A Level H2 Biology Genetics Inheritance Quiz

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A-Level Biology H2 Quiz - Genetics Inheritance

Name: ____________________
Class: ____________________
Date: ____________________
Score: ________ / 65

Duration: 90 Minutes
Total Marks: 65 Marks

Instructions:

• Answer all questions in the spaces provided.
• Use a black or blue pen.
• For calculation questions, show all working clearly.
• Ensure terminology is precise and aligned with the H2 Biology syllabus.

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Section A: Mendelian and Non-Mendelian Inheritance (Questions 1-7)

1. Define the term codominance and provide one biological example. [2]
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2. In a species of flower, red (R) is dominant to white (W). A heterozygous red flower is crossed with a white flower. State the expected phenotypic ratio of the offspring. [2]
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3. Explain why a test cross is used in genetics and describe the expected results if the dominant phenotype parent is homozygous. [3]
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4. A trait is governed by two genes, A and B. If a plant is homozygous recessive for gene A (aa), it remains white regardless of the alleles at gene B. If it has at least one dominant A, gene B determines if the flower is red (B_) or pink (bb). Identify this type of gene interaction and explain the logic. [4]
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5. Distinguish between incomplete dominance and codominance using a specific example for each. [4]
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6. In humans, ABO blood groups are an example of multiple alleles. Explain why an individual with blood group AB is considered to have codominant alleles. [3]
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7. A dihybrid cross (AaBb x AaBb) is performed. If the genes are on different autosomes, what is the expected phenotypic ratio? Explain the basis of this ratio. [4]
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Section B: Sex-Linkage and Pedigree Analysis (Questions 8-13)

8. Why are X-linked recessive disorders more frequently observed in males than in females? [3]
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9. A woman who is a carrier for haemophilia (X^H X^h) marries a man who is normal (X^H Y). Determine the probability that their first son will have haemophilia. [3]
   
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10. In a pedigree chart, how can you distinguish between an autosomal recessive trait and an X-linked recessive trait? [4]
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11. Describe the phenomenon of lethal alleles and explain how they can alter the expected Mendelian ratio in a monohybrid cross. [4]
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12. Explain the concept of epistasis and provide a scenario where recessive epistasis would result in a 9:3:4 ratio. [4]
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13. A male with a dominant X-linked trait marries a female who is homozygous recessive. What percentage of their daughters will express the trait? Justify your answer. [3]
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Section C: Molecular Genetics and Statistical Analysis (Questions 14-20)

14. Explain the role of restriction endonucleases in the creation of RFLPs (Restriction Fragment Length Polymorphisms). [3]
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15. An individual's DNA is digested with a restriction enzyme. If the individual is heterozygous for a specific locus, how many bands will be visible on a gel electrophoresis image for that locus? Explain. [3]
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16. Describe how a mutation in the BRCA2 gene can increase the risk of cancer, focusing on the protein's normal function. [4]
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17. A researcher expects a 3:1 ratio in a cross. The observed results are 72 dominant and 28 recessive. Perform a Chi-squared ($\chi^2$) calculation to determine if the null hypothesis should be accepted. (Critical value for $df=1, p=0.05$ is 3.84). [6]
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18. Explain the difference between penetrance and expressivity in the context of genetic diseases. [4]
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19. Discuss how the Philadelphia chromosome (a translocation between chromosomes 9 and 22) leads to chronic myelogenous leukemia (CML). [5]
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20. Compare the use of PCR (Polymerase Chain Reaction) and Gel Electrophoresis in the diagnosis of a genetic condition. How do they complement each other? [4]
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Answer Key - A-Level Biology H2 Quiz: Genetics Inheritance

1. Codominance: A situation where two different alleles for a gene are both fully expressed in the phenotype of a heterozygote. Example: ABO blood group (Type AB) or roan coat colour in cattle. [2]

