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.

(a) 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?

(b) Write one difference between dominant and recessive trait.

(c) In a cross with two pairs of contrasting characters

RRYY (Round Yellow) X rryy (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

(c) Justify the statement : "It is possible that a trait is inherited but may not be expressed."

(a) When Mendel crossed pure tall (TT) and pure short (tt) pea plants then two observation were :

1. All F₁ plants were tall, showing that the tall trait dominates over the short one.
2. The short trait reappeared in the F₂ generation, showing that it was not lost but only masked in the F₁ generation.

(b)

(c) Mendel performed a dihybrid cross between RRYY (Round Yellow) and rryy (Wrinkled Green) pea plants and he obtained the F₂ generation by self-pollinating the F₁ hybrids (RrYy × RrYy).

Observations in F₂ generation:

• Four types of combinations appeared:

1. Round Yellow (RRYY, RrYY, RRYy, RrYy)
2. Round Green (RRyy, Rryy)
3. Wrinkled Yellow (rrYY, rrYy)
4. Wrinkled Green (rryy)

Ratio of parental combinations:

• Parental combinations (Round Yellow + Wrinkled Green) → 9 + 1 = 10 parts
• New (recombinant) combinations (Round Green + Wrinkled Yellow) → 3 + 3 = 6 parts

So, the overall phenotypic ratio = 9 : 3 : 3 : 1.

Conclusion drawn:

• The inheritance of one pair of contrasting traits does not affect the inheritance of another pair.
• This led to Mendel’s Law of Independent Assortment.

OR

(c) It is possible that a trait is inherited but may not be expressed because some traits are recessive and remain masked by dominant traits in the F₁ generation.

Example:

• When a pure tall (TT) pea plant is crossed with a pure dwarf (tt) plant, all F₁ plants are tall (Tt).
• The dwarfness trait is inherited from the parent but not expressed in F₁ due to the dominance of tallness.
• The trait reappears in F₂ when two recessive alleles come together (tt).

Hence, a trait may be present in the genes but hidden in expression.