ScienceCBSE Class 10 · Heredity

Mendel's Experiments and the Monohybrid Cross

Updated 14 October 2026 · Fact-checked

Mendel crossed pure tall and pure short pea plants. All F1 plants were tall, but on self-pollination one quarter of the F2 plants were short. So each plant carries two copies of a gene for a trait: tallness (T) is dominant, shortness (t) recessive. F2 shows 3 tall to 1 short (TT, Tt, tt in 1:2:1).

Understand Mendel's Experiments and Monohybrid Cross

Why two copies? In sexual reproduction, a child receives almost the same quantity of genetic material from each parent. So every trait can be influenced by both paternal and maternal DNA, and each individual has two versions of each trait. Which one shows? Gregor Mendel worked out the main rules more than a century ago using garden peas, which have clear contrasting characters: round or wrinkled seeds, tall or short plants, violet or white flowers. He was the first to count the individuals showing each trait in every generation.

The experiment. Mendel crossed a tall plant with a short plant.

F1 (first generation): all plants were tall. There were no 'medium-height' plants, so traits do not blend; only one parental trait appeared.
Were these F1 tall plants the same as the tall parents? He let both self-pollinate. The tall parents gave only tall plants. But the F2 progeny of the F1 plants were not all tall: one quarter were short.

So the F1 plants carried the shortness trait, even though only tallness was expressed.

Mendel's explanation. Two copies of a factor (now called a gene) control each trait. They may be identical or different. Using T for tall and t for short:

pure tall parent TT, pure short parent tt;
gametes carry one copy: T from one parent, t from the other;
F1: all Tt, tall;
F1 × F1 (Tt × Tt) gives F2 combinations TT, Tt, Tt, tt.

Both TT and Tt are tall, while only tt is short. A single copy of T is enough to make the plant tall; both copies must be t for shortness. A trait like T is dominant; one like t is recessive.

Phenotypic ratio (what is seen) in F2: 3 tall : 1 short.
Genotypic ratio (gene combinations) in F2: 1 TT : 2 Tt : 1 tt.

How do genes make traits? Cellular DNA is the information source for making proteins; a section of DNA that provides information for one protein is the gene for that protein. Plant height can depend on the amount of a growth hormone, which depends on how efficiently an enzyme makes it. If the gene for that enzyme works well, plenty of hormone is made and the plant is tall; if the gene has an alteration that makes the enzyme less efficient, less hormone is made and the plant is short. Thus genes control traits.

Why germ cells carry one copy. Each cell has two copies of each chromosome, one from each parent. Each germ cell takes one chromosome from each pair, so it has only one copy of each gene. When two germ cells fuse, the normal number is restored. This is how equal genetic contribution of both parents is ensured, and how the stability of the species' DNA is kept.

Mendel's Experiments and Monohybrid Cross: formulas and key facts

Monohybrid cross: $T T \times tt \to F_{1} : T t (all tall)$; Gametes: T and t.
F2 from selfing F1: $T t \times T t \to 1 T T : 2 T t : 1 tt$; Phenotype 3 tall : 1 short.
Fraction showing the recessive trait: $\frac{1}{4} of F_{2}$; Of the tall F2 plants, $\frac{1}{3}$ are TT and $\frac{2}{3}$ are Tt.
Dominant vs recessive: $dominant shows with one copy; recessive shows only with two copies$; TT and Tt tall; tt short.

How to solve Mendel's Experiments and Monohybrid Cross questions

For any one-trait cross, follow these steps.

1Choose a letter: capital for the dominant trait, small for the recessive (T, t).
2Write each parent's two copies (pure tall TT, pure short tt, hybrid Tt).
3List the gametes; each carries one copy (TT gives only T; Tt gives T and t).
4Combine gametes in a checkerboard (Punnett square) to get offspring combinations.
5Count genotypes (TT, Tt, tt) and translate to phenotypes (tall or short).
6For numbers, multiply the total by each fraction: tall = $\frac{3}{4}$ , short = $\frac{1}{4}$ ; TT = $\frac{1}{4}$ , Tt = $\frac{1}{2}$ .
7To work backwards from offspring, ask: did any recessive offspring appear? If so, each parent must carry the recessive copy.

