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A Level H2 Biology Genetics Inheritance Quiz

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Questions

A-Level Biology H2 Quiz - Genetics Inheritance

Name: __________________________
Class: __________________________
Date: __________________________
Score: _______ / 40

Duration: 45 minutes
Total Marks: 40

Instructions:

Answer all questions.
Write your answers in the spaces provided.
The number of marks is indicated in brackets [ ] at the end of each question or part question.
You are advised to spend approximately 2 minutes per mark.

Section A: Multiple Choice and Short Answer (Questions 1–5)

Answer all questions in this section.

1. In a population of snapdragons, flower colour is determined by a single gene with two alleles, $C^R$ (red) and $C^W$ (white). The alleles show codominance. A plant with pink flowers is crossed with a plant with white flowers. What is the expected phenotypic ratio of the offspring?

A. 1 Red : 1 White
B. 1 Red : 2 Pink : 1 White
C. 1 Pink : 1 White
D. All Pink

[1]

2. Which of the following statements correctly describes the difference between a gene and an allele?

A. A gene is a sequence of DNA, while an allele is a protein product.
B. A gene is a locus on a chromosome, while an allele is a specific variant of that gene.
C. Genes are found only on autosomes, while alleles are found only on sex chromosomes.
D. An allele is a group of genes that control the same trait.

[1]

3. Haemophilia is a sex-linked recessive condition caused by an allele on the X chromosome. A woman who is a carrier for haemophilia marries a man who does not have the condition. What is the probability that their first child will be a son with haemophilia?

A. 0%
B. 25%
C. 50%
D. 100%

[1]

4. Define the term test cross.

[2]

5. In mice, black fur (B) is dominant to brown fur (b). A heterozygous black mouse is crossed with a brown mouse. Construct a genetic diagram to show the expected genotypes and phenotypes of the offspring.

Parental Phenotypes: Black $\times$ Brown
Parental Genotypes: ______________ $\times$ ______________

Gametes: ______________ and ______________

Offspring Genotypes: __________________________

Offspring Phenotypes: __________________________
Ratio: __________________________

[3]

Section B: Structured Questions (Questions 6–15)

Answer all questions in this section.

6. The ABO blood group system in humans is controlled by three alleles: $I^A$ , $I^B$ , and $I^O$ . $I^A$ and $I^B$ are codominant, and both are dominant to $I^O$ .

(a) A man with blood group A and a woman with blood group B have a child with blood group O.
(i) State the genotypes of the man and the woman.
Man: __________________________
Woman: __________________________

[2]

(ii) Calculate the probability that their next child will have blood group AB. Show your working.

Probability: __________________________

[2]

7. Explain why male mammals are described as hemizygous for genes located on the X chromosome.

[2]

8. Distinguish between autosomal linkage and sex linkage.

[2]

9. In Drosophila melanogaster (fruit flies), grey body (G) is dominant to ebony body (g), and normal wings (N) are dominant to vestigial wings (n). These two genes are located on the same autosome.

(a) Define the term linkage.

[1]

(b) A homozygous grey-bodied, normal-winged fly was crossed with a homozygous ebony-bodied, vestigial-winged fly. All F1 offspring were grey-bodied with normal wings.
State the genotype of the F1 offspring.

[1]

Grey body, normal wings: 405
Ebony body, vestigial wings: 395
Grey body, vestigial wings: 95
Ebony body, normal wings: 105

(i) Explain why the results do not show a 1:1:1:1 ratio.

[2]

(ii) Calculate the cross-over value (recombination frequency) for these two genes. Show your working.

Cross-over value: __________________________ %

[2]

10. Polydactyly (extra fingers or toes) is caused by a dominant allele (D). A man with polydactyly, whose mother did not have the condition, marries a woman with normal digits.

(a) Deduce the genotype of the man. Explain your answer.

Genotype: __________________________
Explanation: _________________________________________________________________

[2]

(b) What is the probability that their first child will have polydactyly?

[1]

11. Epistasis occurs when one gene masks the expression of another. In Labrador retrievers, coat colour is determined by two genes:

Gene B: Black (B) is dominant to brown (b).
Gene E: Pigment deposition (E) is dominant to no deposition (e). If the genotype is ee, the dog is yellow regardless of the B/b genotype.

(a) State the phenotype of a dog with the genotype BbEe.

[1]

(b) Two dogs with the genotype BbEe are crossed. Calculate the expected phenotypic ratio of the offspring. Show your working using a dihybrid cross or logical deduction.

Ratio (Black : Brown : Yellow): __________________________

[3]

12. Describe the molecular basis of Sickle Cell Anaemia. In your answer, explain how a change in the DNA sequence leads to the altered phenotype of the red blood cell.

