A Level H2 Biology Genetics Inheritance Quiz

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A-Level Biology H2 Quiz - Genetics Inheritance

Name: ________________________
Class: _________________________
Date: __________________________
Score: ________ / 60

Duration: 90 Minutes
Total Marks: 60
Instructions: Answer all questions. Use the space provided. For calculation questions, show all working.

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Section A: Molecular Genetics & Fragment Analysis

Focus: DNA analysis, RFLPs, and chromosomal abnormalities.

1. A restriction enzyme is used to analyze a specific gene locus in four individuals. Individual A shows one band, while Individual B shows two bands of different sizes. Explain the difference in genotypes between Individual A and B. [3]
   
   
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2. With reference to the concept of Restriction Fragment Length Polymorphism (RFLP), explain why a mutation in a non-coding region of DNA can still be used as a genetic marker for a disease. [3]
   
   
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3. A patient is diagnosed with Chronic Myelogenous Leukemia (CML). Describe the chromosomal event that leads to the formation of the Philadelphia chromosome. [3]
   
   
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4. Explain how the fusion protein produced by the BCR-ABL gene leads to the uncontrolled proliferation of white blood cells. [4]
   
   
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5. In a DNA fragment analysis, a homozygous dominant individual and a heterozygous individual are compared. Describe the expected banding pattern for each on an electrophoresis gel. [3]
   
   
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6. Suggest why the use of multiple restriction enzymes might be more effective than using a single enzyme when mapping a previously unknown genetic locus. [2]
   
   
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7. Explain the relationship between the charge of a DNA fragment and its migration speed through an agarose gel matrix. [2]
   
   
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Section B: Mendelian & Non-Mendelian Inheritance

Focus: Autosomal crosses, codominance, and probability.

8. In a species of bird, feather color is controlled by an autosomal gene with two alleles: $C^B$ (black) and $C^W$ (white). Individuals with the genotype $C^B C^W$ exhibit "splashed-white" feathers. Identify the type of inheritance shown here and justify your answer. [3]
   
   
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9. A male bird with splashed-white feathers is mated with a female bird with black feathers. Predict the phenotypic ratio of the offspring. [3]
   
   
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10. Distinguish between autosomal dominant and autosomal recessive inheritance in terms of the phenotype of the parents in a pedigree where the offspring are affected but the parents are not. [3]
    
    
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11. A dihybrid cross is performed between two individuals heterozygous for two genes located on different autosomes. State the expected phenotypic ratio of the F2 generation. [2]
    
    
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12. Explain why the expected 9:3:3:1 ratio in a dihybrid cross may not be observed if the two genes are closely linked on the same chromosome. [3]
    
    
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13. Define "incomplete dominance" and provide a biological example that differs from the "splashed-white" codominance described in Question 8. [3]
    
    
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14. In a test cross, an individual with an unknown dominant phenotype is crossed with a homozygous recessive individual. Explain the purpose of this specific cross. [3]
    
    
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15. If two carriers of an autosomal recessive disorder have a child, what is the probability that the child will be a carrier? Show your working. [3]
    
    
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Section C: Statistical Analysis & Data Evaluation

Focus: Chi-squared tests and cancer risk evidence.

16. A researcher observes 300 offspring from a dihybrid cross. The observed counts are: 170 (dominant/dominant), 60 (dominant/recessive), 65 (recessive/dominant), and 5 (recessive/recessive). State the null hypothesis for a $\chi^2$ test to determine if this fits a 9:3:3:1 ratio. [2]
    
    
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17. Calculate the expected number of offspring for the "recessive/recessive" phenotype in the cross described in Question 16. [2]
    
    
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18. If the calculated $\chi^2$ value is greater than the critical value at $p = 0.05$, explain what conclusion must be drawn regarding the null hypothesis. [2]
    
    
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19. A study shows that individuals with a mutation in the BRCA2 gene have a significantly higher incidence of breast cancer. Discuss why this does not prove that the BRCA2 mutation is the sole cause of the cancer. [4]
    
