BIOLOGY Volume 3 - A Guide to General Biology - 2004

24. VARIATION AND GENETICS

24.1. Mendel's Studies

24.1.2. Test Cross

An F₁ Organism, obtained from a cross between homozygous dominant and homozygous recessive individuals, is heterozygous yet exhibits the dominant phenotype. For the recessive phenotype to be expressed, an organism must be homozygous for the recessive Gene. In the F₂ generation, individuals with the dominant phenotype can be either homozygous or heterozygous. If a breeder needs to determine the genotype of such an individual, the only way to do so is by performing a test cross. By crossing an organism of unknown genotype with an organism homozygous for the recessive gene, this genotype can be determined in a single cross. For example, in the fruit fly Drosophila, long wings are dominant over vestigial wings. An individual with long wings may be homozygous (LL) or heterozygous (Ll). To determine its genotype, a test cross must be performed between this fly and one homozygous for the recessive allele (ll). If all offspring from this cross have long wings, then the individual with the unknown genotype is homozygous for the dominant allele. A 1:1 numerical ratio of offspring with long and vestigial wings indicates that the individual with the unknown genotype is heterozygous (Fig. 24.3).

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Fig. 24.3. Complete genetic explanation of the method used to determine the genotype of an organism displaying a dominant trait. This method is known as a test cross. The phenotypes of the resulting offspring are shown.

24.2. Why is it impossible to use an individual homozygous for the dominant allele (e.g., TT) in a test cross to determine the genotype of an individual with a dominant phenotype? Illustrate your answer with a genetic cross diagram using appropriate genetic symbols.