NCERT grounding
NCERT Class 12 Biology, Chapter 4 (Principles of Inheritance and Variation), develops the monohybrid cross under Section 4.2, Inheritance of One Gene. The text describes how Gregor Mendel crossed tall and dwarf garden-pea plants, observed that the entire F1 generation was tall, and then found that selfing the F1 brought back dwarf plants in the F2 — with three-fourths tall and one-fourth dwarf. From these monohybrid crosses Mendel framed his first two laws: the Law of Dominance and the Law of Segregation.
The NIOS Senior Secondary Biology module (Chapter 22, Principles of Genetics) reinforces the same definitions and adds the alternate name for the second law — the law of purity of gametes — noting that it is "a universal law" applying to all sexually reproducing organisms. Both sources treat the cross between a single pair of contrasting traits as the foundational genetic experiment, so every later topic — dihybrid crosses, incomplete dominance, sex linkage — is read against this baseline.
"Since the Tt plant is heterozygous for genes controlling one character (height), it is a monohybrid and the cross between TT and tt is a monohybrid cross." — NCERT Class 12 Biology, Section 4.2
The monohybrid cross, step by step
A monohybrid cross is a cross between two parents that differ in a single pair of contrasting traits. Mendel's classic version uses garden pea (Pisum sativum) and the trait of stem height: a true-breeding tall plant against a true-breeding dwarf plant. A true-breeding line is one that, after continuous self-pollination, shows stable inheritance of a trait for many generations — so the tall parent reliably produces tall offspring and the dwarf parent reliably produces dwarf offspring.
We assign the symbol T to the allele expressed at the F1 stage (tall) and t to the contrasting allele (dwarf). T and t are alleles — slightly different forms of the same gene for height. The true-breeding tall parent is homozygous TT and the true-breeding dwarf parent is homozygous tt. TT and tt describe the genotype (the genetic constitution); "tall" and "dwarf" describe the phenotype (the visible appearance).
The parental cross and the F1 generation
During gamete formation, meiosis halves the chromosome number, so each parent passes only one allele of the pair into each gamete. The TT parent can produce only T gametes; the tt parent can produce only t gametes. At fertilisation, a T from one parent unites with a t from the other, so every zygote of the first filial (F1) generation has the genotype Tt.
Mendel observed that all F1 plants were tall — none were dwarf, and none were of in-between height. The dwarf trait had simply disappeared from view. The Tt heterozygote looks exactly like the TT parent because T is completely dominant over t. The dwarf allele is still present, hidden, in every F1 plant.
Figure 1. The three-generation pattern of a monohybrid cross. A uniform Tt F1 on selfing gives an F2 of 1 TT : 2 Tt : 1 tt — a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
The F2 generation and how the ratios arise
Mendel then self-pollinated the tall F1 plants. To his surprise, the dwarf trait reappeared: about one-fourth of the F2 plants were dwarf and three-fourths were tall. Crucially, the traits did not blend — every F2 plant was either fully tall or fully dwarf, identical to one of the original parental types.
The F1 plant is Tt. When it forms gametes, the two alleles segregate, so it produces T gametes and t gametes in equal frequency (½ each). On selfing, T and t pollen each have an equal chance of fertilising T and t eggs. The four equally likely fertilisation events are T×T, T×t, t×T and t×t — giving zygotes TT, Tt, Tt and tt. That is the source of the 1 TT : 2 Tt : 1 tt genotypic ratio.
The phenotypic ratio is read off the same four boxes. Because T is completely dominant, TT and Tt are both tall and look identical — externally indistinguishable. The 1 TT and 2 Tt classes therefore merge into 3 tall, while only tt is dwarf — leaving 1 dwarf. Hence the genotypic ratio 1:2:1 collapses to a phenotypic ratio of 3:1.
