Question 23M.2.HL.TZ1.5
| Date | May 2023 | Marks available | [Maximum mark: 6] | Reference code | 23M.2.HL.TZ1.5 |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Deduce, Explain, Identify, Predict | Question number | 5 | Adapted from | N/A |
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.
[Source: photo: Vėlavičienė, N., 2004. [Dolomedes.] [image online] Available at: https://commons.wikimedia.org/wiki/
File:Dolomedes_fimbriatus.jpg. [Accessed 14 March 2022]. Licensed under the Creative Commons Attribution-Share
Alike 3.0 Unported license (https://creativecommons.org/licenses/by-sa/3.0/deed.en).]
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.
[Source: table: Baillie, A.L., Baillie, S.R. and Smith, H., 2019. The heritability of lateral banding in Dolomedes plantarius. Arachnology, [e-journal] 18(3), pp. 237–244. https://doi.org/10.13156/arac.2019.18.3.237.]
Explain the conclusion that can be drawn from Cross 1.
[2]
- allele/trait/gene for banded is dominant/allele for unbanded is recessive;
- because two banded spiders produced some unbanded offspring
OR
because there is a ratio of 3 banded:1 unbanded; - both parents are heterozygous;
Accept answers given in the form of Punnet squares.
SL:
Candidates were moderately successful at answering (a) and (c), but unsuccessful at (b) and (d). In many responses, some understanding was indicated, but the words needed to express the answer were lacking.
HL:
This was relatively well answered, with many candidates scoring both marks.
Deduce reasons for the difference between the results of Cross 2 and Cross 3.
[2]
- (1:1 ratio) in cross 2 as banded parent is heterozygous/has one copy of each allele;
- (no unbanded offspring) in cross 3 as banded parent is homozygous/has two alleles for banded;
- (in crosses 2 and 3) banded parental phenotypes are the same but their genotypes are different;
Many candidates also answered this question well, showing sound understanding of inheritance of dominant and recessive alleles and of genetic ratios.
There were 79 progeny in Cross 4. Predict the expected results by completing the table.
[1]
| 0 | 79 | 0.000/0 |
All three parts of the answer must be correct for the mark to be awarded.
This was one of the less discriminating questions in the exam. It was easy for candidates to make the error of giving the unbanded rather than banded proportion in the right hand column, perhaps because this column was adjacent to the column for the number of unbanded spiderlings.
Identify, using one recognition feature visible in the photo, the phylum in which D. plantarius is classified.
[1]
arthropods (as spider has) segmentation/exoskeleton/jointed limbs/jointed appendages/bilateral symmetry;
There were several complaints from teachers that this was a trick question because the binomial Dolomedes plantarius would have suggested to candidates that the organism was a plant. However, the stem of the question states that D. plantarius is a spider and it was assumed that candidates would know that spiders are animals. This was the least well answered question on the exam, with a mean mark of 0.2. Many candidates did not know that the phylum of a spider is Arthropods and some that did could not then give a recognition feature.