DSE Biology | Mendelian inheritance lesson plan (for tutors)

DSE Biology | Mendelian inheritance lesson plan (for tutors) | When 3:1 breaks down

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students can recite 3:1 but freeze when asked why it is three and not four. This lesson starts from the Punnett square so the ratio becomes something they compute, then explains exactly when it drifts.

Objectives — Students can: (1) fill a Punnett square for Aa x Aa and state 1:2:1 genotypes and 3:1 phenotypes with the probability of each cell; (2) use a test cross to separate AA from Aa; (3) state that 200 offspring give an expectation of 150 dominant and 50 recessive, and explain why small samples, gamete chance and survival shift the observed numbers.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Gametes and meiosis	How many gamete types does Aa make?	Two: A and a, half each
Multiplying probabilities	What is one half times one half?	One quarter
Dominant and recessive	Which trait does Aa show?	The dominant trait

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Opening question	Two brown-eyed parents (Aa): what is the chance of a blue-eyed child?	Guess, then verify
10-24 min	Punnett square	Write gametes in the four cells and count	Produce 1:2:1 and 3:1 themselves
24-36 min	Test cross	AA and Aa look identical; cross with aa and split 120 offspring 1:1	Compute 60 and 60
36-46 min	Sample size	200 offspring expect 150 dominant and 50 recessive, with scatter	Name three causes of drift
46-56 min	Answer format	Genotype, gametes, offspring, ratio in four steps	Rewrite one answer in full
56-60 min	Exit check	Mark on the spot and reteach the weak act	Finish three questions alone

03 Teaching script (read it out)

Scene 1: Replace memorising with a square

Aa x Aa: each parent gives A or a with equal chance, so the four cells read AA, Aa, Aa, aa. Genotypes are 1:2:1 and phenotypes 3:1. Say the probability of every cell out loud: aa is 0.5 x 0.5 = 0.25, a homozygote of either kind is 0.50. Filling the square once beats seeing the diagram ten times.

Ask them to: shade the four cells and count dominant against recessive.

Scene 2: The test cross: the only way to see inside

Phenotypes are identical, so read the offspring. Aa x aa gives a in half the gametes, so 120 offspring split roughly 60 dominant and 60 recessive, while AA x aa gives all dominant. When the paper says “crossed with a recessive individual”, it is asking for the genotype.

Ask them: a test cross giving 1:1 means the parent is what?

Scene 3: Why the ratio drifts: three reasons

200 offspring are not guaranteed 150:50. First, small samples scatter. Second, some genotypes die before birth and rewrite the ratio. Third, environment changes the phenotype, for example temperature altering enzyme activity. Write “expected ratio”, never “exactly this ratio” – that phrasing earns the mark.

Remind: with real counts, compute the expectation first, then compare.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Phenotype ratio of Aa x Aa	Square, then count cells	3:1
Deduction	Parent genotypes from an offspring ratio	1:1 means a test cross	Aa x aa
Explanation	Why observed numbers differ from expectation	Sample, lethality, environment	One sentence each
Application	Reading a pedigree	Find affected children first	Unaffected parents with an affected child means recessive

Sanity check (teach this): The ratio needs one gene, complete dominance, no linkage and no lethality. If the question says incomplete dominance the phenotype ratio becomes 1:2:1 – do not copy 3:1.

05 Independent practice (3 questions)

Question	Answer	What to watch
Expected dominant offspring from 200	150	Multiply by three quarters
Expected recessive from a test cross of 120	60	Aa x aa is half and half
Phenotypes of AA x aa	All dominant	Every offspring receives one A

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating 3:1 as genotypes	Writing “genotypes 3:1”	“3:1 is appearance; genotypes are 1:2:1 and you need both lines.”
Test cross against AA	Using AA as the tester	“Test unknown with recessive; recessive hides nothing.”
Ignoring sample size	Insisting 50 offspring must be 3:1	“The expectation is 3:1; the count scatters, so write expected.”
Mixing cells with fractions	Counting four cells as three parts	“Four cells are four equally likely outcomes, not three.”

07 Homework (paper, section and question numbers)

DSE biology practice paper (inheritance and evolution) — Section B pedigree and ratio questions
Secondary biology chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Genotype ratio of Aa x Aa	1:2:1	All three right: move to linkage
Expected recessive from a test cross of 120	60	One wrong: reteach the test cross
Three reasons ratios drift	Sample, lethality, environment	Two wrong: restart act one

09 Tutor FAQ

Must 9:3:3:1 be memorised?

Only for two independently assorting genes; check the wording before applying it.

Why do recessive traits never disappear?

They hide in heterozygotes and reappear on selfing or test crossing.

What to look for first in a pedigree?

Unaffected parents with an affected child – that is recessive.

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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.