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O Level Biology Genetics Inheritance Quiz
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Questions
O-Level Biology Quiz - Genetics Inheritance
Name: _________________________ Class: _________________________ Date: _________________________ Score: ______ / 50
Duration: 45 minutes Total Marks: 50
Instructions:
- Answer ALL questions in the spaces provided.
- The number of marks is given in brackets [ ] at the end of each question or part question.
- Show all working for calculation questions.
- Use appropriate biological terminology throughout.
Section A: Short Answer Questions (10 marks)
Answer all questions in this section.
1. Define the term allele. [1]
2. Distinguish between the terms genotype and phenotype. [2]
3. State the number of chromosomes present in a human gamete. [1]
4. A person with blood group AB is said to show codominance. Explain what is meant by codominance. [2]
5. Name the process of cell division that produces gametes and state how the chromosome number in the daughter cells compares to the parent cell. [2]
Section B: Structured Questions (20 marks)
Answer all questions in this section.
6. Explain why observed phenotypic ratios in genetic crosses often differ from expected ratios. [2]
7. In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t).
(a) A homozygous tall pea plant is crossed with a homozygous short pea plant.
(i) State the genotype of the F1 generation. [1]
_______________________________________________________________________________
(ii) State the phenotype of the F1 generation. [1]
_______________________________________________________________________________
(b) Two plants from the F1 generation are crossed to produce an F2 generation. Complete the genetic diagram below to show the possible genotypes and phenotypes of the F2 offspring. [4]
Parental phenotypes: Tall × Tall
Parental genotypes: ________ × ________
Gametes: ________ ________ ________ ________
F2 genotypes: _______________________________________________________
F2 phenotypes: ______________________________________________________
Phenotypic ratio: ____________________________________________________
(c) If 120 F2 offspring were produced, calculate the expected number of short-stemmed plants. Show your working. [2]
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
8. Sickle cell anaemia is a genetic disorder caused by a mutation in the gene that codes for haemoglobin. The normal allele is represented by H<sup>A</sup> and the sickle cell allele by H<sup>S</sup>. Individuals who are heterozygous (H<sup>A</sup>H<sup>S</sup>) have some resistance to malaria.
(a) Explain what is meant by a gene mutation. [2]
(b) A man who is heterozygous for the sickle cell allele marries a woman who is also heterozygous. Complete the genetic diagram to determine the probability that their child will have sickle cell anaemia. [4]
Parental genotypes: ________ × ________
Gametes: ________ ________ ________ ________
Offspring genotypes: _________________________________________________
Probability of child with sickle cell anaemia: ____________________________
(c) Explain why the heterozygous condition (H<sup>A</sup>H<sup>S</sup>) is advantageous in regions where malaria is common. [2]
9. The diagram below shows the sex chromosomes of a human male and female.
Male: XY Female: XX
(a) Using a genetic diagram, show how sex is determined in humans. [3]
Parental genotypes: ________ × ________
Gametes: ________ ________ ________ ________
Offspring genotypes: _________________________________________________
Probability of male offspring: _________________________________________
(b) Explain why the father determines the sex of the child. [1]
10. A student investigated the inheritance of seed shape in pea plants. Round seed (R) is dominant to wrinkled seed (r). The student crossed a heterozygous round-seeded plant with a wrinkled-seeded plant and obtained the following results:
| Phenotype | Observed number | Expected number |
|---|---|---|
| Round | 53 | 50 |
| Wrinkled | 47 | 50 |
(a) State the genotypes of the parent plants used in this cross. [1]
(b) Draw a genetic diagram to show the expected offspring genotypes and phenotypes from this cross. [3]
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
(c) Calculate the expected phenotypic ratio from this cross. [1]
(d) Suggest a reason why the observed results differ slightly from the expected results. [1]
Section C: Data-Based and Extended Response Questions (20 marks)
Answer all questions in this section.
