DSE Biology | Genetics and Mendel Teacher Prep Guide | Punnett squares, test crosses, 3-step pedigrees

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students memorise the 3:1 ratio and freeze when asked why. This lesson starts from gametes: count the gametes each parent makes, fill the grid, and the ratio falls out of the counting.

Objectives — Students can: (1) write every gamete from a parental genotype and derive offspring ratios from a Punnett square; (2) use a 1:1 or all-dominant test cross result to infer an unknown genotype; (3) read a pedigree using the rule that an unaffected pair with an affected child proves a recessive allele.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Cell division	How many cells meiosis makes and what happens to chromosome number	Four cells, halved
Alleles	What dominant and recessive mean	State the phenotype of Aa
Probability	Chance of two heads on two coins	One in four

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Start from gametes	Which gametes does Aa make, in what fractions	Write A and a at one half each
8-20 min	Fill the grid	Fill the 4 cells of Aa x Aa, gametes first	Draw it themselves
20-34 min	Read ratios	Genotypes 1:2:1, phenotypes 3:1; why not 1:1	Explain where 3:1 comes from
34-46 min	Test cross	Unknown dominant x recessive homozygote, offspring 1:1	Infer the parent genotype
46-56 min	Pedigrees	Three generations: find the recessive individuals first, then mark carriers	Label the whole chart
56-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Count gametes before ratios

Most students write 3:1 on sight but cannot say where it comes from. The order must be: (1) which gametes each parent makes (Aa gives A and a, half each); (2) pair them into the grid; (3) count phenotypes.
A ratio is an output, not a starting point. Reverse the order and dihybrid or linkage questions collapse.

Students do: draw the Aa x aa grid and explain why a test cross gives 1:1.

Scene 2: The grid is not four separate babies

The 4 cells are expected proportions of offspring, not one litter of four. Each cell has equal probability, so 1:2:1 means parts of four; scale to 400 offspring and you get 100:200:100.
Students must also keep two layers apart: genotype (AA, Aa, aa) and phenotype (dominant, recessive).

Students explain: what it means that Aa shows the dominant phenotype but still carries the recessive allele.

Scene 3: Pedigrees: break in with the unaffected parents

The fastest first move is to find an unaffected pair with an affected child: the trait must be recessive, and both parents must be heterozygotes.
Only then ask whether it is autosomal or sex-linked, using the sex distribution of affected individuals and whether every daughter of an affected father is a carrier.

Students do: circle the unaffected-pair-with-affected-child group on any pedigree before writing a genotype.

04 Question types and worked steps

Type	Example	Steps	Answer
Monohybrid cross	Aa x Aa	Gametes, grid, count phenotypes	Genotypes 1:2:1, phenotypes 3:1
Test cross	Unknown dominant x aa	Read ratio, infer parent genotype	1:1 heterozygous, all dominant homozygous
Pedigree	Three-generation chart	Unaffected pair first, then dominance, then linkage	Recessive; daughters of affected father are carriers
Combined probability	Two independent genes	Solve each, multiply, write ratio	9:3:3:1 is two 3:1 ratios multiplied

Sanity check (teach this): An unaffected pair with an affected child proves a recessive allele and heterozygous parents. A 2:2 offspring ratio must be simplified to 1:1 on the spot.

05 Independent practice (3 questions)

Question	Answer	What to watch
Monohybrid ratio	Write gametes, fill grid, count phenotypes	Answer in two lines: genotypes then phenotypes
Test cross inference	Offspring ratio back to the parent	1:1 means heterozygous; all dominant means homozygous dominant
Pedigree reading	Recessive first, then sex linkage	Every individual labelled; blanks mean unfinished
Combined probability	Multiply independent gene pairs	Work each pair separately, then multiply

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reciting 3:1 without the grid	Writes 3:1 straight away	“Draw the four cells first. Show me how you counted the three.”
Mixing genotype and phenotype	Says Aa is recessive	“Did the a show up in that plant? No, so which trait does it display?”
Leaving 2:2 unsimplified	Answers “two to two”	“Same fact, but a ratio must be in lowest terms. 1:1 is the ratio.”
Guessing sex linkage first	Claims X-linked before dominance is known	“Find the unaffected pair first. Without dominance, sex linkage is a guess.”

07 Homework (paper, section and question numbers)

Secondary Biology chapter index — All chapters and practice
DSE Biology practice paper — Genetics questions
Redo this chapter — Draw two grids, Aa x Aa and Aa x aa, with both ratios

08 Exit check (last 5 minutes)

Question	Answer	Decision
Gametes from Aa	A and a, half each	Answering “A only” means act one again
What 1:1 in a test cross means	The unknown parent is heterozygous	Answering homozygous means act two again
First move on a pedigree	Look for unaffected parents with an affected child	Counting males means act three again

09 Tutor FAQ

Is the grid compulsory?

For two gene pairs, yes. A grid is more reliable than mental arithmetic and shows exactly where a student stalls.

How does 3:1 relate to 9:3:3:1?

The dihybrid ratio is two independent 3:1 ratios multiplied, not a new rule.

Can pedigree work skip steps?

Details can be skipped, but never skip deciding dominance first; everything after that rests on it.

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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.