Question 1

In a monohybrid cross between a homozygous dominant plant (TT) and a homozygous recessive plant (tt), what will be the genotypic ratio of the F₁ generation?

Accepted Answer

This question tests understanding of Mendel's Law of Segregation and basic Punnett square analysis. When a homozygous dominant parent (TT) is crossed with a homozygous recessive parent (tt), all gametes from TT are T and all gametes from tt are t. The F₁ generation receives one T allele from each parent, producing only Tt offspring. Therefore, the genotypic ratio is 100% Tt (or 1:0:0 for TT:Tt:tt). The answer is B.

Question 2

In a monohybrid cross between a homozygous dominant plant (TT) and a homozygous recessive plant (tt), what is the phenotypic ratio of the F₁ generation?

Accepted Answer

This question tests understanding of Mendel's Law of Dominance and the outcome of a monohybrid cross. When a homozygous dominant parent (TT) is crossed with a homozygous recessive parent (tt), all F₁ offspring receive one dominant allele (T) from one parent and one recessive allele (t) from the other parent. Therefore, all F₁ individuals have the genotype Tt. Since the dominant allele (T) masks the recessive allele (t), all F₁ offspring display the dominant phenotype. The phenotypic ratio is 100% dominant or 1:0 (all dominant, no recessive). According to NCERT Class 10 Biology, this demonstrates Mendel's Law of Dominance, which states that when two contrasting traits are crossed, the dominant trait appears in the F₁ generation. Therefore, the answer is B.

Question 3

In a monohybrid cross between a tall pea plant (TT) and a dwarf pea plant (tt), what will be the phenotype ratio in the F2 generation?

Accepted Answer

According to Mendel's Law of Segregation, the F1 generation produces all tall plants (Tt). When F1 plants are self-pollinated, the F2 generation gives a phenotype ratio of 3 tall : 1 dwarf (TT, Tt, Tt, tt). This is because 'T' (tall) is dominant over 't' (dwarf). This is a foundational result from NCERT Class 10 Chapter on Heredity.

Question 4

In a dihybrid cross between two heterozygous pea plants (AaBb × AaBb), where A (tall) is dominant over a (dwarf) and B (yellow seeds) is dominant over b (green seeds), what is the probability of obtaining an offspring that is homozygous recessive for both traits (aabb)?

Accepted Answer

This question tests Mendel's Law of Independent Assortment and the ability to calculate probabilities in a dihybrid cross. For a dihybrid cross AaBb × AaBb, we must find the probability of aabb. Using the multiplication rule: the probability of aa from Aa × Aa is 1/4 (one aa out of four possible genotypes: AA, Aa, Aa, aa). Similarly, the probability of bb from Bb × Bb is 1/4. Since the two traits assort independently, the combined probability is 1/4 × 1/4 = 1/16. Therefore, the answer is B (1/16 or 6.25%).

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