Explain Mendel's law of dominance using monohybrid cross.

Question of 85

Q. Explain Mendel's law of dominance using monohybrid cross.

Assam AhsecAHSEC Higher Secondary (HS) Final Examination 2026Subjective· 5mImportance★★★★★

0% · 0/85 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Mendel's Experimental Design

Let’s begin with something you already know from everyday life. Suppose you mix red and white paint — you get pink. Now imagine you cross a tall pea plant with a short one. Common sense might say you’ll get a medium-sized plant. But Mendel found that you don’t. You get all tall plants in the first generation, and then in the next generation, short plants reappear as if they had never been mixed. That is the puzzle Mendel set out to solve, and his experimental design was the tool that cracked it.

The core idea: controlled, large-scale, and trait-by-trait

Mendel’s experimental design was not just “grow some peas and see what happens.” It was a carefully planned method that had three key features:

He chose a single trait at a time — for example, only seed shape (round vs wrinkled), not seed shape plus flower colour plus plant height all at once. This kept the results clear and interpretable.
He used pure-breeding lines — plants that, when self-pollinated, always produced offspring identical to themselves for that trait. A pure round-seeded plant always gave round seeds; a pure wrinkled-seeded plant always gave wrinkled seeds. This gave him a known starting point.
He counted large numbers — not just a handful of plants, but hundreds or thousands. This allowed patterns to emerge that would be invisible in small samples.

Important

Mendel’s genius was not in discovering that traits are inherited — farmers knew that. His breakthrough was in designing an experiment that could reveal the rules of inheritance by controlling variables, using pure lines, and counting systematically. This is why he is called the father of genetics.

The step-by-step plan Mendel followed

Mendel worked with the garden pea (Pisum sativum) for practical reasons: it was easy to grow, had a short generation time, and — crucially — its flowers normally self-pollinate, so he could control exactly which plants were crossed.

His typical experiment went like this:

Take two pure-breeding parents that differ in one trait — say, a pure tall plant and a pure short plant. These are the P generation (parental generation).
Cross them artificially — he removed the male parts (anthers) from one flower and dusted pollen from the other parent onto its stigma. This ensured no accidental self-pollination.
Grow the offspring — these are the F₁ generation (first filial generation). He observed that all F₁ plants were tall. The short trait had vanished.
Let the F₁ plants self-pollinate — he did not cross them again; he simply let them produce seeds on their own. These seeds grew into the F₂ generation (second filial generation).
Count and record — in the F₂ generation, both tall and short plants appeared, in a ratio of roughly three tall to one short. The short trait had reappeared.

He repeated this exact procedure for each of the seven traits he studied — seed shape, seed colour, flower colour, pod shape, pod colour, flower position, and stem height. Every time, the pattern was the same: one trait dominated in F₁, and both traits reappeared in a 3:1 ratio in F₂.

Why this design was revolutionary

Before Mendel, people believed in blending inheritance — the idea that parental traits mix like paint. Mendel’s design proved that inheritance is particulate: traits are carried by discrete units (what we now call genes) that remain intact across generations. The short trait did not disappear; it was merely hidden in the F₁ plants and reappeared unchanged in F₂.

Note

The NCERT textbook emphasises that Mendel’s success came from his scientific approach: he chose the right organism, studied one trait at a time, kept accurate records, and used mathematics to analyse his results. This was completely new for biology in the 1860s.

A few key terms you will encounter …

Start your 14-day free trial to unlock the full solution →

Monohybrid cross TT x tt -> all tall F1 (Tt); F2 = 3 tall : 1 dwarf (genotype 1:2:1), proving dominance. OR: Plasmodium multiplies asexually in humans (liver + RBCs) and sexually in female Anopheles.

Mendel's Law of Dominance using a monohybrid cross (main question):

The Law of Dominance states that (i) characters are controlled by discrete units called factors (genes), (ii) factors occur in pairs (alleles), and (iii) in a dissimilar pair, one factor (dominant) expresses itself and masks the other (recessive).

