Question

In a homozygous pea plant, axial flowers (A) are dominant over terminal flowers (a). What is the phenotypic and genotypic ratio in the above cross shown in (ii)?

Answer 1

Hint: Gregor Mendel conducted hybridization on pea plants. For the first time in biology, he used some mathematical logic to explain the pattern of inheritance. He used several true-breeding pea lines. Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits.

Complete step by step answer: In the pea plant, “AA” will be the pair of an allele for the dominant gene which is of axial flowers whereas; the “aa” allele will be of the terminal flowers. “AA” and “aa” are called the genotype of the plant while the elaborated terms axial and terminal are the phenotype. The cross will be made as given below:

Gametes	Terminal flowersa	Terminal flowersa
Axial flowersA	Aa	Aa
Axial flowersA	Aa	Aa

Gametes	Terminal flowersa	Terminal flowersa
Axial flowersA	Aa	Aa
Axial flowersA	Aa	Aa

Phenotypic ratio: axial: terminal
                                  3: 1
Genotypic ratio:AA : Aa : aa
                                1: 2: 1
This concludes that, at F2, three-fourth of the flowers is axial and one-fourth of them are terminal.

Additional information: Contrasting traits studied by Mendel in pea are:

Serial number	Characters	Contrasting traits	Chromosome number
1.	Stem height	Tall/dwarf	4
2.	Flower colour	Violet/white	1
3.	Flower position	Axial/terminal	4
4.	Pod shape	Inflated/constricted	4
5.	Pod colour	Green/yellow	5
6.	Seed Shape	Round/wrinkled	7
7.	Seed colour	Yellow/green	1

Note: When there is an inheritance of a single contrasting trait at a time by two parents, it is referred to as monohybrid ratio. When there is a monohybrid cross between two pure breeding varieties all dominant traits are obtained in the F1 generation, in F2 generation upon selfing it gives the monohybrid phenotypic ratio of 3:1 and a genotypic ratio of 1:2:1 which is 1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive.