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ICSEClass XBiologyGenetics03 Mark2025 Easy

QQuestion

A farmer cultivates a unique species of tomato plants. The two traits selected for breeding were: Colour of the fruits and 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).

A farmer cultivates a unique species of tomato plants

(a) Explain the phenotype of the offsprings of the F1 generation obtained from the initial cross.

(b) Mention the phenotypic ratio of the F2 generation offsprings.

(c) State Mendel’s Law of Independent Assortment.

Exam-ready answer • 03 Mark

Answer

Initial cross: TTRR × ttrr

Gametes: TR and tr

F1 genotype: All TtRr

(a) All F1 plants are tall and bear red fruits because T (tallness) is dominant over t (dwarfness), and R (red fruit colour) is dominant over r (yellow fruit colour).

F1 cross: TtRr × TtRr

Each parent produces four kinds of gametes: TR, Tr, tR and tr.

Gametes TR Tr tR tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr TtRr Ttrr
tR TtRR TtRr ttRR ttRr
tr TtRr Ttrr ttRr ttrr

(b) The F2 phenotypic ratio is 9 tall red : 3 tall yellow : 3 dwarf red : 1 dwarf yellow.

(c) Mendel’s Law of Independent Assortment states that during gamete formation, the alleles of one pair of contrasting characters assort independently of the alleles of another pair.

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