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Principles of Inheritance and Variation
Dihybrid Cross and Test Cross
2 questions from 2 papers, each with its answer.
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Appears in
Concepts tested
- Offspring of TTRr x ttrr
- Offspring ratio from RRTt x rrtt
All Questions
2004 AIPMT 1 question
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In a plant red fruit (R) is dominant over yellow fruit (r) and tallness (T) is dominant over shortness (t). If a plant with RRTt genotype is crossed with a plant that is rrttGametes of RRTt: RT and Rt (in equal proportion, 1/2 each).
Gametes of rrtt: rt only.
Offspring: RT $\times$ rt = RrTt (red, tall); Rt $\times$ rt = Rrtt (red, short).
So all offspring have red fruit (Rr), because the RRTt parent is homozygous for red.
For height, Tt $\times$ tt gives 1/2 Tt (tall) and 1/2 tt (short).
Ratio: 1 red tall : 1 red short.
So 50% of the offspring will be tall with red fruit.
2000 AIPMT 1 question
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Due to the cross between TTRr × ttrr the resultant progenies showed how many percent plants tall, red flowered :Gametes of TTRr: TR and Tr (each 1/2). Gametes of ttrr: tr only.
Offspring: TtRr (tall, red) and Ttrr (tall, white) in equal numbers.
All are tall (Tt), and half are red (Rr).
So 50% of the plants are tall and red flowered.