Mendel's Experiments and Monohybrid Cross in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Dominant and recessive traits in a monohybrid cross

Suppose a school garden club crosses pure-breeding tall pea plants with pure-breeding short pea plants, collects the F1 seeds, grows them, lets them self-pollinate, and then sows 1200 F2 seeds, all of which grow.

Predict the F1 plants, the number of tall and short F2 plants expected, and how many of the tall F2 plants are expected to be pure (TT).

Try it first, then show the solutionSolution

Parents TT × tt; gametes T and t; F1 all Tt, so all tall.
F1 × F1: Tt × Tt gives TT : Tt : tt = 1 : 2 : 1.
Tall = $\frac{3}{4} \times 1200 = 900$ ; short = $\frac{1}{4} \times 1200 = 300$ .
TT = $\frac{1}{4} \times 1200 = 300$ (these are one-third of the 900 tall plants); Tt = $\frac{1}{2} \times 1200 = 600$ .
Check: 300 + 600 + 300 = 1200, and 900 + 300 = 1200.

Answer: F1 all tall (Tt). F2: about 900 tall and 300 short; of the tall plants about 300 are pure TT and 600 are Tt.

Memory hook: Three tall to one short, but only one tall in three breeds true.

Tricks to remember Mendel's Experiments and Monohybrid Cross

Short memory aids. Say each one aloud once, then use it on a question the same day.

1-2-1 hides inside 3-1
TT + 2Tt look the same (tall), so 1 : 2 : 1 shows as 3 : 1.
Use it when: Switching between genotypic and phenotypic ratios.
Capital wins
Capital letter (dominant) shows whenever present.
Use it when: Deciding phenotype from genotype.
Skip a generation
A recessive trait that vanishes in F1 reappears in F2.
Use it when: Spotting a recessive trait from data.

Mendel's Experiments and Monohybrid Cross: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Dominant or recessive? · 3 marks · Short answerBoard level

How do Mendel's experiments show that traits may be dominant or recessive? Use the tall and short pea plants.

Try it first, then show the solutionSolution

Pure tall (TT) crossed with pure short (tt) gave only tall F1 plants (Tt); the short trait did not show (1 mark).
Selfing F1 gave F2 in which one quarter were short, so the short trait was carried by the F1 but hidden (1 mark).
The trait that shows in F1 (tall) is dominant; the trait hidden in F1 and reappearing in F2 (short) is recessive: TT and Tt are tall, only tt is short (1 mark).

Answer: Tallness appears in F1 (dominant); shortness is hidden in F1 but reappears in a quarter of F2 (recessive).

Example 2: Cross diagram · 5 marks · Long answerHarder board level

Show with a cross diagram the inheritance of plant height from the parental generation to F2 when a pure tall plant (TT) is crossed with a pure short plant (tt). Give the phenotypic and genotypic ratios of F2.

Try it first, then show the solutionSolution

Parents: TT (tall) × tt (short); gametes T and t (1 mark).
F1: Tt, all tall (1 mark).
F1 selfed: Tt × Tt; gametes T, t from each (1 mark).
Checkerboard: TT, Tt, Tt, tt (1 mark).
Phenotypic ratio 3 tall : 1 short; genotypic ratio 1 TT : 2 Tt : 1 tt (1 mark).

Answer: F1 all Tt (tall); F2 TT : Tt : tt = 1 : 2 : 1; tall : short = 3 : 1.

Example 3: Confirming 1 : 2 : 1 · 3 marks · Short answerHOTS / competency

In the F2 generation, all tall plants look alike. What experiment could confirm that they are of two kinds (TT and Tt) in the ratio 1 : 2? Predict the results.

Try it first, then show the solutionSolution

Let each tall F2 plant self-pollinate and grow its offspring separately (1 mark).
TT plants will give only tall offspring; Tt plants will give tall and short offspring in about 3 : 1 (1 mark).
About one-third of the tall F2 plants should breed true and two-thirds should give some short offspring, confirming TT : Tt = 1 : 2 (and with tt, 1 : 2 : 1) (1 mark).

