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ICSE Question Bank
2016Q7(a)(v)Genetics – Some Basic Fundamentals
Short answerModerate1 mark
7(a)

A homozygous tall plant (T) bearing red coloured (R) flowers is crossed with a homozygous dwarf (t) plant bearing white (r) flowers:

What is the phenotypic ratio obtained in F2 generation?

Topic: Dihybrid F2 phenotypic ratio

Previous-year question practice database

Answer

Exam answer

9 : 3 : 3 : 1

9 Tall with red flowers 3 Tall with white flowers 3 Dwarf with red flowers 1 Dwarf with white flowers

Explanation

9:3:3:1 is the standard DIHYBRID ratio. 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).

Note that the two middle figures are the NEW combinations from part (iv), and together they make up 6 of the 16 offspring — more than a third. Independent assortment is not a rare exception but the normal result.

Compare the MONOHYBRID ratio 3:1, which applies when only one character is followed.

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