Question

In a homozygous pea plant, axial flowers (A) are dominant over terminal flowers (a).

(i) What is the phenotype and genotype of the F1 generation if a plant bearing pure axial flowers is crossed with a plant bearing pure terminal flowers?
(ii) Draw a Punnett square board to show the gametes and offsprings when both the parent plants are heterozygous for axial flowers.
(iii) What is the phenotypic ratio and genotypic ratio of the above cross shown in (ii)?
(iv) State Mendel’s Law of Dominance.
(v) Name two genetic disorders commonly seen in human males.

Answer 1

(i) Phenotype of F1 generation plants: Axial flowers Genotype of F1 generation plants: Aa
(ii) Punnett square board to represent gametes and offspring when both parent plants are heterozygous for axial flowers (Aa).

	A	a
A	AA	Aa
a	aA	aa

(iii) Phenotypic ratio for the above cross – 3 : 1 (3 pea plants with axial flowers : 1 pea plant with terminal flowers)
Genotypic ratio for the above cross – 1 : 2 : 1 (1 homozygous plant with axial flowers : 2 heterozygous plants with axial flowers : 1 homozygous plant with terminal flowers)
(iv) Mendel’s Law of Dominance: Of a pair of contrasting characters present together, only one can express itself, while the other remains suppressed. The one which is expressed is the dominant and the one which is not expressed is the recessive.
(v) Genetic disorders commonly seen in human males:

• Haemophilia
• Colour blindness