DSE Biology | Monohybrid inheritance: Punnett squares, the 3:1 ratio and test crosses

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students can chant 3 to 1 but cannot say whether it is a genotypic or a phenotypic ratio. This lesson fixes one boundary: the square counts gametes, so name the ratio before writing it.

Objectives — By the end students can: 1) write genotypes and gametes from parental phenotypes; 2) use a Punnett square to obtain F1 and F2 ratios; 3) use a test cross to identify an unknown dominant genotype; 4) read a pedigree backwards from a recessive individual.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Dominance	Which phenotype appears when T and t are together	The dominant T
Gametes	Which gametes a Tt individual makes	T and t, half each
Fertilisation	What two gametes form	One genotype with two alleles

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Writing genotypes	Homozygous, heterozygous, one capital is enough	Write six genotypes
8-20 min	Gametes and the square	Tt x Tt in four boxes	Students fill the boxes
20-34 min	Reading ratios	1:2:1 genotypes, 3:1 phenotypes	From 400 offspring to 100
34-46 min	Test cross	Unknown dominant crossed with tt	Design one test cross
46-54 min	Pedigrees	Work back from recessive individuals	Mark every carrier of t
54-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Genotypes and gametes: two letters first

Use a capital T for the dominant allele and a lower-case t for the recessive one. TT and tt are homozygous, Tt is heterozygous. The phenotype depends only on whether a T is present. Each gamete carries one allele, so Tt gives T and t in equal numbers while TT gives only T. Ask students to hold up two fingers and separate them: that is a gamete.
In Tt x Tt both parents contribute T and t, so the four boxes are TT, Tt, Tt and tt. The genotypic ratio is 1:2:1 and the phenotypic ratio is 3:1. From 400 offspring the expected recessive count is 100, which is 400 times one quarter. Always name the ratio: 3:1 is phenotypes, 1:2:1 is genotypes, and both total four parts. Then ask what a 2:1 ratio would suggest, namely a lethal homozygous dominant.

A test cross mates the unknown dominant individual with a homozygous recessive tt parent. All dominant offspring means the parent was TT; a 1:1 split means it was Tt. In a pedigree find the recessive individuals first, since they must be tt, then work backwards: every tt has at least one parent carrying t. Ask students to mark only people who must carry t and the answer appears.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Punnett ratios	List gametes, fill boxes, count	Tt x Tt gives 3:1
Deduction	Parental genotypes from offspring	All dominant means TT x tt, 1:1 means Tt x tt	100 recessive in 400
Explanation	Why a test cross uses tt	The recessive parent hides nothing	tt contributes only t
Inference	Pedigree genotypes	Work back from the recessive individual	Mark carriers of t

Sanity check (teach this): Three checks: a single-gene ratio must total four parts, a 3:1 phenotype ratio only comes from two heterozygotes, and a recessive individual is always tt. After computing 400 offspring, 300 and 100 must still total 400.

05 Independent practice (3 questions)

Question	Answer	What to watch
Proportion of homozygous dominant in the F2 of Tt x Tt	1 in 4	3:1 is phenotypes; only TT is homozygous dominant
Phenotypic ratio of Tt x tt	1:1	A test cross splits half and half
Expected recessive count in 400 F2 offspring	100	400 times one quarter
Recessive proportion in the F2 of TT x tt selfed	1 in 4	The F1 is all Tt

06 Common mistakes and what to say

Mistake	Student writes	Say this
Calling 3:1 a genotypic ratio	Answering 3:1 genotypes	Genotypes are 1 to 2 to 1; 3 to 1 is the phenotype ratio.
Using the wrong parent in a test cross	Crossing with TT	TT hides everything, so cross with tt instead.
Writing a gamete as Tt	Answering Tt as the gamete	A gamete carries one allele, so it is T or t.
Assuming outsiders are TT	Filling unknown people as TT	Leave unknown outsiders as a question mark or possible Tt.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — Redo the three Punnett square questions
Tutor preparation guide hub — Pair with the cell division chapter

08 Exit check (last 5 minutes)

Question	Answer	Decision
Phenotypic ratio of Tt x Tt	3:1	All three right: move to dihybrid crosses
Genotype used in a test cross	tt	One wrong: reteach gametes
Recessive count in 400 F2 offspring	100	Two wrong: restart act one

09 Tutor FAQ

Must students memorise 9:3:3:1?

Only for dihybrid crosses; a single gene stops at 3:1.

Why is a recessive phenotype always homozygous?

One dominant allele is enough to show the dominant trait.

Must a test cross use a recessive parent?

Yes, a tt parent contributes only t, so offspring reveal the unknown gametes.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.