    
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20. Explain the term "incomplete penetrance" in the context of genetic diseases and how it complicates the prediction of phenotypes from genotypes. [4]
    
    
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Answer Key - A-Level Biology H2 Quiz: Genetics Inheritance

1. Individual A is homozygous (one allele size, one band). Individual B is heterozygous (two different alleles, each producing a different sized fragment, resulting in two bands). [3]
2. Genetic markers do not need to cause the disease; they only need to be linked (close proximity) to the disease-causing mutation. Because they are inherited together, the RFLP pattern of the non-coding region correlates with the presence of the disease allele. [3]
3. A reciprocal translocation occurs between chromosomes 9 and 22. [1] A piece of chromosome 9 swaps with a piece of chromosome 22. [1] This results in a shortened chromosome 22, known as the Philadelphia chromosome. [1]
4. The translocation creates a fusion gene (BCR-ABL). [1] This gene produces a fusion protein with constitutive (always active) tyrosine kinase activity. [1] This leads to continuous phosphorylation of signaling proteins. [1] This triggers uncontrolled cell division/proliferation of myeloid cells. [1]
5. Homozygous dominant: One single band (both alleles are identical and cut at the same sites). [1.5] Heterozygous: Two distinct bands (each allele is cut differently, producing fragments of different lengths). [1.5]
6. A single enzyme might not find a recognition site in a specific allele, or might cut too frequently. Multiple enzymes increase the likelihood of creating a unique "fingerprint" of fragments for different alleles. [2]
7. DNA is negatively charged (phosphate backbone). [1] It migrates toward the positive electrode; smaller fragments move faster through the gel matrix than larger ones. [1]
8. Codominance. [1] Both alleles ($C^B$ and $C^W$) are fully expressed in the phenotype. [1] The "splashed" appearance shows both black and white feathers rather than a blend. [1]
9. Parents: $C^B C^W$ (splashed) x $C^B C^B$ (black). Offspring: 50% $C^B C^B$ (black), 50% $C^B C^W$ (splashed). Ratio: 1 black : 1 splashed-white. [3]
10. In autosomal recessive, affected offspring can have unaffected parents (parents are carriers). [2] In autosomal dominant, every affected offspring must have at least one affected parent. [1]
11. 9:3:3:1 [2]
12. Linked genes do not assort independently. [1] They are inherited together as a unit more frequently than expected. [1] This results in a higher frequency of parental phenotypes and a lower frequency of recombinant phenotypes. [1]
13. Incomplete dominance: The phenotype is an intermediate blend of the two parents (e.g., red x white = pink flowers). [2] Unlike codominance, neither allele is fully expressed; instead, a third, blended phenotype appears. [1]
14. To determine the genotype of the dominant individual. [1] If any offspring show the recessive phenotype, the parent must be heterozygous. [1] If all offspring are dominant, the parent is likely homozygous dominant. [1]
15. Parents: $Aa \times Aa$. Possible offspring: $AA, Aa, aA, aa$. Carriers are $Aa$ and $aA$. Probability = $2/4$ or 50%. [3]
16. Null Hypothesis: There is no significant difference between the observed results and the expected 9:3:3:1 ratio (any difference is due to chance). [2]
17. Total = 300. Expected ratio for recessive/recessive = $1/16$. Calculation: $300 \times (1/16) = \mathbf{18.75}$. [2]
18. The null hypothesis is rejected. [1] The difference between observed and expected frequencies is statistically significant and not due to chance. [1]
19. It is correlational evidence, not necessarily causative. [1] Environmental factors (diet, smoking) may contribute. [1] Other modifier genes may be required for the cancer to develop. [1] Not everyone with the mutation develops cancer (incomplete penetrance). [1]
20. Incomplete penetrance is when an individual has the genotype for a trait but does not express the phenotype. [2] It complicates predictions because the presence of a "disease" allele does not guarantee the disease will manifest, making genetic counseling probabilistic rather than certain. [2]