Question

(a) How would you find out whether a given tall garden pea plant is homozygous or heterozygous? Substantiate your answer with the help of Punnett squares.

(b) Given below are the $F_2$-phenotypic ratios of two independently carried monohybrid crosses:
(i) 1:2:1
(ii) 3:1
Mention what does each ratio suggest. [5 marks]

Answer 1

(a) To know whether a given tall garden pea plant is homozygous or heterozygous, it must be crossed with a homozygous recessive plant. This is called a test cross. [0.5 mark]
In the following test cross, the given pea plant with unknown genotype is crossed with a homozygous recessive short pea plant. [0.5 mark]

If all flowers in the progeny are tall then the original plant was homozygous tall otherwise it was heterozygous tall.
[1 + 1 + 0.5 mark]

(b) (i) When the phenotypic ratio is 1:2:1 , this means there are three different phenotypes. [0.5 mark]
This can happen due incomplete dominance or codominance. The heterozygotes have a phenotype that is a mix of the individual dominant and recessive phenotypes. [0.5 mark]
Here, the genotypic ratios were exactly as we would expect in any Mendelian monohybrid cross, but the phenotypic ratios had changed from the 3:1 dominant : recessive ratio. [0.5 mark]

(ii) The ratio 3:1 is the normal phenotypic ratio of a monohybrid cross between a homozygous dominant with a homozygous recessive parent. The F1 generation is self crossed to give this ratio. [1 mark]