11. Down syndrome is a condition caused by a chromosome mutation where an individual has 47 chromosomes instead of 46.
(a) State the type of chromosome mutation that causes Down syndrome. [1]
(b) Explain how this chromosome mutation can occur during gamete formation. [3]
(c) Name one mutagenic agent that can increase the risk of chromosome mutations. [1]
12. Variation within a species can be continuous or discontinuous.
(a) Distinguish between continuous variation and discontinuous variation. Give one example of each in humans. [4]
(b) Explain how both genetic and environmental factors contribute to continuous variation in human height. [3]
13. Natural selection is a mechanism that drives evolution.
(a) Outline the process of natural selection. [3]
(b) Using the development of antibiotic resistance in bacteria as an example, explain how natural selection can lead to evolution. [3]
14. A man with blood group A marries a woman with blood group B. Their first child has blood group O.
(a) State the genotypes of the man and the woman. [2]
(b) Using a genetic diagram, determine the probability that their next child will have blood group AB. [3]
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
15. Explain how meiosis contributes to genetic variation in sexually reproducing organisms. [3]
Section D: Extended Response and Application Questions (10 marks)
Answer all questions in this section.
16. Describe the differences between mitosis and meiosis. [4]
17. Explain what is meant by the term sex-linked inheritance. Use a named example of a sex-linked genetic disorder in humans to support your answer. [3]
18. A pure-breeding red-flowered plant is crossed with a pure-breeding white-flowered plant. All the F1 offspring have pink flowers.
(a) Name the type of inheritance shown by flower colour in this plant. [1]
(b) Two pink-flowered F1 plants are crossed. Predict the phenotypic ratio of the F2 offspring. Show your working using a genetic diagram. [3]
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
19. Discuss the social and ethical implications of genetic screening for inherited disorders. [3]
20. Explain how mutations can be both harmful and beneficial to organisms, using specific examples. [3]
END OF QUIZ
Check your answers carefully before submitting.
Answers
O-Level Biology Quiz - Genetics Inheritance: Answer Key and Marking Scheme
Total Marks: 50
Section A: Short Answer Questions (10 marks)
1. Define the term allele. [1]
- Answer: An allele is an alternative form of a gene / a different version of a gene that occupies the same position (locus) on homologous chromosomes.
- Marking: Award [1] for correct definition. Accept "alternative form of a gene" or "different version of a gene at the same locus."
2. Distinguish between the terms genotype and phenotype. [2]
- Answer: Genotype refers to the genetic makeup / combination of alleles an organism possesses. Phenotype refers to the observable characteristics / physical traits expressed by an organism.
- Marking: Award [1] for correct definition of genotype, [1] for correct definition of phenotype. Must show clear distinction.
3. State the number of chromosomes present in a human gamete. [1]
- Answer: 23
- Marking: Award [1] for 23. Do not accept 46 or 23 pairs.
4. A person with blood group AB is said to show codominance. Explain what is meant by codominance. [2]
- Answer: Codominance is a condition where both alleles in a heterozygous individual are fully expressed / neither allele is dominant or recessive, so both traits appear in the phenotype.
- Marking: Award [1] for "both alleles expressed" and [1] for "in the heterozygous condition" or equivalent explanation. Accept reference to AB blood group as example.
5. Name the process of cell division that produces gametes and state how the chromosome number in the daughter cells compares to the parent cell. [2]
- Answer: Meiosis. The daughter cells have half the number of chromosomes as the parent cell (haploid vs diploid).
- Marking: Award [1] for "meiosis" and [1] for "half the chromosome number" or "haploid."
Section B: Structured Questions (20 marks)
6. Explain why observed phenotypic ratios in genetic crosses often differ from expected ratios. [2]
- Answer: Observed ratios may differ from expected ratios due to random fertilisation of gametes / chance. The expected ratios are theoretical probabilities, and small sample sizes can lead to deviations. Additionally, some genotypes may have reduced viability.
- Marking: Award [1] for "random fertilisation/chance" and [1] for "small sample size" or "theoretical vs actual." Accept any two valid reasons.
7. Pea plant stem height cross.
(a)(i) State the genotype of the F1 generation. [1]
- Answer: Tt
- Marking: Award [1] for Tt. Must be heterozygous.
(a)(ii) State the phenotype of the F1 generation. [1]
- Answer: Tall
- Marking: Award [1] for "Tall."
(b) Complete the genetic diagram. [4]
- Answer:
- Parental genotypes: Tt × Tt
- Gametes: T, t, T, t
- F2 genotypes: TT, Tt, Tt, tt
- F2 phenotypes: Tall, Tall, Tall, Short
- Phenotypic ratio: 3 Tall : 1 Short
- Marking: Award [1] for correct parental genotypes (Tt × Tt), [1] for correct gametes, [1] for correct F2 genotypes, [1] for correct phenotypes and ratio.