Monohybrid cross (pea plant height):

Parents (P): pure Tall (TT) x pure Dwarf (tt).
Gametes: T and t.
F1 generation: all Tt = Tall. Although the dwarf allele (t) is present, only the dominant Tall (T) is expressed — demonstrating dominance.
F1 x F1 (self-pollination): Tt x Tt. Using a Punnett square, gametes T, t x T, t give:
Genotypes: 1 TT : 2 Tt : 1 tt (ratio 1:2:1).
Phenotypes: 3 Tall : 1 Dwarf (ratio 3:1).
The dwarf trait, absent in F1, reappears in F2, proving that the recessive factor was not lost or blended — alleles remain discrete and separate, and one simply dominates the other in the heterozygote.

OR — Stages in the life cycle of the malarial parasite (Plasmodium):

The parasite is digenetic — needs two hosts.

Infection of humans: an infected female Anopheles mosquito injects sporozoites into the human blood while biting.
Liver (exo-erythrocytic) phase: sporozoites reach the liver, multiply asexually (schizogony), and release merozoites. …

Start your 14-day free trial to unlock the full solution →

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set EG1 markMCQ
$Q. How many years did Gregor Mendel conduct experiment on plants of garden pea?$
(a) Five years
(b) Two years
(c) Seven years
(d) Eleven years
›Reveal solutionSolution

Mendel conducted his hybridisation experiments on garden pea for about seven years — option (c).

Gregor Johann Mendel carried out his classic breeding experiments on the garden pea, Pisum sativum, in the monastery garden at Brno over roughly seven years (1856–1863). He chose pea because it has clear contrasting traits, is normally self-pollinating, has a short life cycle and is easy to hybridise. From these …

Start your 14-day free trial to see the full solution →
CBSE 2026Set ANNUAL1 mark
$Q. What is the cross called, when a dominant phenotype plant is crossed with the recessive parent plant instead of self-crossing?$
›Reveal solutionSolution

The cross of a dominant-phenotype individual with the homozygous recessive parent is a test cross.

Concept: A dominant phenotype can be produced by either a homozygous (TT) or a heterozygous (Tt) genotype, which cannot be told apart by appearance. …

Start your 14-day free trial to see the full solution →
CBSE 2025Set ANNUAL1 markMCQ
$Q. Which of the following claims would be accurate if garden pea plants from Mendel's Garden were sampled ?$
(a) Round seeds were more prevalent than wrinkled seeds
(b) Wrinkled seeds were more prevalent than round seeds
(c) Both round and wrinkled seeds were equally prevalent
(d) The response depended on the time of day the sample was taken
›Reveal solutionSolution

In Mendel's monohybrid cross, round (dominant) seeds outnumbered wrinkled (recessive) seeds roughly 3:1 in the F2 generation.

Mendel crossed true-breeding round-seeded pea plants with true-breeding wrinkled-seeded plants. The F1 generation was entirely round-seeded (round being the dominant trait), and when F1 plants were self-pollinated, the F2 generation segregated in an approximate 3:1 ratio of round to wrinkled seeds (Mendel actually recorded 5474 round : 1850 wrinkled, very close to 3:1). So if a sample were drawn from Mendel's garden pea plants (a mixed population across generations, dom …

Start your 14-day free trial to see the full solution →
CBSE 2024Set BOTANY1 markMCQ
$Q. Fill in the blank selecting the appropriate alternative: Out of seven pairs of contrasting characters of pea plant used by Mendel, ____ had dominant green and recessive yellow.$
(i) flower colour
(ii) pod colour
(iii) seed colour
(iv) stem colour
›Reveal solutionSolution

Pod colour is the trait among Mendel's seven where green is dominant and yellow is recessive.

Mendel studied seven pairs of contrasting characters in garden pea (Pisum sativum): seed shape (round dominant/wrinkled recessive), seed colour (yellow dominant/green recessive), flower colour (violet dominant/white recessive), pod shape (inflated dominant/constricted recessive), pod colour (green dominant/yellow recessive), flower position (axial dominant/terminal recessive), and stem height (t …

Start your 14-day free trial to see the full solution →
CBSE 2023Set A1 mark
$Q. Fill in the blank: ______ conducted hybridization experiments on garden peas for seven years.$
›Reveal solutionSolution

Mendel, an Austrian monk, performed pea (Pisum sativum) hybridisation experiments at the monastery in Brno and laid the foundation of genetics.

