A test cross, also known as a back cross or tester cross, is a genetic breeding experiment used by geneticists and breeders to determine the genotype of an organism displaying a dominant trait. This experiment is particularly useful for identifying whether the organism is homozygous dominant (two dominant alleles) or heterozygous (one dominant and one recessive allele) for that trait.
Here’s how a test cross works:
- Selection of the Organism: The organism whose genotype is to be determined is referred to as the “unknown” or “test” individual. It typically exhibits a dominant phenotype for a specific trait of interest. The dominant phenotype is the outward expression of the dominant allele.
- Selection of the Mate: To perform a test cross, the unknown organism is crossed (mated) with a known recessive individual that displays the recessive phenotype for the same trait. The known recessive individual is referred to as the “tester” or “recessive tester.”
- Observation of Offspring: The resulting offspring of the test cross are observed and their phenotypes are recorded.
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- If all the offspring display the dominant phenotype, it suggests that the unknown organism is homozygous dominant (DD), meaning it has two dominant alleles for that trait.
- If approximately half of the offspring display the dominant phenotype, and the other half display the recessive phenotype, it suggests that the unknown organism is heterozygous (Dd), having one dominant allele and one recessive allele for the trait.
The reason behind this is the principles of Mendelian genetics. When a homozygous dominant organism (DD) is crossed with a homozygous recessive organism (dd), all of the offspring (F1 generation) will inherit one dominant allele and one recessive allele (Dd). These offspring will display the dominant phenotype because the dominant allele masks the presence of the recessive allele. When these F1 heterozygotes are crossed with the recessive tester (dd), the resulting F2 generation will have a 1:1 ratio of dominant to recessive phenotypes if the original unknown organism was heterozygous (Dd).
In summary, a test cross is a genetic experiment used to determine the genotype of an organism displaying a dominant trait by crossing it with a known recessive individual. The offspring’s phenotypes reveal whether the unknown organism is homozygous dominant (two dominant alleles) or heterozygous (one dominant and one recessive allele) for that trait.