(c) Calculate expected number of short-stemmed plants from 120 F2 offspring. [2]
- Answer: Expected ratio is 3:1, so 1/4 of offspring are short. 120 × 1/4 = 30 plants.
- Marking: Award [1] for identifying 1/4 proportion, [1] for correct answer of 30.
8. Sickle cell anaemia.
(a) Explain what is meant by a gene mutation. [2]
- Answer: A gene mutation is a change in the sequence of nucleotides / bases in DNA that makes up a gene. This can result in a change to the protein produced / altered polypeptide.
- Marking: Award [1] for "change in DNA sequence/nucleotide sequence" and [1] for "change in protein produced" or "alteration in gene."
(b) Complete the genetic diagram. [4]
- Answer:
- Parental genotypes: H<sup>A</sup>H<sup>S</sup> × H<sup>A</sup>H<sup>S</sup>
- Gametes: H<sup>A</sup>, H<sup>S</sup>, H<sup>A</sup>, H<sup>S</sup>
- Offspring genotypes: H<sup>A</sup>H<sup>A</sup>, H<sup>A</sup>H<sup>S</sup>, H<sup>A</sup>H<sup>S</sup>, H<sup>S</sup>H<sup>S</sup>
- Probability of child with sickle cell anaemia (H<sup>S</sup>H<sup>S</sup>): 1/4 or 25%
- Marking: Award [1] for correct parental genotypes, [1] for correct gametes, [1] for correct offspring genotypes, [1] for correct probability.
(c) Explain why the heterozygous condition is advantageous in malaria regions. [2]
- Answer: Heterozygous individuals (H<sup>A</sup>H<sup>S</sup>) have some normal haemoglobin and some sickle cell haemoglobin. This provides resistance to malaria because the malaria parasite cannot survive as well in red blood cells containing sickle cell haemoglobin, while the individual does not suffer from full sickle cell anaemia.
- Marking: Award [1] for "resistance to malaria" and [1] for explanation linking to parasite survival or selective advantage.
9. Sex determination.
(a) Genetic diagram for sex determination. [3]
- Answer:
- Parental genotypes: XX × XY
- Gametes: X, X, X, Y
- Offspring genotypes: XX, XX, XY, XY
- Probability of male offspring: 1/2 or 50%
- Marking: Award [1] for correct parental genotypes, [1] for correct gametes and offspring genotypes, [1] for correct probability.
(b) Explain why the father determines the sex of the child. [1]
- Answer: The father produces both X and Y sperm, while the mother produces only X eggs. The sex of the child depends on whether an X or Y sperm fertilises the egg.
- Marking: Award [1] for explanation linking to sperm type (X or Y) determining sex.
10. Seed shape investigation.
(a) State the genotypes of the parent plants. [1]
- Answer: Rr (heterozygous round) and rr (wrinkled)
- Marking: Award [1] for both correct genotypes.
(b) Genetic diagram for the cross. [3]
- Answer:
- Parental genotypes: Rr × rr
- Gametes: R, r, r, r
- Offspring genotypes: Rr, Rr, rr, rr
- Offspring phenotypes: Round, Round, Wrinkled, Wrinkled
- Expected ratio: 1 Round : 1 Wrinkled
- Marking: Award [1] for correct gametes, [1] for correct offspring genotypes, [1] for correct phenotypes.
(c) Calculate the expected phenotypic ratio. [1]
- Answer: 1:1 (Round : Wrinkled)
- Marking: Award [1] for 1:1.
(d) Suggest a reason for the difference between observed and expected results. [1]
- Answer: Random fertilisation / chance / small sample size.
- Marking: Award [1] for any valid reason.
Section C: Data-Based and Extended Response Questions (20 marks)
11. Down syndrome.
(a) State the type of chromosome mutation. [1]
- Answer: Non-disjunction / aneuploidy / trisomy 21
- Marking: Award [1] for any of the above.
(b) Explain how this chromosome mutation can occur during gamete formation. [3]
- Answer: During meiosis, homologous chromosomes (chromosome pair 21) fail to separate properly (non-disjunction). This results in one gamete receiving two copies of chromosome 21 and another receiving none. If the gamete with two copies fuses with a normal gamete during fertilisation, the zygote will have three copies of chromosome 21 (trisomy 21).
