When Mendel crossed tall and dwarf plants, the F2 generation showed a 3:1 ratio of tall to short plants. From this result, he concluded:
I. Law of dominance
II. Law of independent assortment
III. Incomplete dominance
IV. Law of segregation
Choose the correct option.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → I and IV
Mendel crossed tall and dwarf pea plants. All F1 plants were tall, and in the F2 tall and dwarf plants appeared in the ratio 3:1. From such monohybrid crosses he drew two laws.
I. Law of dominance — Concluded from this cross. Only the tall character showed in the F1, because the factor for tallness dominates the factor for dwarfness. The law of dominance also explains why both characters appear in the F2 in the proportion 3:1.
II. Law of independent assortment — Not concluded from this cross. This law needs a dihybrid cross, in which two characters are followed together (giving 9:3:3:1). A cross for one character, height, cannot show it.
III. Incomplete dominance — Not concluded from this cross. With incomplete dominance the F1 would be intermediate (like the pink flowers of snapdragon) and the F2 would show 1:2:1. Here the F1 was fully tall, so tallness showed complete dominance.
IV. Law of segregation — Concluded from this cross. The dwarf character, hidden in the F1, came back unchanged in the F2. So the two factors do not blend; they separate during gamete formation, and each gamete gets only one of them.
NCERT Extract:
Inheritance of One Gene – “… Based on his observations on monohybrid crosses Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses. Today these rules are called the Principles or Laws of Inheritance: the First Law or Law of Dominance and the Second Law or Law of Segregation.”
Law of Dominance – “… The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.”
Law of Independent Assortment – “… Based upon such observations on dihybrid crosses (crosses between plants differing in two traits) Mendel proposed a second set of generalisations that we call Mendel’s Law of Independent Assortment.”