A farmer cultivates a unique species of tomato plants. The two traits selected for breeding were the colour of the fruits and the height of the plants. The farmer initially crosses a homozygous tall plant bearing red fruits (TTRR) with a dwarf plant bearing yellow fruits (ttrr), and the genotype of all the offsprings of the F1 generation was TtRr. Next, he crosses two F1 generation plants (TtRr).
Mention the phenotypic ratio of the F2 generation offsprings.
Topic: Dihybrid cross — F2 phenotypic ratio
Answer
9 : 3 : 3 : 1
9 Tall plants with red fruits 3 Tall plants with yellow fruits 3 Dwarf plants with red fruits 1 Dwarf plant with yellow fruits
9:3:3:1 is the standard DIHYBRID ratio and must be learnt — but write out WHAT each number refers to, since the question asks for the PHENOTYPIC ratio and the phenotypes are the answer.
The order is always: both dominant (9), first dominant with second recessive (3), first recessive with second dominant (3), both recessive (1).
WHERE IT COMES FROM: a TtRr × TtRr cross gives 16 combinations in the Punnett square. If you are asked to draw it, the four gametes from each parent are TR, Tr, tR and tr.
NOTE THE TWO NEW COMBINATIONS — tall with yellow, and dwarf with red. Neither appeared in the parents, and their appearance is the proof of INDEPENDENT ASSORTMENT: height and colour were inherited independently of each other.
Compare the MONOHYBRID ratio, 3:1, which applies when only one character is studied.