Answer: Self each tall F2 plant: one-third breed true (TT), two-thirds give 3 tall : 1 short (Tt).

Example 4: Numbers in F2 · 3 marks · Short answerHarder board level

An F1 hybrid tall pea plant (Tt) is self-pollinated and gives 640 seeds that all grow. How many plants are expected to be (a) tall, (b) short, (c) heterozygous tall (Tt)? Show working.

Try it first, then show the solutionSolution

Tt × Tt gives TT : Tt : tt = 1 : 2 : 1 (1 mark).
(a) Tall = $\frac{3}{4} \times 640 = 480$ ; (b) short = $\frac{1}{4} \times 640 = 160$ (1 mark).
(c) Tt = $\frac{2}{4} \times 640 = 320$ . Check: TT 160 + Tt 320 = 480 tall (1 mark).

Answer: (a) 480 tall (b) 160 short (c) 320 Tt.

Example 5: How a gene makes a plant tall · 3 marks · Short answerHarder board level

Explain, with the example of plant height, how a gene controls a trait. Why does a Tt plant look tall rather than of medium height?

Try it first, then show the solutionSolution

A gene is a section of DNA that provides information for one protein, such as an enzyme (1 mark).
Height can depend on the amount of a growth hormone. An efficient enzyme makes a lot of hormone (tall); an altered gene gives a less efficient enzyme, less hormone, and a short plant (1 mark).
In Tt, one working copy (T) is enough to give the tall trait, so the plant is tall; Mendel saw no medium-height plants in F1 (1 mark).

Answer: Gene → enzyme → amount of growth hormone → height. One T copy is enough for tallness.

Where students lose marks in Mendel's Experiments and Monohybrid Cross

The slips examiners see most often, and what to write instead.

Mistake: Confusing phenotype and genotype ratios
Students give 1 : 2 : 1 when asked for the ratio of tall to short.
Wrong working
Tall : short in F2 = 1 : 2 : 1.
Right working
Tall : short = 3 : 1; TT : Tt : tt = 1 : 2 : 1.
Do this: Label each ratio as phenotypic or genotypic.
Usually costs: 1 mark.
Mistake: Writing two letters in a gamete
Students copy the parent genotype into the gamete.
Wrong working
Gametes of Tt: Tt and Tt.
Right working
Gametes of Tt: T and t (one copy each).
Do this: Each gamete gets one copy of each gene.
Usually costs: 1-2 marks in cross diagrams.
Mistake: Saying F1 plants are medium height
Students expect blending.
Wrong working
F1 plants are of medium height.
Right working
All F1 plants are tall; there is no blending.
Do this: Remember Mendel saw no halfway characteristics.
Usually costs: 1 mark.
Mistake: Deciding dominance from one family
Students over-read small data.
Wrong working
Daughter is O like her mother, so O is dominant.
Right working
One child is not enough to decide; both possibilities fit.
Do this: Test both hypotheses against the data before concluding.
Usually costs: Most of the marks on 'is this enough to decide dominance?' questions, such as the NCERT blood group exercise.

Exam tips

Draw a full cross: parents, gametes, F1, F1 gametes, checkerboard, ratios. Each line carries a mark.
Use a capital and small form of the same letter (T, t), not different letters.
State the conclusion in words: dominant trait, recessive trait, no blending.
Numericals: write the fraction, multiply, and check the totals add up.
Mendel's biography is in a box and is not assessed; focus on the experiments.