2. Ratio: 1 Red : 1 White. (Rr x rr $\rightarrow$ 50% Rr, 50% rr). [2]

3. Test Cross: Crossing an individual of unknown genotype (dominant phenotype) with a homozygous recessive individual. If the parent is homozygous, 100% of offspring will show the dominant phenotype. [3]

4. Recessive Epistasis: Gene A is epistatic to Gene B. If the genotype is aa, the biochemical pathway is blocked, preventing any colour (white). If A_ is present, Gene B determines the specific colour (Red/Pink). [4]

5. Incomplete Dominance: Heterozygote shows an intermediate phenotype (e.g., Red x White $\rightarrow$ Pink). Codominance: Heterozygote shows both parental traits distinctly (e.g., Blood group AB expresses both A and B antigens). [4]

6. ABO Blood Group: The $I^A$ and $I^B$ alleles are codominant. An individual with genotype $I^A I^B$ produces both A and B antigens on the surface of red blood cells, rather than a blend. [3]

7. Ratio: 9:3:3:1. Based on the Law of Independent Assortment; alleles for different genes segregate independently during meiosis, creating four distinct phenotypic classes. [4]

8. X-linked Recessive: Males are hemizygous (XY). They only have one X chromosome; if they inherit the recessive allele, the trait is expressed. Females (XX) need two copies of the recessive allele to express the trait. [3]

9. Probability: 50% of sons. The son must inherit the Y from the father and either $X^H$ or $X^h$ from the mother. Probability of inheriting $X^h$ is 1/2. [3]

10. Distinction: In X-linked recessive, the trait is significantly more common in males. In autosomal recessive, the trait appears with equal frequency in both sexes. Also, an affected father will always pass the allele to daughters, but never to sons. [4]

11. Lethal Alleles: Alleles that cause death of the organism. If homozygous lethal, the expected 3:1 ratio becomes 2:1 because the homozygous recessive class dies in utero. [4]

12. Epistasis: Interaction where one gene masks or modifies the expression of another. Recessive epistasis (9:3:4) occurs when the homozygous recessive state of one gene (e.g., aa) masks the effect of the second gene. [4]

13. Percentage: 100%. The father is $X^A Y$ and the mother is $X^a X^a$. All daughters receive $X^A$ from the father and $X^a$ from the mother, making them all $X^A X^a$ (dominant trait expressed). [3]

14. RFLPs: Restriction enzymes cut DNA at specific recognition sites. Mutations can create or destroy these sites, changing the length of the resulting DNA fragments. [3]

15. Bands: 2 bands. Each allele produces a fragment of a different size due to different restriction sites; since the individual is heterozygous, both distinct sizes are present. [3]

16. BRCA2: Normally codes for a protein involved in repairing double-strand DNA breaks. A mutation leads to a non-functional protein $\rightarrow$ accumulation of DNA mutations $\rightarrow$ increased risk of oncogene activation/tumor suppressor loss. [4]

17. Chi-Squared Calculation:

    • Expected: 75 (3/4 of 100), 25 (1/4 of 100).
    • $\chi^2 = \sum \frac{(O-E)^2}{E} = \frac{(72-75)^2}{75} + \frac{(28-25)^2}{25} = \frac{9}{75} + \frac{9}{25} = 0.12 + 0.36 = 0.48$.
    • $0.48 < 3.84$. Null hypothesis is accepted. [6]

18. Penetrance: The proportion of individuals with a specific genotype who actually express the phenotype. Expressivity: The degree or intensity to which the phenotype is expressed in those who do show it. [4]

19. Philadelphia Chromosome: Translocation between chromosomes 9 and 22 creates a fusion gene BCR-ABL. This produces a constitutively active tyrosine kinase protein. This protein signals the cell to divide continuously, bypassing normal growth checkpoints, leading to leukemia. [5]

20. PCR vs Gel: PCR is used to amplify a specific target sequence of DNA to create enough material for analysis. Gel electrophoresis is then used to separate these amplified fragments by size to identify the presence of specific alleles/mutations. [4]