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 1)

2026-10-31

Ten original questions move from chromosome number and gene definition to dominant and recessive alleles, 3:1 and 1:1 monohybrid ratios and the chance of a boy.

Direct answer　Write the parental genotypes before drawing a grid. The 3:1 and 1:1 ratios are expected values, so multiply the total count by the fraction.

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)

FoundationHuman body cells contain 46 chromosomes. How many pairs is that?
FoundationWhat is a gene?
FoundationHow do dominant and recessive alleles differ?
FoundationHow many chromosomes does a human gamete carry?
IntermediateA Tt x Tt cross produces 400 offspring. Using the 3:1 ratio, how many are tall and how many short?
IntermediateWhat are the genotypes of the F1 from a TT x tt cross?
IntermediateA Tt x tt cross gives 240 offspring. How many are expected tall and short?
ChallengeA couple plans children. What is the chance of a boy, and how many boys among 300 births?
ChallengeArrange DNA, gene and chromosome from smallest to largest.
Error redoA pupil treats phenotype and genotype as the same thing. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	23 pairs (46 divided by 2 = 23).	One chromosome of each pair comes from the father and one from the mother.
2	A section of DNA that controls one characteristic.	A gene sits on a chromosome and is the basic unit of inheritance.
3	A dominant allele shows even in a heterozygote; a recessive allele shows only in a homozygote.	Upper case letters are used for dominant alleles and lower case for recessive ones.
4	23, one from each of the 23 pairs.	Meiosis halves the chromosome number and fertilisation restores the pairs.
5	300 tall and 100 short (400 times three quarters and one quarter).	The 3:1 ratio is expected, so real counts may differ slightly.
6	All Tt, so all are tall.	Each parent donates one allele, so every F1 individual is heterozygous.
7	120 each, in a 1:1 ratio (240 divided by 2).	A cross with a homozygous recessive parent is a test cross used to reveal an unknown genotype.
8	Fifty per cent each, so about 150 boys (300 times 50 per cent).	Sex depends on whether the father contributes an X or a Y sperm; the mother always gives X.
9	DNA, then gene, then chromosome.	A gene is a length of DNA, and DNA with protein forms a chromosome.
10	Genotype is the allele combination, such as Tt; phenotype is the observable trait, such as tall.	Different genotypes can share one phenotype: TT and Tt are both tall.

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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