Practice questions on Mendel's Experiments and Monohybrid Cross

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In Mendel's cross between pure tall and pure short pea plants, the proportion of short plants in the F2 generation is:
(a) 0 (b) one-quarter (c) one-half (d) three-quarters
Hint
Only tt is short.
Answer
(b) one-quarter.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): The F1 plants of a cross between pure tall and pure short pea plants are all tall.
Reason (R): Tallness is a recessive trait.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Which trait shows in F1?
Answer
(c). Tallness is dominant, which is why all F1 plants are tall.
Q3 · 3 marks · Short answerHOTS / competency
Suppose a pea plant with violet flowers is crossed with a pea plant with white flowers, and only two of the seeds are grown: one plant has violet flowers and the other white. (a) Is this enough to decide which colour is dominant? Test both possibilities. (b) Describe a cross that would settle the question.
Hint
Try both possibilities and see if each fits the two plants. Then think of what Mendel did with pure-breeding plants.
Answer
(a) No. If violet were dominant, the violet parent could carry one violet and one white copy (Vv) and the white parent would be vv; their offspring could be violet (Vv) or white (vv), which fits. If white were dominant, the white parent could be Ww and the violet parent ww, which also gives both colours. Both explanations fit, and two plants are far too few (2 marks). (b) Cross a pure-breeding violet plant with a pure-breeding white plant (each one gives only its own colour when self-pollinated for generations). The colour shown by all the F1 plants is the dominant one; the other colour should reappear in about one-quarter of the F2 (1 mark).
Q4 · 3 marks · Short answerHOTS / competency
Suppose a tall pea plant with violet flowers is crossed with a short pea plant with white flowers. All the offspring have violet flowers, but about half of them are short. Using T/t for height and W/w for flower colour (violet dominant), work out the genetic make-up of the tall parent. Explain each step.
Hint
The short white parent is ttww. Half short means the tall parent gives t half the time; all violet means it never gives w.
Answer
The short white parent is ttww and gives only tw gametes. Half the offspring are short (tt), so the tall parent must give t in half its gametes: it is Tt. All offspring are violet, so the tall parent must always give W: it is WW. Tall parent = TtWW.
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a student crosses two tall pea plants and obtains 300 seeds; when grown, 225 plants are tall and 75 are short.
(a) What is the ratio of tall to short plants? (1 mark)
(b) What were the genotypes of the two parents? (1 mark)
(c) How many of the tall offspring are expected to be pure tall? Show working and explain why they cannot be picked out just by looking. (2 marks)
Hint
225 : 75 simplifies to a familiar ratio.
Answer
(a) 225 : 75 = 3 : 1. (b) Both Tt (only Tt × Tt gives short offspring from two tall parents in a 3 : 1 ratio). (c) TT = $\frac{1}{4} \times 300 = 75$ , i.e. one-third of the 225 tall plants. TT and Tt both look tall because one T copy is enough, so only a breeding test (self-pollination) can tell them apart.
Q6 · 2 marks · Very short answerHarder board level
Explain how a child ends up with the same amount of genetic material from the father as from the mother.
Hint
Think about chromosomes in germ cells.
Answer
Each body cell has two copies of each chromosome, one from each parent. Each germ cell receives only one chromosome from each pair. When a male and a female germ cell fuse, the zygote gets one set from each parent, restoring the normal number with equal contribution from both.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.F1 of pure tall × pure short peas?Show answer
All tall (Tt).
2.F2 phenotypic ratio of a monohybrid cross?Show answer
3 : 1.
3.F2 genotypic ratio?Show answer
1 TT : 2 Tt : 1 tt.
4.What did Mendel call genes?Show answer
Factors.
5.What is a gene?Show answer
A section of DNA that provides information for one protein.
6.Name three contrasting pea characters Mendel used.Show answer
Round/wrinkled seeds, tall/short plants, white/violet flowers.

Mendel's Experiments and Monohybrid Cross: frequently asked questions

What is a monohybrid cross?

A cross that follows one pair of contrasting traits, such as tall and short, through F1 and F2 generations.

Why are all F1 plants tall in Mendel's cross?

Each F1 plant gets T from the tall parent and t from the short parent. Tallness is dominant, so one T copy is enough to make the plant tall.

What is the difference between dominant and recessive traits?

A dominant trait shows even when only one copy of its gene is present. A recessive trait shows only when both copies are recessive.

Why do we get a 3 : 1 ratio in F2?

Tt × Tt gives TT, Tt, Tt and tt. Three of the four combinations contain T and are tall; only tt is short.

How do genes control traits?

A gene carries information for a protein such as an enzyme. In pea plants, an efficient enzyme can make more growth hormone, giving a tall plant.