What is the commonest genetics mistake?

Applying the 3:1 ratio before writing the parental genotypes; test crosses and homozygous parents then break it.

Do units matter in magnification questions?

Yes: mm and micrometres differ by a factor of 1000, so a missed conversion is out by three orders of magnitude.

Secondary Biology: Inheritance and Mendel (Paper 4)

2026-10-31

Ten original questions splitting 960 Tt×Tt seeds into 240, 480 and 240, counting the tall class, working a tt×tt cross, handling the 9/16 double dominant class, the AB chance from IAi×IBi, and the children of a colour-blind father and a homozygous normal mother.

Direct answer　Write the parental genotypes first, then use 1:2:1 for genotypes and 3:1 for phenotypes.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: Secondary Biology: Inheritance and Mendel
Level	4 basic, 3 advanced, 2 challenge and 1 redo question

02　Questions (10 items)

FoundationIn 960 seeds from a Tt×Tt cross, how many are expected to be homozygous tall TT?
FoundationIn the same 960 seeds, how many are expected to be short?
FoundationHow many chromosomes does a gamete produced by meiosis carry?
FoundationWhat phenotypes appear in the offspring of a tt×tt cross?
IntermediateHow many of the 960 seeds from Tt×Tt are tall?
IntermediateWhat is the genotype ratio TT:Tt:tt among Tt×Tt offspring, and what numbers does it give out of 960 seeds?
IntermediateTwo independently inherited traits each follow a 3:1 ratio. Out of 640 offspring, how many show both dominant traits?
ChallengeParents with blood groups IAi and IBi have a child. What is the chance the child is AB?
ChallengeA colour-blind father (XᶜY) has children with a homozygous normal mother (XX). What about the daughters and the sons?
Error redoKnowing that Tt×Tt gives 3:1, a student says 720 of the 960 seeds are short. What is the correct number?

03　Answers and one-line notes

#	Answer	One-line note
1	240 seeds, the pure-breeding tall quarter.	The offspring genotypes come in a 1:2:1 ratio, so a quarter of the total is each homozygous class.
2	240 seeds, since tt is one quarter of the offspring.	Shortness is recessive and shows only in tt, which is one quarter of the offspring.
3	Twenty-three, half of the 46 in a body cell.	Meiosis halves the chromosome number so that fertilisation restores the full 46.
4	All short, that is 100 per cent.	Both parents carry only the recessive allele, so no dominant offspring can appear.
5	720 seeds.	The dominant class covers both homozygote and heterozygote, so add the first two parts of 1:2:1.
6	One to two to one, that is 240, 480 and 240 seeds.	The 3:1 figure is the phenotypic ratio; 1:2:1 is the genotypic ratio and the two must not be mixed up.
7	360 offspring, that is 9/16 of 640.	With independent assortment the double dominant class is 9/16, obtained from three quarters times three quarters.
8	One in four.	Four gamete combinations are possible and only IAIB gives group AB, so the chance is one quarter.
9	All daughters are carriers but not colour-blind, and all sons are normal.	The father passes his affected X only to daughters; sons receive his Y, so they cannot inherit colour blindness from him.
10	240 seeds, because the 3:1 ratio is tall to short.	The error comes from reversing the order of the ratio; state which trait comes first before calculating.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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