Why is the first filial generation uniform?

Each pure parent supplies only one allele, so all offspring are heterozygous and show the dominant trait.

Why does a test cross use a recessive parent?

A recessive parent hides nothing, so the offspring reveal what gametes the tested plant supplies.

S5 Biology: Mendelian Genetics (Paper 1)

2026-11-01

The first paper builds the basics: alleles, dominance, the three to one phenotypic and one to two to one genotypic ratios of a monohybrid cross, and the purpose of a test cross.

Direct answer　A Tt by Tt cross gives three to one phenotypes and one to two to one genotypes; a tt test cross reveals whether a dominant plant is pure or hybrid; gene pairs on different chromosomes assort independently.

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: S5 Biology: Mendelian Genetics
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationWhat do we call different versions of a gene controlling the same character?
FoundationA pure tall TT plant is crossed with a pure dwarf tt plant. What are all the offspring like?
FoundationA Tt plant is tall. Which allele is dominant?
FoundationWhat is the phenotypic ratio from a cross of Tt with Tt?
IntermediateA Tt by Tt cross yields 400 plants. How many are expected to be tall and dwarf?
IntermediateWhat is the genotypic ratio from Tt by Tt?
IntermediateTo find out whether a tall plant is TT or Tt, what should it be crossed with?
ChallengeA Tt by Tt cross gives 1 600 plants. About how many are dwarf?
ChallengeWhen T/t and G/g sit on different chromosome pairs, which law governs their inheritance?
Error redoA student says the three to one ratio is the genotypic ratio. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Alleles	Different versions at the same locus are alleles, such as T and t both governing stem height.
2	All Tt and all tall	Each parent supplies only one kind of allele, so all offspring are Tt and, since T is dominant, all are tall.
3	T is dominant and t is recessive	The trait shown in a heterozygote is set by the dominant allele, so the recessive t is masked.
4	Tall to dwarf, three to one	Three of the four combinations carry T, and only tt is dwarf, giving a three to one ratio.
5	300 tall and 100 dwarf	Four hundred times three quarters gives 300, and one quarter gives 100.
6	TT to Tt to tt, one to two to one	The four squares read TT, Tt, Tt and tt, with the middle two both heterozygous.
7	A tt test cross; all tall offspring means TT, any dwarf means Tt	A recessive partner hides nothing, so the offspring reveal the gametes of the plant under test.
8	About 400	Dwarfs make up one quarter, and 1 600 divided by four gives 400, with some natural deviation.
9	The law of independent assortment	Different chromosome pairs move to the poles at random in meiosis, so the two gene pairs assort independently.
10	Three to one is the phenotypic ratio; the genotypic ratio is one to two to one	Phenotype only looks at appearance, where TT and Tt are both tall, while genotype separates all three classes.

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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