Biology · Principles of Inheritance and Variation · NEET
'Mono' means ONE. A monohybrid cross studies only ONE character (one pair of contrasting traits), for example plant height (tall vs dwarf) OR seed colour (yellow vs green) — but only one at a time. If you study TWO characters together, it becomes a dihybrid cross. Weak students often think 'monohybrid' means one plant; it means one trait being tracked.
Both are correct, but they describe different things. 3:1 is the PHENOTYPE ratio (what you SEE): 3 tall : 1 dwarf. 1:2:1 is the GENOTYPE ratio (the gene make-up): 1 TT : 2 Tt : 1 tt. NEET loves to trap you here. If the question says 'phenotypes' the answer is 3:1; if it says 'genotypes' the answer is 1:2:1. The 3 tall = 1 TT + 2 Tt combined.
Because tall (T) is dominant and dwarf (t) is recessive. The F1 is Tt — it carries one dwarf allele, but the dominant T masks it, so every F1 plant looks tall. The dwarf trait is hidden, not lost. It reappears in F2 when two Tt plants are selfed and some offspring get tt. This is Mendel's Law of Dominance.
During gamete formation the two alleles of the F1 (Tt) separate — this is the Law of Segregation. Each gamete carries only T or only t. When F1 x F1 (Tt x Tt) is done, some gametes carry t and can combine (t + t = tt), giving a dwarf plant again. So the recessive trait was hidden in F1 but reappears in 1/4 of F2 plants.
A monohybrid cross studies inheritance of one trait, usually F1 x F1 selfing, and gives a 3:1 F2 ratio. A test cross is a specific cross of an organism showing the dominant trait with a HOMOZYGOUS RECESSIVE (tt) partner to find out if the dominant one is TT or Tt. A test cross gives a 1:1 ratio if the tested plant was Tt. So test cross = a tool to find genotype; monohybrid cross = the basic one-trait experiment.
A tall true breeding garden pea plant is crossed with a dwarf true breeding garden pea plant. When the F1 plants were selfed the resulting genotypes were in the ratio
Match List-I (Type of cross) with List-II (Phenotypic ratio). A. Monohybrid Cross B. Dihybrid Cross C. Incomplete dominance D. Test Cross | I. 1:1 II. 1:2:1 III. 3:1 IV. 9:3:3:1
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A cross that follows the inheritance of a single character (one pair of contrasting traits) between two parents, such as tall vs dwarf pea plants.
The F2 phenotypic ratio is 3:1 (3 dominant : 1 recessive), for example 3 tall : 1 dwarf.
The F2 genotypic ratio is 1:2:1 (1 homozygous dominant : 2 heterozygous : 1 homozygous recessive), i.e. 1 TT : 2 Tt : 1 tt.
By tracking just one trait at a time, results were simple and clear. This let him discover the Law of Dominance and the Law of Segregation before moving to two-trait (dihybrid) crosses.
The monohybrid cross explains Mendel's Law of Dominance (F1 shows only the dominant trait) and the Law of Segregation (alleles separate during gamete formation, so the recessive trait reappears in F2).