F2 of a monohybrid cross
The 3:1 figure is the phenotypic ratio (tall : dwarf). The 1:2:1 figure is the genotypic ratio (TT : Tt : tt). Both describe the same F2 — they differ only because dominance hides the Tt genotype behind a tall phenotype.
NCERT also expresses the 1:2:1 outcome mathematically. Since the F1 produces T and t gametes each at frequency ½, the F2 is the binomial expansion (½T + ½t)2 = ¼ TT + ½ Tt + ¼ tt. This confirms that the ratio is a direct consequence of random fertilisation acting on equally frequent gametes.
The Punnett square
The Punnett square is a graphical grid for calculating the probability of all possible offspring genotypes in a cross. It was developed by the British geneticist Reginald C. Punnett. The possible gametes of one parent are written along the top row and those of the other parent down the left column; each inner box shows the genotype formed by combining that row and column gamete.
Figure 2. The 2×2 Punnett square for the F1 selfing. Of the four equally likely boxes, three carry at least one T allele (tall) and one is tt (dwarf) — the visual proof of the 3:1 and 1:2:1 ratios.
The Law of Dominance
Mendel's first law, the Law of Dominance, makes three statements: characters are controlled by discrete units called factors; factors occur in pairs; and in a dissimilar pair of factors, one member dominates (dominant) the other (recessive). This law explains why only one parental trait appears in the F1, why both reappear in the F2, and why the F2 phenotypes settle into a 3:1 proportion.
Why is one allele dominant at all? NCERT explains dominance at the level of gene products. A gene often carries the information to produce an enzyme. The normal (unmodified) allele makes a functional enzyme; a modified allele may make a non-functional enzyme or no enzyme. In a heterozygote, the single functional allele is usually enough to give the normal phenotype — so the functional, unmodified allele behaves as dominant and the modified allele as recessive. Dominance, therefore, is not an inherent property of an allele; it depends on the gene product and the phenotype being examined.
The Law of Segregation
Mendel's second law, the Law of Segregation, states that the two alleles of a gene pair separate (segregate) during gamete formation, so a gamete receives only one of the two factors. A homozygous parent produces gametes that are all alike; a heterozygous parent produces two kinds of gametes, each carrying one allele, in equal proportion. Because each gamete is "pure" — it never carries a blended or mixed factor — this law is also called the law of purity of gametes.
Why segregation produces the 1:2:1 ratio
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Step 1
Alleles separate
In meiosis the T and t of the Tt parent move into different gametes.
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Step 2
Equal gamete classes
Half the gametes carry T, half carry t — a 1:1 gamete ratio.
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Step 3
Random fertilisation
Every T and t gamete pairs at random with another T or t gamete.
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Step 4
1:2:1 zygotes
Four equal events give ¼ TT, ½ Tt, ¼ tt — the genotypic ratio.
The reappearance of the dwarf trait in the F2 is the direct evidence for segregation. If alleles blended, a Tt F1 selfed would never recover pure dwarf plants. Mendel confirmed purity experimentally: dwarf F2 plants, when selfed, bred true through F3 and F4 generations — proving their genotype was the unmixed homozygous tt.
The alleles of a pair segregate from each other such that a gamete receives only one of the two factors.
Mendel's Law of Segregation
Test cross and back cross
A tall pea plant in the F2 can be either TT or Tt — its appearance alone cannot tell you which. To resolve this, Mendel devised the test cross: cross the individual of unknown genotype (showing the dominant phenotype) with a homozygous recessive (tt) partner. The recessive parent contributes only t gametes, so the offspring phenotype is decided entirely by the gamete coming from the tested plant.
If the tested tall plant is TT, the test cross TT × tt yields all Tt offspring — every one tall. If the tested plant is Tt, the cross Tt × tt yields Tt and tt in a 1 tall : 1 dwarf ratio. The appearance of even a single dwarf offspring proves the tested plant was heterozygous.
Figure 3. A test cross resolves an unknown dominant genotype. All tall offspring point to TT; a 1:1 tall-to-dwarf split exposes a Tt heterozygote.
