DSE Biology | Genetics and Mendel: Tutor Lesson Plan

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students recite 3:1 but cannot say why, and fall apart at two genes. This lesson starts from the Punnett square: write the gametes, fill the grid, and only then read the ratio, so the ratio becomes a result rather than a chant.

Objectives — Students can: (1) tell a genotype from a phenotype; (2) build the Tt x Tt square and get 3:1, so 300 tall and 100 short out of 400; (3) use a test cross Tt x tt to get 1:1 and explain why it reveals homozygosity; (4) work RrYy x RrYy to 9:3:3:1, which is 180:60:60:20 out of 320; (5) find that two children of two carriers are both affected with probability 1/16, or 6.25 %.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Dominance	T is tall and t is short; what is Tt?	Tall
Gametes	Which gametes does Tt make?	T and t, half each
Probability	Two independent events each with probability 1/4: both?	1/16
Simplifying	Simplify 300 to 100.	3 to 1

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Traits and alleles	Introduce height and earlobes	Classify their own earlobes
8-18 min	Genotype and phenotype	Show TT, Tt and tt	Mark the pure-breeding one
18-30 min	Monohybrid cross	Draw Tt x Tt and count 3:1	Draw and count their own
30-40 min	Test cross	Tt x tt gives 1:1 and its purpose	Explain why it tests purity
40-50 min	Dihybrid cross	Gametes RY, Ry, rY, ry then a 4×4 grid	Write all four gametes
50-60 min	Probability and exit	Two children both affected 1/16	Three questions in eight minutes

03 Teaching script (read it out)

Scene 1: Ratios come from gamete combinations, not from memorised slogans

Tt x Tt gives gametes T and t in equal numbers. The square holds TT, Tt, Tt and tt. TT and Tt both look tall, so tall to short is 3:1, while the genotype ratio is 1:2:1. Make students say which ratio answers which question; both appear in the same paper item and mixing them loses all the marks. Out of 400 offspring that is 300 tall and 100 short.

Ask the student to point at each cell and name its genotype before counting anything.

Scene 2: A test cross tests for pure breeding

A dominant individual may be TT or Tt and look identical. Cross it with tt: all tall offspring means TT, a 1:1 split means Tt. Out of 200 offspring, 100 tall and 100 short identifies the heterozygote. The scoring sentence is that the recessive parent can only donate t, so any short offspring proves the dominant parent carries t.

Ask whether 100 tall offspring proves TT; the answer is that it supports it with high probability, not certainty.

Scene 3: Dihybrid crosses start with four gametes, then a 4×4 grid

RrYy makes RY, Ry, rY and ry, each with probability 1/4. Crossing two such parents gives 16 cells: 9 double dominant, 3 round wrinkled, 3 green round, 1 green wrinkled, the 9:3:3:1 ratio. For 320 offspring divide by 16 to get 20, then multiply by 9, 3, 3 and 1, which is more reliable than reciting the ratio. Warn that linkage on one chromosome shifts the ratio.

Require the four gametes in writing before any grid is drawn.

04 Question types and worked steps

Type	Example	Steps	Answer
Monohybrid cross	Tt x Tt	Gametes, 2×2 grid, count phenotypes, simplify	3:1 (300 to 100)
Test cross	Tt x tt	Recessive gives only t, fill the grid, read 1:1, state the conclusion	1:1 (100 to 100)
Dihybrid cross	RrYy x RrYy	Four gametes, 4×4 grid, allocate by sixteenths, simplify	9:3:3:1 (180:60:60:20)
Probability	Two children of two carriers	Each child 1/4, multiply independent events, convert to a percentage	1/16 = 6.25 %

Sanity check (teach this): Totals check: 300 plus 100 is 400 and 180 plus 60 plus 60 plus 20 is 320, so both allocations are complete. A 1/16 chance is smaller than the 25 % for one child, which is the right direction. A result above 25 % means the student conflated both affected with at least one affected, which is 1 – (3/4)^2 = 43.75 %.

05 Independent practice (3 questions)

Question	Answer	What to watch
Bb x Bb: how many of 160 offspring show the recessive trait?	40	Do they use 1/4 rather than 1/2
How many gamete types does Aabb make?	Two: Ab and ab	Do they treat the genes independently
Two carriers have a boy and a girl: chance both are affected?	1/16 = 6.25 %	Did they multiply sex in unnecessarily

06 Common mistakes and what to say

Mistake	Student writes	Say this
Swapping phenotype and genotype ratios	“Tt x Tt gives 1:2:1 tall”	“1:2:1 counts genotypes; 3:1 counts what you can see, and the question asks what you see.”
Only two gametes for a dihybrid	“RrYy makes RY and ry”	“Two genes assort independently: write RY, Ry, rY and ry, a quarter each.”
Adding probabilities	“Each child is 1/4, so both is 1/2”	“Two children are independent events, so multiply: 1/4 times 1/4 is 1/16.”
Claiming certainty from four offspring	“Four tall offspring means TT”	“A Tt parent still gives four tall young one time in sixteen, so quote the sample size and the probability.”

07 Homework (paper, section and question numbers)

Biology chapter index — Revise the cell cycle first, then questions 1 and 2
DSE Biology paper — Do the genetics section in 12 minutes and draw every square
Cell cycle and mitosis — Question 3 compares meiosis, so separate the two purposes first

08 Exit check (last 5 minutes)

Question	Answer	Decision
Phenotype ratio of Tt x Tt	3:1	Error: written as 1:2:1
Recessive count among 160 Bb x Bb offspring	40	Error: used 1/2
Phenotype ratio of RrYy x RrYy	9:3:3:1	Error: middle classes missing
Two children of two carriers both affected	1/16 = 6.25 %	Error: probabilities added

09 Tutor FAQ

What does Mendel law assume?

Genes on different chromosomes or far apart, no linkage, no lethality and a large sample; break any one and the ratio shifts.

Why are real counts never exactly 3:1?

Because it is probability, and small samples fluctuate; exam questions give large samples so a chi-square or a judgement is possible.

Can a recessive trait skip a generation?

Yes. A carrier Aa looks normal but passes a, so two carriers can produce aa again, which is the skipped generation.

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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.