using pea plants. He cross-bred plants with opposite characteristics (e.g., tall/short, round/wrinkled) and tracked how these traits appeared in subsequent generations.

Part (i): Observations in F₁ Progeny from Pure Tall and Pure Short Cross.
When Mendel crossed pure tall (TT) and pure short (tt) pea plants, the F₁ generation exhibited the following:
1. All F₁ offspring were tall, indicating the tall trait's dominance over the short trait.
2. The short trait was not visible in the F₁ generation, signifying its recessive nature, masked by the dominant tall trait.

Part (ii): Difference Between Dominant and Recessive Traits.
The primary distinction is:
- Dominant trait: Expressed when at least one dominant allele is present (e.g., T for tall). It overrides the recessive allele's effect.
- Recessive trait: Only expressed when two recessive alleles are present (e.g., t for short). It is suppressed by a dominant allele.

Part (iii) (A): Dihybrid Cross (RRYY × rryy) and F₂ Generation.
In a cross involving two contrasting character pairs (RRYY × rryy), Mendel observed four combinations in F₂. He obtained the F₂ generation through self-pollination of the F₁ progeny (RrYy × RrYy). The ratio of parental combinations in F₂ was 9:3:3:1. This outcome aligns with Mendel's Law of Independent Assortment, leading to the conclusions that traits are inherited independently and follow predictable inheritance patterns (9:3:3:1 ratio).

Part (iii) (B): Justification: "A Trait Can Be Inherited But Not Expressed."
This statement applies to recessive traits. Alleles for a recessive trait can be passed down but may not manifest if a dominant allele is also present, masking its expression. For instance, inheriting the allele for blue eyes (recessive) from both parents might not result in blue eyes if a dominant brown eye allele is also inherited, thereby preventing the expression of the blue eye trait.

Conclusion:
Mendel's experiments and findings formed the bedrock of genetics, illuminating the principles of dominant and recessive traits and independent inheritance.