IB DP Biology SL Prediction Paper 2 for 2025 Exams
IB DP Biology SL Prediction Paper 2- April/May 2025 Exam
IB DP Biology SL Prediction Paper 2: Prepare for the IB exams with subject-specific Prediction questions, model answers. All topics covered.
Prepared by IB teachers: Access our IB DP Biology SL Prediction Paper 2 with model answer. Students: Practice with exam-style papers for IB DP Biology SL Exam
Question 1
The spider Dolomedes plantarius exhibits variation in its banded trait, which is investigated through genetic crosses to understand inheritance patterns.
The spider Dolomedes plantarius usually has white bands down the left and right sides of its body, but some individuals lack these bands. The photograph shows the banded form of D. plantarius with a ball of spiderlings.
Crosses were performed to investigate the inheritance of this trait, by allowing specific males and females to mate. Numbers of banded and unbanded spiderlings that hatched out from all the eggs laid by the female were recorded. Results are shown in the table.
Question a – Topic B8.1: Inheritance
Explain the conclusion that can be drawn from Cross 1.
▶️ Answer/Explanation
Answer:
- The allele for banded is dominant, and the allele for unbanded is recessive.
- This is shown by the 3:1 ratio of banded (153) to unbanded (51) offspring from two banded parents.
- Both parents in Cross 1 are heterozygous (Bb).
Detailed Explanation:
- Cross 1 results (153 banded : 51 unbanded) approximate a 3:1 phenotypic ratio, typical of a monohybrid cross between two heterozygous individuals (Bb × Bb).
- Let B = banded (dominant), b = unbanded (recessive). The Punnett square is:
B b B BB Bb b Bb bb Phenotypes: 75% banded (BB or Bb), 25% unbanded (bb).
- Total offspring = 204; expected: 153 banded (75%), 51 unbanded (25%), matching the observed results, confirming both parents are Bb.
Question b – Topic B8.1: Inheritance
Deduce reasons for the difference between the results of Cross 2 and Cross 3.
▶️ Answer/Explanation
Answer:
- In Cross 2 (banded × unbanded = 72 banded : 68 unbanded), the banded parent is heterozygous (Bb).
- In Cross 3 (banded × unbanded = 136 banded : 0 unbanded), the banded parent is homozygous dominant (BB).
- The banded parents have the same phenotype but different genotypes.
Detailed Explanation:
- Cross 2: Banded (Bb) × Unbanded (bb). Punnett square:
B b b Bb bb Expected 1:1 ratio (50% banded, 50% unbanded). Observed 72:68 ≈ 1:1.
- Cross 3: Banded (BB) × Unbanded (bb). Punnett square:
B B b Bb Bb All offspring banded (100% Bb). Observed 136:0 matches this.
- The difference arises from the banded parent’s genotype: heterozygous (Bb) in Cross 2 allows unbanded offspring, while homozygous (BB) in Cross 3 ensures all banded offspring.
Question c – Topic B8.1: Inheritance
There were 79 progeny in Cross 4. Predict the expected results by completing the table.
▶️ Answer/Explanation
Answer:
| Phenotype | Number of Spiderlings |
|---|---|
| Banded | 79 |
| Unbanded | 0 |
Detailed Explanation:
- Cross 4: Unbanded (bb) × Banded (unknown genotype). From Cross 3, a banded parent (BB) × unbanded (bb) produced all banded offspring.
- Assuming the banded parent in Cross 4 is also BB (consistent with Cross 3), all progeny are Bb (banded). Punnett square:
B B b Bb Bb - Total progeny = 79; all expected to be banded (79:0).
Question d – Topic B8.1: Inheritance
The proportion of banded and unbanded individuals in a population of D. plantarius can change. Suggest how such a change could occur.
▶️ Answer/Explanation
Answer:
- Natural selection could favor one phenotype (e.g., banded) over the other.
- Better camouflage of one type (e.g., banded) increases its survival rate.
Detailed Explanation:
- Natural Selection: If banded spiders are better camouflaged against predators in their habitat (e.g., white bands blend with water edges), they may survive and reproduce more, increasing the B allele frequency.
- Survival Advantage: Conversely, unbanded spiders might be more visible, reducing their survival rate. Over generations, this shifts the population proportion via differential survival and reproduction.
- Alternative: Sexual selection could also play a role—e.g., if females prefer banded males, the B allele would spread faster.