A back cross is the broader term: it is any cross of the F1 progeny with either of its parents. A test cross is a specific kind of back cross — one made with the homozygous recessive parent for the purpose of revealing genotype. Therefore every test cross is a back cross, but not every back cross is a test cross (a cross back to the dominant homozygous parent is a back cross but is uninformative about genotype).
Test cross
F1 × tt
crossed with recessive parent
- Partner is always homozygous recessive
- Purpose: determine an unknown genotype
- Heterozygote test cross gives a 1:1 ratio
- Every test cross is also a back cross
Back cross
F1 × parent
crossed with either parent
- Partner is any one of the two parents
- Purpose: recover or fix parental traits
- Cross to dominant parent gives all dominant
- Not every back cross reveals genotype
Why these patterns matter beyond peas
The monohybrid framework is the template for the entire genetics unit. The dihybrid 9:3:3:1 ratio is literally the product of two independent 3:1 monohybrid ratios. Exceptions are read against this baseline too: in incomplete dominance (snapdragon flower colour), the F2 phenotypic ratio shifts to 1:2:1 because the heterozygote is visibly distinct — yet the genotypic ratio is still 1:2:1, proving segregation is intact. The Law of Segregation never fails; only the Law of Dominance has exceptions.
Worked examples
A true-breeding tall pea plant is crossed with a true-breeding dwarf pea plant. The F1 is selfed. State the genotypic and phenotypic ratios of the F2 generation.
The parents are TT (tall) and tt (dwarf). TT gives only T gametes; tt gives only t gametes — so the entire F1 is Tt and tall. Selfing Tt × Tt gives gametes T and t at ½ each from both sides. The Punnett square gives 1 TT : 2 Tt : 1 tt — the genotypic ratio 1:2:1. Since T is completely dominant, TT and Tt are both tall, so 3 plants are tall and 1 is dwarf — the phenotypic ratio 3:1.
In a plant, black seed colour (BB or Bb) is dominant over white seed colour (bb). A black-seeded plant of unknown genotype is given. Which genotype should it be crossed with to identify the unknown genotype, and what result confirms each possibility?
It should be crossed with a homozygous recessive plant, bb (white) — a test cross. The bb partner supplies only b gametes. If the unknown plant is BB, the cross BB × bb gives all Bb offspring, all black-seeded. If the unknown plant is Bb, the cross Bb × bb gives Bb and bb in a 1 black : 1 white ratio. The appearance of any white-seeded offspring proves the unknown plant is heterozygous Bb.
A monohybrid cross produces an F2 of 1 RR : 2 Rr : 1 rr, but the observed phenotypic ratio is 1:2:1 instead of 3:1. What does this tell you about the alleles?
A 1:2:1 phenotypic ratio means the heterozygote Rr is phenotypically distinct from both homozygotes — so R is not completely dominant over r. This is incomplete dominance, as in the snapdragon, where RR is red, rr is white and Rr is pink. The genotypic ratio is still 1:2:1, confirming that the Law of Segregation continues to hold; only the Law of Dominance is the exception here.
A heterozygous tall pea plant (Tt) is crossed with a homozygous tall plant (TT). Is this a test cross? What proportion of offspring will be dwarf?
This is a back cross (F1 crossed with a parent), but it is not a test cross, because a test cross requires the homozygous recessive parent. Crossing Tt × TT gives gametes T, t from one side and only T from the other, producing TT and Tt offspring in a 1:1 ratio — but all are tall. Zero offspring are dwarf. This is why a back cross to the dominant parent cannot reveal whether the F1 was heterozygous.
Common confusion & NEET traps
The monohybrid cross looks simple, but NEET items consistently exploit a few predictable slips. The most common is conflating the two ratios; the next is treating "test cross" and "back cross" as synonyms. Work through the callouts below before attempting practice questions.