Mendelian Inheritance
In a monohybrid cross between two heterozygous tall pea plants, what fraction of the offspring are expected to be homozygous recessive dwarf?
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About This Question
- Subject
- biology
- Chapter
- genetics and evolution
- Topic
- mendelian inheritance
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
1/4
Mendel's law of segregation states that the two alleles of a gene separate during gamete formation, so a heterozygous tall plant (Tt) produces T and t gametes in equal proportion. Crossing Tt with Tt gives the genotypic ratio 1 TT : 2 Tt : 1 tt, which simplifies to a phenotypic ratio of 3 tall : 1 dwarf. The dwarf plants are homozygous recessive (tt), representing 1 out of the 4 boxes in the Punnett square, hence 1/4 of the offspring. The option 1/2 corresponds to the heterozygous Tt class, not the recessive phenotype. The fraction 3/4 represents all tall plants, which is the dominant phenotype rather than the recessive one. The value 1/16 belongs to a dihybrid cross ratio (9:3:3:1) and is irrelevant to a single-gene cross. As described in NCERT Class 12, Chapter 5 (Principles of Inheritance and Variation), a monohybrid cross always yields a 3:1 phenotypic and 1:2:1 genotypic ratio in the F2 generation. As a consistency check, the four genotypic fractions (1/4 + 2/4 + 1/4) sum to 1, confirming that 1/4 dwarf is the correct recessive proportion.
This easy difficulty biology question is from the chapter genetics and evolution, covering the topic of mendelian inheritance. It appeared in the 2025 exam.
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