- Marking: Award [1] for "non-disjunction during meiosis," [1] for "gamete receives extra chromosome 21," [1] for "fertilisation produces trisomy 21."
(c) Name one mutagenic agent that can increase the risk of chromosome mutations. [1]
- Answer: Ionising radiation (e.g., X-rays, gamma rays) / certain chemicals / advanced maternal age (accepted as risk factor).
- Marking: Award [1] for any valid mutagen or risk factor.
12. Variation.
(a) Distinguish between continuous and discontinuous variation with examples. [4]
- Answer:
- Continuous variation: A range of phenotypes between two extremes, with no distinct categories. It is usually controlled by many genes (polygenic) and influenced by the environment. Example: Human height / skin colour / body mass.
- Discontinuous variation: Distinct, separate categories with no intermediates. It is usually controlled by a single gene and is not significantly influenced by the environment. Example: Human blood groups / ability to roll tongue / attached or free earlobes.
- Marking: Award [1] for correct description of continuous variation, [1] for correct example; [1] for correct description of discontinuous variation, [1] for correct example.
(b) Explain how genetic and environmental factors contribute to continuous variation in human height. [3]
- Answer: Genetic factors: Height is a polygenic trait, meaning it is controlled by multiple genes. Individuals inherit a combination of alleles from their parents that determine their potential height range. Environmental factors: Nutrition (diet quality and quantity during growth), health during childhood, and exercise can influence whether an individual reaches their full genetic potential height. For example, malnutrition can stunt growth.
- Marking: Award [1] for "polygenic inheritance/multiple genes," [1] for "nutrition/diet," [1] for "health/disease/exercise" or other valid environmental factor. Must mention both genetic and environmental contributions.
13. Natural selection.
(a) Outline the process of natural selection. [3]
- Answer: 1. There is variation within a population. 2. Organisms produce more offspring than can survive, leading to competition for resources. 3. Individuals with advantageous traits/adaptations are more likely to survive and reproduce. 4. These individuals pass on their advantageous alleles to their offspring. 5. Over many generations, the frequency of these advantageous alleles increases in the population.
- Marking: Award [1] for "variation," [1] for "competition/struggle for survival," [1] for "survival of the fittest/reproduction of individuals with advantageous traits" and "passing on alleles."
(b) Using antibiotic resistance in bacteria as an example, explain how natural selection leads to evolution. [3]
- Answer: Within a bacterial population, there is genetic variation due to mutation. Some bacteria may possess a gene/allele that confers resistance to a specific antibiotic. When the antibiotic is applied, susceptible bacteria are killed, but resistant bacteria survive. These resistant bacteria reproduce, passing on the resistance allele to their offspring. Over time, the frequency of the resistance allele increases, and the population evolves to become predominantly antibiotic-resistant.
- Marking: Award [1] for "variation/mutation producing resistance," [1] for "antibiotic kills susceptible bacteria, resistant survive," [1] for "resistant bacteria reproduce and pass on allele, leading to evolution of population."
14. Blood group inheritance.
(a) State the genotypes of the man and the woman. [2]
- Answer: Man: I<sup>A</sup>i; Woman: I<sup>B</sup>i. (Child with blood group O has genotype ii, so both parents must carry the recessive i allele.)
- Marking: Award [1] for each correct genotype. Must show heterozygous genotypes with i allele.
(b) Genetic diagram for probability of blood group AB child. [3]
- Answer:
- Parental genotypes: I<sup>A</sup>i × I<sup>B</sup>i
- Gametes: I<sup>A</sup>, i, I<sup>B</sup>, i
- Offspring genotypes: I<sup>A</sup>I<sup>B</sup>, I<sup>A</sup>i, I<sup>B</sup>i, ii
- Probability of blood group AB (I<sup>A</sup>I<sup>B</sup>): 1/4 or 25%
- Marking: Award [1] for correct parental genotypes, [1] for correct gametes and Punnett square/genetic diagram, [1] for correct probability.