[4]

13. A pedigree chart shows the inheritance of a rare genetic disorder. The disorder appears in every generation, and affected fathers pass the trait to all their daughters but none of their sons.

(a) Identify the mode of inheritance.

[1]

(b) Explain your reasoning for part (a).

[2]

14. In a certain plant species, height is controlled by two unlinked genes (A and B). Each dominant allele adds 5 cm to a base height of 20 cm.

Genotype aabb = 20 cm
Genotype AaBb = 30 cm

(a) What is the height of a plant with genotype AABB?

__________________________ cm

[1]

(b) Two plants with genotype AaBb are crossed. What proportion of the offspring will have a height of 30 cm? Show your working.

Proportion: __________________________

[2]

15. Explain the significance of chiasmata formation during Prophase I of meiosis in terms of genetic variation.

[3]

Section C: Data Analysis and Extended Response (Questions 16–20)

Answer all questions in this section.

16. The following data was obtained from a cross between two heterozygous pea plants for seed shape (Round R, Wrinkled r) and seed colour (Yellow Y, Green y). The expected ratio for a dihybrid cross is 9:3:3:1.

Phenotype	Observed (O)	Expected (E)
Round, Yellow	315	312.75
Round, Green	101	104.25
Wrinkled, Yellow	108	104.25
Wrinkled, Green	32	34.75

(a) State the null hypothesis for this investigation.

[1]

(b) Complete the table below to calculate the Chi-squared ( $\chi^2$ ) value. Use the formula: $\chi^2 = \sum \frac{(O-E)^2}{E}$

Phenotype	O	E	O - E	$(O-E)^2$	$\frac{(O-E)^2}{E}$
Round, Yellow	315	312.75	2.25	5.06	0.016
Round, Green	101	104.25	-3.25	10.56	_______
Wrinkled, Yellow	108	104.25	3.75	14.06	_______
Wrinkled, Green	32	34.75	-2.75	7.56	_______
Total					$\chi^2 =$ _______

[3]

(c) The critical value for 3 degrees of freedom at p=0.05 is 7.82.
(i) Compare your calculated $\chi^2$ value with the critical value.

[1]

(ii) State your conclusion regarding the null hypothesis.

[1]

17. Cystic Fibrosis (CF) is an autosomal recessive disorder caused by mutations in the CFTR gene.

(a) Explain why two unaffected parents can have a child with Cystic Fibrosis. Use a genetic diagram to support your answer.

[3]

(b) Suggest why the frequency of the CF allele remains relatively high in some populations, despite the severe nature of the disease.

[2]

18. Fig. 18.1 (not shown) displays the results of gel electrophoresis for a family being tested for Huntington’s Disease, which is caused by a dominant allele (H). The normal allele is (h). The gel shows DNA fragments separated by size.

Lane 1: DNA Ladder
Lane 2: Father (Affected) - Shows one band at position X.
Lane 3: Mother (Unaffected) - Shows one band at position Y.
Lane 4: Child 1 - Shows bands at X and Y.
Lane 5: Child 2 - Shows one band at Y.

(a) Identify which band position (X or Y) corresponds to the Huntington’s allele. Explain your answer.

Band: __________________________
Explanation: _________________________________________________________________

[2]

(b) What is the genotype of Child 2?

[1]

19. Discuss the ethical implications of pre-implantation genetic diagnosis (PGD) in IVF treatment. Include one argument for and one argument against its use.

Argument For:

Argument Against:

[4]

20. In cats, the gene for coat colour is located on the X chromosome. The allele for black fur ( $X^B$ ) is codominant with the allele for orange fur ( $X^O$ ). Heterozygous females ( $X^B X^O$ ) are tortoiseshell (patches of black and orange). Male cats can only be black or orange.

(a) Explain why male cats cannot be tortoiseshell.

[2]

(b) A black female cat is crossed with an orange male cat.
(i) State the genotypes of the parents.
Female: __________________________
Male: __________________________

[1]

(ii) Predict the phenotypes of the offspring.

[2]

End of Quiz

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Answers

A-Level Biology H2 Quiz - Genetics Inheritance (Answer Key)

Total Marks: 40

Section A: Multiple Choice and Short Answer

1. C
[1]
Reasoning: Pink is $C^R C^W$ . White is $C^W C^W$ . Cross: $C^R C^W \times C^W C^W$ . Offspring: 50% $C^R C^W$ (Pink), 50% $C^W C^W$ (White). Ratio 1:1.

