Question 4
A homozygous tall plant was crossed with a homozygous short plant. An offspring of the F1 generation was then crossed with another homozygous short plant. With the aid of a suitable genetic diagram each, determine the:
(a) genotypes of the F1 generation;
(b) phenotypic ratio of the offspring of the cross between the F1 generation and the homozygous short plant;
(c) genotypic ratio of the offspring in 4(b). [20 marks]
Observation
This question was not popular among candidates as majority avoided it because it has to do with genetics and crossing. Majority of candidates lost marks because they did not put the cross sign (X) for the parental genotype and the gametes. Gametes should be clearly separated.
Some candidates also could not distinguish among phenotypic and genotypic ratio. Instead of writing All Tt in 4(a), some candidates wrote All Tall.
Some candidates also do not know the Punette Square as just a few used it.
The expected answers are:
(b) Phenotypic ratio of the offspring of the cross between the F1 generation and the homozygous short plant
(c) Genotypic ratio of the offspring
2:2 or 1:1