Why is a dihybrid double recessive not one quarter?

One quarter applies to a single gene pair, and independent assortment adds another quarter, giving one sixteenth.

Why does risk differ between sons and daughters?

A son has only the X from his mother, so a carrier mother gives him a one in two risk, while a daughter also gets a normal X from her father.

S5 Biology: Mendelian Genetics (Paper 4)

2026-11-01

Tutor Prep Guide · Primary English

The fourth paper drills the arithmetic of inheritance: expected counts for one and two gene pairs, chances across several births, carrier fractions and the risk to sons in X-linked crosses, plus the double-recessive misconception.

Direct answer　One gene pair gives a quarter for the recessive, two independent pairs multiply to a sixteenth; independent births multiply; two thirds of tall plants are heterozygous; a son takes his X from his mother alone.

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)

FoundationA Tt by Tt cross yields 800 plants. How many dwarf plants are expected?
FoundationA TtGg by ttgg cross yields 320 plants. How many are expected to be ttgg?
FoundationAmong the tall offspring of a Tt by Tt cross, what fraction is heterozygous?
FoundationTwo Aa parents have 600 children. What is the expected normal to affected ratio and the numbers?
IntermediateA cross of TtGg with TtGg yields 1 600 plants. How many ttgg plants are expected?
IntermediateWhat is the chance that three children from Tt by Tt parents are all dwarf?
IntermediateOf 400 children from Aa by Aa parents, about how many are normal carriers?
ChallengeNormal parents already have one affected child. Among four more children, what is the chance that exactly two are affected?
ChallengeA carrier mother XCXc and a normal father XCY have 400 sons. How many are expected to be affected?
Error redoA student says a dihybrid 9:3:3:1 ratio means ttgg is one quarter. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	200 plants	Dwarf tt takes one quarter, and 800 times one quarter gives 200 plants.
2	80 plants	The ttgg class is one quarter, so 320 times one quarter gives 80 plants.
3	Two thirds	Tall plants cover one TT and two Tt squares out of three, so two thirds are heterozygous.
4	Three to one, that is 450 normal and 150 affected	The dominant to recessive ratio is three to one, so 450 are normal and 150 are affected.
5	100 plants	Two independent gene pairs give ttgg one sixteenth, and 1 600 over 16 gives 100.
6	One in sixty-four, about 1.6 per cent	Each child is dwarf with probability one quarter, and three independent events multiply to one over sixty-four.
7	About 200	About 300 are normal, split evenly between AA and Aa, so roughly 200 are carriers.
8	Twenty-seven over one hundred and twenty-eight, about 21.1 per cent	Each birth is affected with probability one quarter, six orders are possible, so six times one quarter squared times three quarters squared.
9	200 sons	A son takes his only X from his mother, so half receive Xc and 400 splits into 200.
10	In a 9:3:3:1 ratio the double recessive is one sixteenth, not one quarter	One quarter is the ratio for a single gene pair, and independent assortment adds another quarter to give one sixteenth.

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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