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 the experiment by Mendel:
Mendel conducted experiments using pea plants to study heredity. His experiments involved crossing pea plants with contrasting traits, such as tall vs. short, round vs. wrinkled, etc., and he analyzed the inheritance patterns of these traits in successive generations.

Part (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?
When Mendel crossed pure tall (TT) and pure short (tt) pea plants, the following two observations were made in the F₁ progeny:
1. All F₁ progeny were tall, showing that the tall trait is dominant over the short trait.
2. The short trait was not expressed in F₁ generation, which indicated that it was recessive and masked by the dominant tall trait.

Part (ii): Write one difference between dominant and recessive trait.
The key difference between dominant and recessive traits is:
- Dominant trait: A trait that is expressed in the presence of at least one dominant allele (e.g., T for tall in pea plants). It masks the effect of the recessive allele.
- Recessive trait: A trait that is expressed only when two recessive alleles are present (e.g., t for short in pea plants). It is masked in the presence of a dominant allele.

Part (iii) (A): In a cross with two pairs of contrasting characters (RRYY × rryy), 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.
In this experiment, Mendel crossed pea plants with two pairs of contrasting characters: round-yellow (RRYY) and wrinkled-green (rryy). This is a dihybrid cross, involving two traits: seed shape (round vs. wrinkled) and seed color (yellow vs. green).
To obtain F₂ generation, Mendel self-pollinated the F₁ progeny (RrYy × RrYy). The results of this cross gave 4 types of combinations in the F₂ generation, and the ratio of the parental combinations (round-yellow, round-green, wrinkled-yellow, and wrinkled-green) was:
- 9:3:3:1
This ratio is the result of independent assortment of alleles, as per Mendel's second law (Law of Independent Assortment). The conclusions drawn from this experiment were:
- Each trait is inherited independently of the other.
- The inheritance of two traits follows a predictable pattern (9:3:3:1 ratio).

Part (iii) (B): Justify the statement: “It is possible that a trait is inherited but may not be expressed.”
The statement refers to the concept of recessive traits. A trait can be inherited, meaning the alleles for that trait are passed on to the offspring, but it may not be expressed if it is recessive and the individual carries a dominant allele that masks its expression. For example, a person may inherit the allele for blue eyes (a recessive trait) from both parents, but the brown eye allele (dominant) from one of the parents may mask the expression of the blue eye trait. Thus, the blue eye allele is inherited but not expressed in the phenotype.

Conclusion:
This experiment and the principles discovered by Mendel laid the foundation for the study of genetics and inheritance patterns. Mendel’s work emphasized the role of dominant and recessive traits and the independent inheritance of traits.