O Level Biology Genetics Inheritance Quiz

O-Level Biology Quiz - Genetics Inheritance (Answer Key)

Section A: Fundamental Concepts

1. Allele: An alternative form of a gene. [1]
2. Genotype vs Phenotype: Genotype is the genetic makeup/combination of alleles of an organism [1]; Phenotype is the observable physical characteristics/traits of an organism [1]. [2]
3. Homozygous vs Heterozygous: Homozygous means having two identical alleles for a trait (e.g., TT or tt) [1]; Heterozygous means having two different alleles for a trait (e.g., Tt) [1]. [2]
4. Dominant allele: An allele that is expressed in the phenotype even if only one copy is present (masks the recessive allele). [1]
5. Recessive trait reappearance: In F1, the dominant allele masks the recessive one, so the trait is not visible [1]. The F1 individuals are heterozygous carriers [1]. When F1 individuals are crossed, there is a 25% chance of offspring inheriting two recessive alleles (homozygous recessive), allowing the trait to be expressed [1]. [3]
6. Continuous vs Discontinuous: Continuous variation shows a range of phenotypes with no clear categories (e.g., height) [1]; Discontinuous variation shows distinct categories with no intermediates (e.g., blood group) [1]. [2]
7. Example: Height, skin colour, mass. [1]

Section B: Application and Analysis

8. (a) Parent 1: Tt; Parent 2: tt. [1] (b) Diagram: Parents: Tt x tt Gametes: (T), (t) x (t), (t) Offspring: Tt, Tt, tt, tt. [2] Ratio: 1 Tall : 1 Short (or 50% Tall, 50% Short). [1] [3]
9. (a) Both the $I^A$ and $I^B$ alleles are expressed equally [1], resulting in a phenotype that shows both antigens on the red blood cells [1]. [2] (b) A, B, AB, and O. [2]
10. (a) Either X or Y. [1] (b) The mother only provides X chromosomes [1]. The father can provide either an X or a Y chromosome; if he provides Y, the child is male; if X, the child is female [1]. [2]
11. (a) Males only have one X chromosome [1]. If they inherit the recessive allele, there is no second X chromosome to provide a dominant allele to mask it [1]. [2] (b) 50% (or 1/2). [1]
12. (a) Mitosis: 2; Meiosis: 4. [1] (b) Mitosis: Genetically identical to parent and each other [1]. Meiosis: Genetically different from parent and each other [1]. [2]
13. Crossing over: Exchange of genetic material between homologous chromosomes during prophase I of meiosis [1]. Significance: Increases genetic variation in offspring [1], which allows a species to better adapt to changing environments [1]. [3]
14. (a) A permanent change in the nucleotide sequence of DNA. [1] (b) Change in DNA sequence $\rightarrow$ change in the sequence of amino acids in the polypeptide [1] $\rightarrow$ change in the 3D shape/folding of the protein [1] $\rightarrow$ protein may lose function or gain new function, altering the phenotype [1]. [3]
15. Gene mutation: Change in a small segment of DNA/nucleotide sequence (e.g., Sickle cell anaemia) [2]. Chromosome mutation: Change in the number or structure of whole chromosomes (e.g., Down syndrome/Trisomy 21) [2]. [4]

Section C: Synthesis and Evaluation

16. Natural Selection (Antibiotics):
    • Variation exists in the bacterial population due to random mutation [1].
    • Some bacteria possess a gene for antibiotic resistance [1].
    • When antibiotics are applied, non-resistant bacteria are killed, while resistant ones survive [1].
    • Survivors reproduce asexually and pass the resistance gene to offspring [1].
    • Over time, the frequency of the resistant phenotype increases in the population [1]. [5]
17. Environmental influence: The phenotype is a result of the interaction between genotype and environment [1]. Example: A plant may have genes for tall growth (genotype), but if grown in nutrient-poor soil (environment), it will remain stunted (phenotype) [2]. This shows the environment can limit the expression of the genetic potential [1]. [4]
18. (a) Average beak size will increase. [1] (b) Variation in beak size exists [1]. Birds with larger beaks are better adapted to eat the hard seeds and are more likely to survive [1]. These survivors reproduce and pass the "large beak" alleles to offspring [1]. Over generations, the large-beak phenotype becomes more common [1]. [4]
19. Asexual disadvantage: Offspring are genetically identical (clones) [1]. There is no genetic variation in the population [1]. If the environment changes (e.g., new disease, climate change), all individuals are equally susceptible [1]. This increases the risk of the entire population being wiped out [1]. [4]
20. Relationship: DNA is the chemical molecule that makes up genetic material [1]. A gene is a specific sequence of nucleotides on DNA that codes for a protein [1]. Chromosomes are long strands of DNA coiled around proteins [1]. [3]