Gregor Johann Mendel chose the garden pea plant for his experiments because it had several contrasting, easily distinguishable traits (like round/wrinkled seeds, tall/dwarf plants), was self-pollinating (allowing pure-line breeding) but could also be cross-pollinated artificially, and had a short generation time. Over seven years of carefully planned cro …

Start your 14-day free trial to see the full solution →
CBSE 2022Set GO1 mark
$Q. How many pairs of contrasting traits were studied by Mendel in pea plant?$
›Reveal solutionSolution
Mendel selected 7 pairs of contrasting characters in pea.

Concept. For his hybridisation experiments Mendel chose the garden pea and worked with seven pairs of contrasting (alternative) traits, each showing a clear dominant and recessive form:

Seed shape — round / wrinkled

Seed (cotyledon) colour — yellow / green

Flower colour — violet / white …
Start your 14-day free trial to see the full solution →
CBSE 2018Set ANNUAL1 markMCQ
$Q. How many pairs of contrasting characters were studied by Mendel?$
(a) 5 pairs
(b) 6 pairs
(c) 7 pairs
(d) 8 pairs
›Reveal solutionSolution

Mendel selected seven distinct, easily distinguishable contrasting character-pairs in the garden pea for his hybridisation experiments.

Gregor Mendel conducted his classic breeding experiments on the garden pea (Pisum sativum) because it is self-pollinating, easy to cross-pollinate artificially, and has several traits with clear-cut contrasting (either/or) forms. He specifically chose and studied seven pairs of contrasting characters: (1) seed shape - round/wrinkled, (2) seed colour - yellow/green, (3) pod shape - inflated/constricted, (4) pod colour - green/yellow, (5) flower colour - violet/white, (6) flower position - axial/terminal, and (7) s …

Start your 14-day free trial to see the full solution →
CBSE 2018Set ANNUAL1 mark
$Q. Why is self pollination procedure essential among the different progeny hybrids for the selection of new recombinants in plant breeding programme?$
›Reveal solutionSolution

Repeated self-pollination of hybrid progeny fixes new gene combinations into a stable, true-breeding (homozygous) form, which is essential before a useful recombinant variety can be released.

In a plant breeding programme, after two parent varieties are crossed to produce hybrid progeny showing new combinations of desirable traits (recombinants), these progeny are still heterozygous and will continue to segregate/throw a mixture of different genotypes in subsequent generations if allowed to cross-pollinate freely. To 'fix' the desired new trait combination into a stable, uniform, true-breeding variety, breeders subject the selected progeny to repeated cycles of self-pollination across several successive generations. Self-pollination increases homozygosity generation after generation, so that eventually a line is …

Start your 14-day free trial to see the full solution →
CBSE 2018Set ANNUAL1 mark
$Q. Write the answer in one word/sentence: Write botanical name of maize.$
›Reveal solutionSolution

The botanical name of maize is Zea mays.

Maize, commonly called corn, is a monocot cereal crop belonging to the grass family (Poaceae). Its scientific binomial name, following Linnaeus' system of nomenclature, is Zea mays, where Zea is …

Start your 14-day free trial to see the full solution →
CBSE 2017Set ANNUAL1 markMCQ
$Q. Which one of the following trait of pea plant studied by Mendel is dominant?$
(a) Green pod colour
(b) Yellow pod colour
(c) White flower colour
(d) Terminal flower position
›Reveal solutionSolution

Of Mendel's seven pea traits, green pod colour is dominant over yellow.

Mendel studied seven contrasting traits in garden pea (Pisum sativum), each with a dominant and a recessive form:

Trait Dominant Recessive
Pod colour Green Yellow
Flower colour Violet White
Flower position Axial Terminal
Stem height Tall Dwarf
…

Start your 14-day free trial to see the full solution →

Trait	Dominant	Recessive
Pod colour	Green	Yellow
Flower colour	Violet	White
Flower position	Axial	Terminal
Stem height	Tall	Dwarf
…

←Previous chapterCh 3Reproductive Health

Q.Explain Mendel's law of dominance using monohybrid cross.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days