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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."

Genetics

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Answer

(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)

Dominant TraitRecessive Trait
Expressed in the F1 generation, even when one allele is present.Expressed only when both alleles are of the same type (homozygous).
Example: Tallness in pea plant (T)Example: Dwarfness in pea plant (t)

(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.

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