12 min readPathway sheet1-mark MCQ, a 3-mark cross with a Punnett square, and part of a 5-mark answer on Mendel's laws; also a NEET favourite

Mendel's monohybrid cross: dominance and segregation

Mendel crossed pure pea plants that differed in one trait. The 3:1 ratio in F2 showed that genes come in pairs and separate during gamete formation.

Heredity and variationHeredity: passing of characters from parents to offspring. Variation: how much offspring differ from their parents

Mendel's materialGarden pea (Pisum sativum), 7 pairs of contrasting traits, experiments from 1856 to 1863

Monohybrid F2 ratiosPhenotype 3 tall : 1 dwarf; genotype 1 TT : 2 Tt : 1 tt

Test crossCross a dominant-looking plant with the recessive parent: all dominant offspring means TT, a 1:1 ratio means Tt

Heredity, variation and Mendel's method

Heredity is the passing of characters from parents to offspring. Variation is how much offspring differ from their parents. Gregor Mendel studied heredity in the garden pea for seven years, from 1856 to 1863. He used true-breeding lines: plants that show the same trait over many generations of self-pollination. He chose 14 such varieties, in 7 pairs that differed in just one character. He counted large numbers of plants and used statistics, so his results were reliable. Mendel called the units of inheritance factors; we now call them genes. Slightly different forms of one gene, such as T (tall) and t (dwarf), are alleles. The allele make-up, such as Tt, is the genotype; the visible trait, such as tall, is the phenotype. TT and tt are homozygous (both alleles alike); Tt is heterozygous.

The seven pairs of contrasting traits in pea

Character	Dominant trait	Recessive trait
Stem height	Tall	Dwarf
Flower colour	Violet	White
Flower position	Axial	Terminal
Pod shape	Inflated	Constricted
Pod colour	Green	Yellow
Seed shape	Round	Wrinkled
Seed colour	Yellow	Green

The monohybrid cross, generation by generation

A **monohybrid cross** follows one pair of contrasting traits, here tall and dwarf stems.

1
Parents (P): true-breeding tall (TT) × true-breeding dwarf (tt). The anthers of one parent are removed (emasculation) and its stigma is dusted with pollen from the other.
2
Gametes: TT makes only T gametes; tt makes only t gametes.
3
F1: every plant is Tt and tall. The dwarf trait does not show at all.
Law of dominance
4
F1 selfed: Tt × Tt. Each plant makes T and t gametes in equal numbers (1/2 each).
Law of segregation
5★
F2 from the Punnett square: 1 TT : 2 Tt : 1 tt. Phenotypic ratio 3 tall : 1 dwarf; genotypic ratio 1 : 2 : 1.
6
F3 (selfing F2 plants): tt plants always give dwarf plants and TT always give tall, but Tt plants give 3 : 1 again.
So only 1 in 3 tall F2 plants is pure tall

★ key step

Law of dominance vs law of segregation

Law of dominanceCharacters are controlled by factors that occur in pairs. In a dissimilar pair, one factor (the dominant one) shows and the other (the recessive one) stays hidden. It explains why all F1 plants are tall.

Law of segregationThe two alleles of a pair do not blend. They separate during gamete formation, so each gamete gets only one. A homozygote makes one kind of gamete; a heterozygote makes two kinds in equal proportion. It is also called the law of purity of gametes.

Dominance explains why F1 looks like one parent; segregation explains why the hidden dwarf trait comes back in F2.

A test cross and a probability question

Problem

A tall pea plant of unknown genotype is crossed with a dwarf plant. Of 120 offspring, 61 are tall and 59 are dwarf. (a) What is the genotype of the tall plant? (b) If this tall plant is self-pollinated, what fraction of its offspring will be pure tall?

Working

Dwarf is recessive, so the dwarf parent is tt and gives only t gametes.
If the tall plant were TT, every offspring would be Tt and tall.
Instead we see about 1 tall : 1 dwarf (61 : 59).
So the tall plant gives T and t gametes equally: it is Tt.
Selfing Tt × Tt gives 1 TT : 2 Tt : 1 tt.
Pure tall (TT) is 1 out of 4 offspring. Among the tall offspring alone, it is 1 out of 3.

Answer

(a) Tt (heterozygous). (b) 1/4 of all offspring are pure tall; among the tall ones, 1/3 are pure.

How to draw a Punnett square for a monohybrid cross

Write the parents with phenotype and genotype: Tall (TT) × Dwarf (tt), and then the F1: Tall (Tt).
For F1 × F1, write each parent's gametes in circles: T and t.
Draw a 2 × 2 grid. Write the male gametes across the top and the female gametes down the left side.
Fill each box by joining the gamete above it and the gamete beside it: TT, Tt, Tt, tt.
Below the grid, write the phenotypic ratio 3 : 1 and the genotypic ratio 1 : 2 : 1.
Title it 'Monohybrid cross (Punnett square)'. The grid is named after Reginald C. Punnett.

In real life

Seed producers use test crosses to make sure a new pea, rice or brinjal variety is true breeding before they sell it, so farmers get the same crop every season.

Exam angle

2 marks: “What is a test cross? Why is it done?” Crossing an organism showing the dominant phenotype with the homozygous recessive parent. It tells the unknown genotype: all offspring dominant means homozygous (TT); 1 : 1 means heterozygous (Tt).

3 marks: “Explain the law of segregation using a monohybrid cross between tall and dwarf pea plants.” P: TT × tt; gametes T and t; F1 Tt, all tall; F1 selfed, each makes T and t equally; Punnett square; F2 3 tall : 1 dwarf and 1 TT : 2 Tt : 1 tt. The alleles stay unchanged in Tt and separate at gamete formation.

5 marks: “State Mendel's law of dominance and law of segregation. Work out a monohybrid cross up to F2 and give both ratios.” State both laws; P, gametes, F1; selfing; labelled Punnett square; phenotypic 3 : 1 and genotypic 1 : 2 : 1; F3 shows 1/3 of tall F2 plants breed true.

Remember it

Monohybrid: 3:1 is what you see (phenotype), 1:2:1 is what you are (genotype).