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Difficulty: MediumBasic Genetics Terminology and Concepts

In classic Mendelian genetics, a testcross is carried out by mating an organism exhibiting a dominant phenotype with an individual that is homozygous dominant for the trait in question.

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Answer

The statement is False because a testcross requires mating with a homozygous recessive individual, not a homozygous dominant one.
The statement is false because a testcross relies on a homozygous recessive tester (aaaa) so that the offspring phenotypes directly reveal the gametic contribution and genotype of the dominant parent.

Step-by-Step Solution

1
Identify the biological purpose of a testcross.
A testcross determines whether an organism expressing a dominant trait is homozygous dominant (AAAA) or heterozygous (AaAa).
Organisms with genotypes AAAA and AaAa are phenotypically identical.
2
Determine the required genotype of the tester parent.
The tester individual must be homozygous recessive (aaaa).
A homozygous recessive tester produces only recessive gametes (aa), allowing hidden recessive alleles from the tested parent to be expressed in the offspring's phenotype.
3
Evaluate the effect of using a homozygous dominant tester (AAAA).
Crossing either AAAA or AaAa with AAAA yields 100% dominant phenotype offspring.
The dominant allele from the AAAA tester masks any recessive allele contributed by a heterozygous parent.

Key Concept

Definition and methodology of a genetic testcross
Estimated Time:1m 0s
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Basic Genetics Terminology and Concepts

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