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Experiments were conducted to obtain data on the traits ‘seed shape’ in plants and ‘feather colour’ in chickens - HSC - SSCE Biology - Question 30 - 2019 - Paper 1
Question 30
Experiments were conducted to obtain data on the traits ‘seed shape’ in plants and ‘feather colour’ in chickens. In each case, the original parents were pure breedin... show full transcript
Worked Solution & Example Answer:Experiments were conducted to obtain data on the traits ‘seed shape’ in plants and ‘feather colour’ in chickens - HSC - SSCE Biology - Question 30 - 2019 - Paper 1
Step 1
Explain the phenotypic ratios of the F2 generation in both the plant breeding experiments
Answer
Graph A shows a phenotypic ratio of 3:1, indicative of a typical dominant/recessive allele interaction. In this experiment, the pure breeding parents must have been homozygous for seed shape, with one parent being RR (round seeds) and the other rr (wrinkled seeds).
The first generation (F1) offspring would be all heterozygous Rr. When these F1 individuals are self-pollinated to produce the F2 generation, the following Punnett square can be constructed:
R | r
--------
R | RR | Rr
--------
r | Rr | rr
From the table, we can see that the seed shape ratio in the F2 generation will be:
- 1 RR (round seeds)
- 2 Rr (round seeds)
- 1 rr (wrinkled seeds)
This provides us with the 3:1 ratio of round to wrinkled seeds.
Step 2
Explain the phenotypic ratios of the F2 generation in both the chicken breeding experiments
Answer
Graph B exhibits a phenotypic ratio of 1:2:1, which is typical of co-dominance. In this case, the feather color traits represent two alleles that are co-dominant. Let's say one parent is homozygous for black feathers (FF) and the other is homozygous for white feathers (WW).
The F1 generation will express both alleles and be heterozygous (FW), exhibiting black feathers. When the F1 generation is interbred to derive the F2 generation, the following Punnett square can be set up:
F | W
--------
F | FF | FW
--------
W | FW | WW
The ratios derived from this Punnett square would be:
- 1 FF (Black feathers)
- 2 FW (Black feathers)
- 1 WW (White feathers)
This results in a total distribution where we find 50% black feathers and 50% with white feathers, emphasizing the co-dominance of the alleles.