15. Explain how meiosis contributes to genetic variation. [3]
- Answer: Meiosis contributes to genetic variation through two main processes: 1. Crossing over: During prophase I, homologous chromosomes exchange segments of DNA, creating new combinations of alleles on a chromosome. 2. Independent assortment: During metaphase I, homologous pairs line up randomly at the equator, resulting in different combinations of maternal and paternal chromosomes in the gametes. Additionally, random fertilisation of gametes further increases variation.
- Marking: Award [1] for "crossing over" with explanation, [1] for "independent assortment" with explanation, [1] for mentioning "random fertilisation" or clear explanation of how these processes create new allele combinations.
Section D: Extended Response and Application Questions (10 marks)
16. Describe the differences between mitosis and meiosis. [4]
- Answer:
- Mitosis produces two genetically identical daughter cells; meiosis produces four genetically non-identical daughter cells.
- Mitosis involves one division; meiosis involves two successive divisions.
- Mitosis maintains the diploid chromosome number; meiosis halves the chromosome number (haploid).
- Mitosis is used for growth, repair, and asexual reproduction; meiosis is used for the production of gametes for sexual reproduction.
- No crossing over in mitosis; crossing over occurs in prophase I of meiosis.
- No independent assortment in mitosis; independent assortment occurs in metaphase I of meiosis.
- Marking: Award [1] for each correct difference up to [4] marks. Accept any four valid comparisons.
17. Explain what is meant by the term sex-linked inheritance. Use a named example. [3]
- Answer: Sex-linked inheritance refers to the inheritance of genes located on the sex chromosomes (X or Y chromosomes). Most sex-linked traits are X-linked, as the X chromosome is larger and carries more genes. Males (XY) are more likely to express X-linked recessive traits because they have only one X chromosome. Example: Haemophilia / Red-green colour blindness. A recessive allele on the X chromosome causes the disorder. A male with the allele on his X chromosome will express the trait, while a female would need two copies of the recessive allele to express it.
- Marking: Award [1] for "genes located on sex chromosomes," [1] for explanation of why males are more affected (hemizygous), [1] for correct named example with brief explanation.
18. Incomplete dominance in flower colour.
(a) Name the type of inheritance. [1]
- Answer: Incomplete dominance / partial dominance.
- Marking: Award [1] for "incomplete dominance."
(b) Predict the phenotypic ratio of the F2 offspring with genetic diagram. [3]
- Answer:
- Let R = allele for red, W = allele for white. Pink F1 plants are RW.
- Parental genotypes: RW × RW
- Gametes: R, W, R, W
- F2 genotypes: RR, RW, RW, WW
- F2 phenotypes: Red, Pink, Pink, White
- Phenotypic ratio: 1 Red : 2 Pink : 1 White
- Marking: Award [1] for correct parental genotypes and gametes, [1] for correct F2 genotypes, [1] for correct phenotypic ratio.
19. Discuss the social and ethical implications of genetic screening for inherited disorders. [3]
- Answer: Social implications: Genetic screening can allow for early diagnosis and treatment/preparation for inherited disorders, potentially improving quality of life. However, it may lead to discrimination by employers or insurance companies based on genetic risk. Ethical implications: It raises issues of privacy and confidentiality of genetic information. It can lead to difficult decisions regarding termination of pregnancy if a severe disorder is detected. There may be psychological impacts on individuals and families. Access to screening may be unequal, raising issues of fairness.
- Marking: Award [1] for a valid social implication (positive or negative), [1] for a valid ethical implication, [1] for further elaboration or a second distinct point. Accept any reasoned discussion points.
20. Explain how mutations can be both harmful and beneficial to organisms, using specific examples. [3]
- Answer: Harmful mutations: Most mutations are harmful because they disrupt normal gene function. Example: The mutation causing sickle cell anaemia (H<sup>S</sup> allele) produces abnormal haemoglobin, leading to sickle-shaped red blood cells that can block blood vessels and cause pain and organ damage. Beneficial mutations: Occasionally, a mutation can provide a survival advantage in a particular environment. Example: The same sickle cell allele (H<sup>S</sup>) in the heterozygous state provides resistance to malaria, which is beneficial in malaria-endemic regions. Another example is mutations in bacteria conferring antibiotic resistance, which is beneficial for bacterial survival.
- Marking: Award [1] for explanation of harmful mutation with example, [1] for explanation of beneficial mutation with example, [1] for clear contrast or a second valid example/elaboration.
END OF ANSWER KEY