Question 1 · MCQ · Analyse · 1 mark
An unseen recessive allele
Mendelian inheritance
In pea plants, T is dominant for tallness and t is recessive. Which plant is tall but can pass a recessive t allele to its offspring?
(A) TT only
(B) Tt
(C) tt only
(D) No tall plant
Show answer and solution
Answer: (B) Tt
Worked solution: A Tt plant is tall because T is dominant, but its gametes can carry either T or t. A TT plant cannot contribute t.
Question 2 · MCQ · Analyse · 1 mark
A test cross gives two phenotypes
Mendelian inheritance
A tall pea plant is crossed with a dwarf tt plant. Many offspring include tall and dwarf plants in an approximately 1:1 ratio. What is the likely genotype of the tall parent?
(A) TT
(B) tt
(C) Tt
(D) It has no alleles for height
Show answer and solution
Answer: (C) Tt
Worked solution: Tt × tt produces Tt and tt offspring in equal expected proportions. TT × tt would give only tall Tt offspring in this simple model.
Question 3 · MCQ · Analyse · 1 mark
Human gametes and chromosomes
Sex determination
In the standard XX-XY model of human sex determination, which statement is correct?
(A) Eggs carry X; sperm can carry X or Y.
(B) Eggs carry Y; all sperm carry X.
(C) Every egg carries both X and Y.
(D) Both parents contribute an intact pair of sex chromosomes in each gamete.
Show answer and solution
Answer: (A) Eggs carry X; sperm can carry X or Y.
Worked solution: Gametes carry one sex chromosome. Eggs carry X, whereas sperm may carry X or Y; fertilisation gives the usual XX or XY combination.
Question 4 · MCQ · Analyse · 1 mark
A scar in the next generation
Inherited traits and variation
A parent has a scar caused by an accident. Why is the same scar not normally inherited by the child?
(A) Children inherit no DNA.
(B) Every acquired change is always inherited.
(C) A scar changes every gamete into an identical scarred cell.
(D) The injury does not normally alter the DNA passed through the gametes.
Show answer and solution
Answer: (D) The injury does not normally alter the DNA passed through the gametes.
Worked solution: The scar is an acquired change in body tissue. It does not normally change the hereditary information carried by reproductive cells.
Question 5 · A-R · Analyse · 1 mark
A recessive trait reappears
Mendelian inheritance
Assertion (A): Two tall heterozygous pea plants can produce dwarf offspring.
Reason (R): Each heterozygous parent can contribute a recessive t allele, forming a tt offspring.
(a) Both A and R are true, and R correctly explains A.
(b) Both A and R are true, but R does not correctly explain A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show answer and solution
Answer: (A) Both A and R are true, and R correctly explains A.
Worked solution: Tt × Tt can produce TT, Tt and tt. The tt offspring expresses the recessive dwarf phenotype.
Question 6 · A-R · Analyse · 1 mark
Does dominant mean common
Mendelian inheritance
Assertion (A): A dominant trait must always be the most common trait in every population.
Reason (R): Dominance describes how an allele is expressed in a heterozygote.
(a) Both A and R are true, and R correctly explains A.
(b) Both A and R are true, but R does not correctly explain A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show answer and solution
Answer: (D) A is false, but R is true.
Worked solution: Dominance concerns expression when two different alleles occur together. Population frequency depends on additional factors and need not be high for a dominant allele.
Question 7 · VSA · Apply · 2 marks
Reading a simple cross
Mendelian inheritance
For Tt × Tt in the simple dominant-recessive pea-height model, what fraction of offspring is expected to be tt and what phenotype do they show?
Show answer and solution
Answer: One quarter; dwarf.
Worked solution: Each parent contributes t with probability 1/2. Thus the expected fraction tt is (1/2) × (1/2) = 1/4.
Question 8 · VSA · Apply · 2 marks
Why the chromosome number stays stable
Genes and chromosomes
Explain how gamete formation and fertilisation together maintain the usual chromosome number across human generations.
Show answer and solution
Answer: Gametes have half the usual chromosome number; fertilisation restores the pair of sets.
Worked solution: Each gamete has 23 chromosomes in the usual human model. Fusion of egg and sperm restores 46 chromosomes, rather than doubling the chromosome number every generation.
Question 9 · SA · Analyse · 3 marks
Predicting an F1 generation
Mendelian inheritance
A pure tall TT pea plant is crossed with a pure dwarf tt plant. Give the gametes from each parent, the F1 genotype, and the F1 phenotype. Explain why the dwarf trait is not seen in F1.
Show answer and solution
Answer: Gametes: T and t; all F1 are Tt and tall. The dominant T allele masks t in a heterozygote.
Worked solution: Each TT parent gamete carries T and each tt parent gamete carries t. Every combination is Tt. Under complete dominance, T produces tallness in Tt, while t is retained and can be inherited even though dwarfness is not expressed.
Question 10 · SA · Analyse · 3 marks
Evaluating a claim about the mother
Sex determination
A claim says that only the mother determines whether a child is XX or XY. Use the usual chromosome model to evaluate the claim and explain whether the outcome is a deliberate parental choice.
Show answer and solution
Answer: The claim is incorrect: the egg contributes X, while the fertilising sperm contributes X or Y. The outcome is not a deliberate choice by either parent.
Worked solution: An X-bearing sperm with an X-bearing egg forms XX; a Y-bearing sperm with an X-bearing egg forms XY. Which sperm fertilises the egg is a chance event in this model, so blaming either parent is scientifically unjustified.
Question 11 · LA · Analyse · 5 marks
Building and interpreting an F2 cross
Mendelian inheritance
For Tt × Tt, (i) state the two possible gametes from each parent; (ii) list the four equally likely combinations; (iii) give the genotype ratio; (iv) give the phenotype ratio; and (v) explain why a small family need not match these ratios exactly.
Show answer and solution
Answer: T and t; TT, Tt, Tt, tt; genotype 1:2:1; tall:dwarf 3:1; ratios describe probabilities.
Worked solution: The cross has four equally likely combinations in the simple model. One is TT, two are Tt and one is tt. TT and Tt are tall, while tt is dwarf. These are expected proportions over many offspring; random fertilisation means a small sample can deviate from 3:1.
Question 12 · Case Study · Analyse · 4 marks
Testing a predicted ratio
Mendelian inheritance
In the simple Mendelian model, two Tt pea plants produce 160 offspring. T gives tallness and t gives dwarfness with complete dominance. Treat the model ratios as expected proportions, not guaranteed counts.
Answer the following:
(i) What fraction is expected to be dwarf? [1 mark]
(ii) What is the expected number of dwarf plants? [1 mark]
(iii) What is the expected number of heterozygous tall plants, and why? [2 marks]
Show answer and solution
Answer: (i) 1/4. (ii) 40. (iii) 80, because half of the expected genotypes are Tt.
Worked solution: Tt × Tt gives TT:Tt:tt = 1:2:1. Expected dwarf number is 160 × 1/4 = 40; expected heterozygous number is 160 × 1/2 = 80. Actual counts can differ through chance.
