DSE Biology: Genetics and Mendel — Tutor Lesson Plan (monohybrid cross)

2026-10-27

Tutor lesson-prep guides · Secondary biology

Students memorise 3:1 but cannot draw the cross. One monohybrid cross separates genotype from phenotype and the ratio falls out.

Objectives — the student can distinguish genotype from phenotype, complete a monohybrid cross, and explain and use the 3:1 ratio.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Cell division	What does meiosis produce?	Gametes with half the chromosome number
Chromosomes	How many in a human body cell?	46
Probability	Two coins both heads	One quarter

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Vocabulary	Gene, allele, genotype, phenotype	Sort four terms
8-22 min	The cross	Aa times Aa	Draw one
22-36 min	Where 3:1 comes from	Four boxes, one is aa	Explain
36-48 min	Test cross	Aa times aa	Ratio
48-56 min	Probability	Predict offspring	One question
56-60 min	Exit check	Three questions marked	Finish alone

03 Teaching script (read it out)

Scene 1: Genotype and phenotype differ

Aa and AA look the same but have different genotypes.
Genotype uses letters, phenotype describes the trait.
Writing that Aa is recessive is the commonest error.

Student states both for Aa.

Scene 2: Four boxes do the work

Aa times Aa: gametes are A and a from each parent, giving AA, Aa, Aa, aa.
Genotypes 1:2:1, phenotypes 3:1.
The ratio is counted, not memorised.

Student draws the grid and labels it.

Scene 3: The test cross reveals the unknown

To find whether a dominant individual is AA or Aa, cross it with aa.
Any recessive offspring proves the parent was Aa.

Warn: state that aa offspring appeared.

04 Question types and worked steps

Type	Example	Steps	Answer
Vocabulary	Genotype and phenotype of Aa	Aa; dominant trait	Both stated
Cross	Aa times Aa	AA, Aa, Aa, aa	3:1
Test cross	Dominant crossed with aa gives recessive offspring	Parent was Aa	Because aa appeared
Probability	Probability of aa from Aa times Aa	One quarter	One box in four

Sanity check (teach this): One phenotype can come from two genotypes, so a phenotype alone never fixes the genotype.

05 Independent practice (3 questions)

Question	Answer	What to watch
Phenotype ratio of Aa times Aa	3 : 1	Count the boxes
Phenotype ratio of Aa times aa	1 : 1	Two boxes each
Probability of aa from Aa times Aa	1/4	One box in four

06 Common mistakes and what to say

Mistake	Student writes	Say this
Genotype and phenotype mixed	calls Aa recessive	Aa is dominant in phenotype.
Ratio wrong	says 1:1 for Aa times Aa	It is 3:1.
Gametes omitted	grid with no gametes	Write A and a on the edges.
Test cross reasoning skipped	just says Aa	Because recessive offspring appeared.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — Use the three questions on this page

08 Exit check (last 5 minutes)

Question	Answer	Decision
Phenotype of Aa	Dominant	All right: genetics introduced
Phenotype ratio of Aa times Aa	3 : 1	One wrong: redraw the cross
Probability of aa	One quarter	Two wrong: repeat scene 2

09 Tutor FAQ

Teach dihybrid crosses now?

Not yet; stabilise the monohybrid first.

Memorise the ratios?

No, the four boxes produce them.

Pedigrees?

Mark genotypes first, then phenotypes.

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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.