▶️ Answer/Explanation
(a)
12 chromosomes
(Each gamete has half the number of chromosomes as a body cell, so 24 ÷ 2 = 12)
(b)
Key:
- P = purple flower allele (dominant)
- p = white flower allele (recessive)
Cross: F₁ self-pollination: Pp × Pp
| P | p | |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
- Genotypes: PP, Pp, Pp, pp
- Phenotypes:
- PP → Purple
- Pp → Purple
- pp → White
- Phenotypic ratio: 3 Purple : 1 White
Observed numbers in F₂ generation:
- Purple = 97
- White = 38
This is roughly consistent with the 3:1 ratio.
(c)
The actual ratio may differ from the expected due to random chance in inheritance.
Other possible reasons:
- Small sample size
- Environmental factors
- Slight experimental error during pollination or classification
Markscheme
(a) 12;
(b)
a. male and female gamete genotypes/alleles shown as P and p (or others following convention with a suitable key) in a punnet grid;
b. \(F_2\) genotypes shown as PP, Pp, pP and pp;
c. \(F_2\) phenotypes indicated for each genotype on the Punnett grid / 3 purple to 1 white ratio indicated;
(c)
chance / OWWTE;
▶️ Answer/Explanation
(a)
- The banded trait is dominant, and the unbanded trait is recessive
- Evidence: Two banded parents produced both banded and unbanded offspring
- This suggests both parents were heterozygous (Bb)
The offspring show a 3:1 ratio (approx. 46:16) of banded:unbanded
(b)
- Cross 2 produced a 1:1 ratio of banded to unbanded (37:38):
→ The banded parent was heterozygous (Bb) - Cross 3 produced only banded offspring (63:0):
→ The banded parent was homozygous dominant (BB) - So, although both crosses involved unbanded × banded, the difference is due to the banded parent’s genotype (Bb vs BB)
(c)
- Both parents are unbanded, so both must be homozygous recessive (bb)
- All offspring will also be unbanded (bb)
| Cross number | Parental phenotypes | Banded | Unbanded | Proportion banded |
|---|---|---|---|---|
| 4 | Unbanded × Unbanded | 0 | 79 | 0.000 |
(d)
- Through natural selection, one phenotype might be favored
- For example:
- Better camouflage could lead to greater survival
- Sexual selection: one form might be more attractive to mates
- Environmental changes might make one type more advantageous
Markscheme
(a)
a. allele/trait/gene for banded is dominant/allele for unbanded is recessive;
b. because two banded spiders produced some unbanded offspring
OR
because there is a ratio of 3 banded:1 unbanded;
c. both parents are heterozygous;
(b)
a. (1:1 ratio) in cross 2 banded parent is heterozygous/has one copy of each allele;
b. (no unbanded offspring) in cross 3 as banded parent is homozygous/has two alleles for banded;
c. (In crosses 2 and 3) banded parental phenotypes are the same, but their genotypes are different;
(c)
(d)
a. natural selection could favour one type over the other;
b. greater survival of one type
OR
one type better camouflaged than the other type;
c. one type more attractive during courtship/mate selection;
▶️ Answer/Explanation
a.i.
Things like radiation or harmful chemicals can raise how often mutations happen.
a.ii.
One type is when a base in the DNA is swapped for a different one — that’s a base substitution.
b.
They have bodies covered in a hard outer layer called an exoskeleton.
Their legs and other parts are jointed.
b.ii.
Hypothesis A is better because it follows the idea of natural selection. Some lice already had resistance due to random mutations. When the shampoo was used, the non-resistant lice died and the resistant ones survived and reproduced. Hypothesis B is less accurate because it suggests the shampoo caused the mutation, but mutations don’t happen because of need — they happen by chance.
Markscheme
a.i.
a. radiation
b. chemical mutagens/carcinogens/papilloma virus/cigarette smoke
a.ii.
base substitution/insertion/deletion/frameshift
b.
a. jointed appendages
b. «chitinous» exoskeleton
c. segmented body
OR
bilateral symmetry
OR
mouth \(AND\) anus
OR
paired appendages
b.ii.
a. «scientists would accept» hypothesis \(A\) as the better one as mutations are random
b. scientists would reject hypothesis B because characteristics acquired during the lifetime of the individual being inherited is Lamarckian/not part of the evolution by natural selection theory/not all mutations are heritable
c. «the resistance» mutation would be present in the population initially and not caused by the shampoo «as hypothesis B states»
d. both hypotheses include variation in the population of lice «resistant and non-resistant»
e. variation is necessary for natural selection to occur
f. frequency of the best adapted increases and these individuals reproduce/pass on resistance to their offspring, so the resistant population increases «so hypothesis \(\mathrm{A}\) is better»
OWTTE can be used for any of the answers in this part.
▶️ Answer/Explanation
a.
X: The phylum of plant X is Filicinophyta. It has fronds and reproduces using spores instead of seeds.
Y: The phylum of plant Y is Coniferophyta. It produces seeds in cones and has needle-like leaves.
b.
One cause of mutation is exposure to radiation, such as UV light or X-rays. These can damage the DNA by changing the base sequence in a gene, which may lead to a new or altered trait.
c.
- To place a species in a clade, scientists compare DNA base sequences or amino acid sequences. Fewer differences in sequences mean species are more closely related.
- They also look at homologous structures, like similar bone patterns, to show shared ancestry.
- All members of a clade share a common ancestor, and this is shown through these genetic and structural similarities.
d.
The family most closely related to Diprotodontoidea is Vombatidae (wombats), because they branch off from the same recent common point on the cladogram.
Markscheme
a.
\(\mathrm{X}\): Filicinophyta ✔
\(\mathrm{Y}\): Coniferophyta/Conifera/Gymnosperms ✔
b.
Not chromosomal.
c.
a. a clade is a group of organisms that have evolved from a common ancestor ✔
b. identify the base sequences of a gene ✔
c. identify amino acid sequence of a protein ✔
d. comparing homologous structures ✔
e. the fewer the differences, the closer they diverged in time from a common ancestor ✔
Accept vice versa.
d.
Vombatidae/wombats ✔