When a dominant phenotype organism is crossed with the homozygous recessive genotype organism, it is known as a test-cross. It is used to determine the zygosity of an organism with an unknown genotype. It is used to test whether an individual is pure (homozygous) or hybrid (heterozygous).

In a typical test cross, an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-fertilisation. The progenies of such a cross can easily be analysed to predict the genotype of the test organism. The figure below shows the results of a typical test cross where the violet colour of a flower (W) is dominant over the white colour (w).

Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 4

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Explain the Law of Dominance using a monohybrid cross.

Important Questions on Principles of Inheritance and Variation

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 5

Define a test-cross.

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 5

Design a test-cross.

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 6

Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 7

When a cross is made between a tall plant with yellow seeds and a tall plant with green seeds (TtYy), what proportion of phenotype in the offspring is expected to be tall and green?

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12th Andhra Pradesh Board

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 7

When a cross is made between a tall plant with yellow seeds and a tall plant with green seeds (TtYy), what proportion of phenotype in the offspring is expected to be dwarf and green?

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 8

Explain the following term with examples:

Co-dominance

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Intermediate Second Year Botany>Chapter 9 - Principles of Inheritance and Variation>Questions>Q 8

Explain the following term with examples:

Incomplete dominance