2. B
[1]
Reasoning: A gene is the locus/sequence; an allele is a specific variant form of that gene.

3. B
[1]
Reasoning: Mother (Carrier): $X^H X^h$ . Father (Normal): $X^H Y$ .
Possible sons: $X^H Y$ (Normal), $X^h Y$ (Haemophilic).
Probability of being a son is 0.5. Probability of son being haemophilic is 0.5.
Total probability = $0.5 \times 0.5 = 0.25$ (25%).
Note: If the question asks "Of their sons, what proportion...", it would be 50%. But "first child" implies from all possible births.

4. Definition of Test Cross
[2]

Crossing an individual with a dominant phenotype (unknown genotype) [1]
With an individual that is homozygous recessive [1]

5. Genetic Diagram
[3]

Parental Genotypes: Bb $\times$ bb [1]
Gametes: B, b (from parent 1) and b (from parent 2) [1]
Offspring Genotypes: Bb, bb [0.5]
Offspring Phenotypes: Black, Brown [0.5]
Ratio: 1:1 [1]

Section B: Structured Questions

6. ABO Blood Groups
(a)(i) Genotypes:

Man: $I^A I^O$ [1]
Woman: $I^B I^O$ [1]
(Must have $I^O$ to produce O child)

(a)(ii) Probability of AB:

Cross: $I^A I^O \times I^B I^O$
Possible offspring: $I^A I^B$ (AB), $I^A I^O$ (A), $I^B I^O$ (B), $I^O I^O$ (O)
Probability of AB ( $I^A I^B$ ) = 1/4 or 25% [2]
(1 mark for correct working/diagram, 1 mark for answer)

7. Hemizygous
[2]

Males have only one X chromosome (XY) [1]
Therefore, they possess only one allele for X-linked genes (cannot be homozygous or heterozygous) [1]

8. Autosomal vs Sex Linkage
[2]

Autosomal linkage: Genes are located on the same non-sex chromosome (autosome) [1]
Sex linkage: Genes are located on the sex chromosomes (X or Y) [1]

9. Drosophila Linkage
(a) Definition:

Genes located on the same chromosome [1]
They tend to be inherited together (do not assort independently) [1] (Accept "located close together on same chromosome")

(b) F1 Genotype:

GgNn (or $GN/gn$ to show linkage) [1]

(c)(i) Explanation of Ratio:

Genes are linked, so parental combinations (Grey/Normal and Ebony/Vestigial) are more frequent [1]
Recombinants (Grey/Vestigial and Ebony/Normal) are produced by crossing over during Prophase I of meiosis, which is a less frequent event [1]

(c)(ii) Cross-over Value:

Total offspring = $405 + 395 + 95 + 105 = 1000$
Recombinants = $95 + 105 = 200$
Calculation: $(200 / 1000) \times 100 = 20\%$ [2]
(1 mark for correct numerator/denominator, 1 mark for final answer)

10. Polydactyly
(a) Genotype and Explanation:

Genotype: Dd [1]
Explanation: The man has the dominant phenotype, so he has at least one D. His mother was normal (dd), so he must have inherited a d allele from her. [1]

(b) Probability:

Cross: Dd (Man) $\times$ dd (Woman)
Offspring: 50% Dd, 50% dd
Probability: 50% or 0.5 [1]

11. Epistasis (Labradors)
(a) Phenotype of BbEe:

Black [1] (Has B for black pigment and E for deposition)

(b) Phenotypic Ratio:

Cross: BbEe $\times$ BbEe
Standard dihybrid ratio: 9 B_E_ : 3 B_ee : 3 bbE_ : 1 bbee
B_E_ (Black) = 9
bbE_ (Brown) = 3
B_ee and bbee (Yellow, because ee masks B/b) = $3 + 1 = 4$
Ratio: 9 Black : 3 Brown : 4 Yellow [3]
(1 mark for identifying Black group, 1 for Brown, 1 for combining Yellow groups)

12. Sickle Cell Molecular Basis
[4]

Mutation is a base substitution in the DNA of the beta-globin gene [1]
This changes the mRNA codon, resulting in Valine replacing Glutamic Acid at position 6 [1]
Valine is hydrophobic (non-polar), whereas Glutamic Acid is hydrophilic (polar) [1]
Under low oxygen, hydrophobic patches interact, causing haemoglobin to polymerize/aggregate, distorting the RBC into a sickle shape [1]

13. Pedigree Analysis
(a) Mode of Inheritance:

X-linked Dominant [1]

(b) Reasoning:

Affected father passes to all daughters (daughters receive father's only X) [1]
Affected father passes to no sons (sons receive father's Y) [1]
(Note: If it were autosomal dominant, sons and daughters would have equal chance. If X-linked recessive, affected father would have carrier daughters, not necessarily affected, unless mother was carrier/affected).

