A test cross is a genetic experiment designed to reveal the unknown genetic makeup of an organism. It is a fundamental tool in genetics used to determine whether an individual displaying a dominant trait carries two copies of the dominant gene or one copy of the dominant and one copy of the recessive gene.
The Genetic Puzzle It Solves
Organisms inherit two copies of each gene, one from each parent. Some genes have different versions, called alleles, which can be dominant or recessive. A dominant allele expresses its trait even if only one copy is present, while a recessive allele only expresses its trait if two copies are inherited. For example, a plant might have a dominant allele for tallness and a recessive allele for dwarfness.
A plant that appears tall could either have two copies of the dominant tall allele (homozygous dominant) or one copy of the tall allele and one copy of the dwarf allele (heterozygous). Both combinations result in a tall plant because the tall allele is dominant. This means the outward appearance, or phenotype, does not always indicate the underlying genetic makeup, or genotype. The test cross solves this genetic ambiguity.
Performing the Cross
To perform a test cross, the individual with the unknown dominant genotype is bred with a homozygous recessive individual, often called the “tester.” The homozygous recessive tester always expresses the recessive phenotype, meaning it carries two identical copies of the recessive allele. For instance, if testing a tall plant, the tester would be a dwarf plant.
The homozygous recessive tester can only pass on recessive alleles to its offspring. This allows the alleles from the unknown parent to be expressed without interference from another dominant allele. By observing the offspring’s traits, geneticists can deduce the unknown parent’s genetic makeup.
Decoding the Results
The phenotypes of the offspring from a test cross provide the answer to the unknown parent’s genotype. There are two primary outcomes. If all offspring display the dominant phenotype, the unknown parent must have been homozygous dominant. This occurs because the homozygous recessive tester contributes only recessive alleles, so the dominant trait in all offspring must come from the unknown parent.
Alternatively, if approximately half of the offspring show the dominant phenotype and the other half show the recessive phenotype (a 1:1 ratio), the unknown parent was heterozygous. In this scenario, the heterozygous parent contributed either a dominant or a recessive allele to each offspring with equal probability. Recessive offspring express the recessive trait because they received a recessive allele from both the unknown parent and the homozygous recessive tester.
Real-World Relevance
Test crosses have practical applications in various fields. In agriculture, they are used by plant and animal breeders to identify individuals with desired traits and eliminate undesirable ones. For example, a breeder might use a test cross to confirm if a valuable animal displaying a dominant trait is homozygous, ensuring all its offspring inherit that trait.
Test crosses also assist in identifying carriers of recessive genetic disorders in animals, even if the carriers themselves do not show symptoms. By crossing an individual with an unknown genotype, breeders can determine if it carries a recessive disease allele. This information is then used to make informed breeding decisions, helping to reduce the incidence of genetic diseases within populations.