Monohybrid Cross
ஒற்றைப்பண்பு கலப்பு
Most students fail to distinguish between phenotype and genotype ratios in a monohybrid cross. If you confuse the 3:1 ratio with the 1:2:1 ratio, you lose 4 marks instantly. Precision is the difference between a medical seat and a repeat year.
The idea
A monohybrid cross involves crossing two organisms differing in one trait, such as stem height in Pisum sativum. Mendel’s Law of Dominance states that in a heterozygous condition, the dominant allele masks the expression of the recessive allele. The F1 generation shows only the dominant phenotype. The F2 generation reveals the phenotypic ratio of 3:1 and the genotypic ratio of 1:2:1.
Why it matters
It exists to simplify the study of inheritance patterns, allowing for the clear observation of dominance, recessiveness, and segregation of alleles.
NEET lens
NEET examiners frequently swap phenotypic and genotypic ratios to trap you. Always draw the Punnett square if the cross involves codominance or incomplete dominance, as the ratios will deviate from the standard 3:1. Focus on the F2 generation data; that is where 90% of the questions originate.
Clinical significance
Mendelian inheritance patterns explain autosomal dominant and recessive disorders in humans. Conditions like Cystic Fibrosis or Huntington’s disease follow these precise segregation rules. When these single-gene inheritance patterns are disrupted, clinical pathology manifests.
Introduction
A monohybrid cross is a genetic cross between two parents that differ in a single character, used to study the inheritance pattern of that trait and to derive Mendel's Law of Segregation.
Core concept
Mendel's monohybrid cross with pea height: P generation: TT (tall, true-breeding) × tt (short, true-breeding). F1: all Tt (tall) — tall is dominant over short. F1 × F1: Tt × Tt. Gametes: T or t (each parent). F2 genotypes: TT : Tt : tt = 1:2:1. F2 phenotypes: 3 tall : 1 short = 3:1. Explanation: T = dominant (tall), expressed in TT and Tt. t = recessive (short), expressed only in tt. Punnet square for Tt × Tt: TT (1), Tt (2), tt (1). Law of Segregation derived: alleles (T and t) segregate during gamete formation → each gamete gets only one allele → reunite at fertilisation. Reciprocal cross: tt × TT also → all Tt in F1 (confirms Mendelian, not maternal inheritance).
Go deeper
Monohybrid cross ratios: F2 phenotypic ratio: 3:1 (dominant : recessive). F2 genotypic ratio: 1:2:1 (TT : Tt : tt). Test cross: Tt × tt → 1 Tt (tall) : 1 tt (short) = 1:1. Backcross: F1 × either parent. Monohybrid ratio deviates from 3:1 when: incomplete dominance (1:2:1 phenotypic), co-dominance (1:2:1), lethal alleles (2:1), epistasis (modified ratios).
From Class 10
A monohybrid cross studies the inheritance of a single character — for example, crossing tall pea plants with short pea plants to see how height is inherited.