14. Polygenic Inheritance
(a) Height of AABB:

Base 20 + 4 dominant alleles (5 each) = $20 + 20 = 40$ cm [1]

(b) Proportion of 30 cm offspring:

30 cm requires 2 dominant alleles (Base 20 + 10).
Cross AaBb $\times$ AaBb.
Genotypes with 2 dominant alleles: AAbb, aaBB, AaBb.
Probabilities:
- AAbb: $1/4 \times 1/4 = 1/16$
- aaBB: $1/4 \times 1/4 = 1/16$
- AaBb: $2/4 \times 2/4 = 4/16$
Total: $1/16 + 1/16 + 4/16 = 6/16$ or $3/8$ [2]
(1 mark for identifying correct genotypes, 1 mark for calculation)

15. Chiasmata and Variation
[3]

Chiasmata are points where non-sister chromatids touch/cross over [1]
Exchange of genetic material (alleles) between non-sister chromatids occurs [1]
This produces new combinations of alleles on the chromatids (recombinants), increasing genetic variation in gametes [1]

Section C: Data Analysis and Extended Response

16. Chi-Squared Test
(a) Null Hypothesis:

There is no significant difference between the observed and expected results [1]
(Or: The genes assort independently / follow the 9:3:3:1 ratio)

(b) Table Completion:

Round, Green: $10.56 / 104.25 = 0.101$ [1]
Wrinkled, Yellow: $14.06 / 104.25 = 0.135$ [1]
Wrinkled, Green: $7.56 / 34.75 = 0.218$ [1]
Total $\chi^2$ : $0.016 + 0.101 + 0.135 + 0.218 = 0.47$ [1]
(Allow small rounding errors, e.g., 0.46 - 0.48)

(c)(i) Comparison:

Calculated value (0.47) is less than the critical value (7.82) [1]

(c)(ii) Conclusion:

Accept the null hypothesis [1]
The difference between observed and expected is not significant (due to chance) [1] (Note: Only 1 mark allocated in Q16(c)(ii) in prompt, but usually 2 parts. Prompt says [1]. So just "Accept null" is sufficient, or "Results fit 9:3:3:1 ratio")

17. Cystic Fibrosis
(a) Unaffected Parents, Affected Child:

CF is recessive [1]
Parents are heterozygous carriers (Ff) [1]
Diagram: Ff $\times$ Ff $\rightarrow$ ff (25% chance) [1]

(b) High Frequency:

Heterozygote advantage [1]
Carriers may have resistance to other diseases (e.g., Cholera or Typhoid) [1]

18. Gel Electrophoresis (Huntington's)
(a) Identification:

Band X [1]
Father is affected and has Band X. Mother is unaffected and has Band Y. Since Huntington's is dominant, the affected allele must be present in the father and absent in the mother. Child 1 has both and is affected (implied or stated in context of dominant trait inheritance patterns). Correction: The prompt implies Child 1 has X and Y. If X is dominant (Huntington's), Child 1 is affected. If Y is normal, Child 2 (YY) is normal.
Explanation: The father (affected) passes the disease allele. He only has band X. Therefore X is the disease allele. [1]

(b) Genotype of Child 2:

hh (Homozygous recessive / Normal) [1] (Assuming H/h notation, or just "Normal homozygote")

Father (Hh) $\times$ Mother (hh)
Offspring: 50% Hh, 50% hh
Probability: 50% or 0.5 [1]

19. Ethical Implications of PGD
[4]

Argument For: Prevents suffering/serious genetic disease in the child; allows parents to have a healthy biological child; reduces emotional/financial burden of caring for a severely disabled child. [2] (1 for point, 1 for elaboration)
Argument Against: "Designer babies" / slippery slope to selecting for non-medical traits (e.g., gender, intelligence); destruction of embryos (ethical status of embryo); potential reduction in genetic diversity; psychological impact on "selected" vs "rejected" embryos. [2] (1 for point, 1 for elaboration)

20. Cat Coat Colour (Sex Linkage/Codominance)
(a) Why males cannot be tortoiseshell:

Males have only one X chromosome (XY) [1]
Therefore, they can only possess one allele for coat colour ( $X^B$ or $X^O$ ), not both [1]
Tortoiseshell requires both alleles ( $X^B X^O$ ) to be expressed (codominance).

(b)(i) Parental Genotypes:

Female: $X^B X^B$ [0.5]
Male: $X^O Y$ [0.5]

(b)(ii) Offspring Phenotypes:

Daughters: $X^B X^O$ (Tortoiseshell) [1]
Sons: $X^B Y$ (Black) [1]