Testcross Explained

Summary

Key Points

• —A test cross is a method used to determine the genotype of an organism that displays a dominant phenotype. 0:03
• —In pea plants, the dominant allele results in yellow peas, while the recessive allele results in green peas, meaning a yellow pea plant can be either homozygous dominant or heterozygous. 0:29
• —To perform a test cross, the organism with the unknown dominant phenotype is crossed with an organism that has a homozygous recessive phenotype. 0:59
• —The phenotypes of the resulting offspring provide the necessary information to deduce the genotype of the parent with the dominant phenotype. 1:09
• —This outcome occurs because the homozygous dominant parent can only pass on dominant alleles to its offspring, making all offspring heterozygous. 1:41
• —If the unknown parent is homozygous dominant, all offspring from the test cross will display the dominant phenotype (e.g., 100% yellow peas). 2:29
• —By analyzing the phenotypic ratios of the offspring, one can definitively determine whether the parent with the dominant phenotype was homozygous dominant or heterozygous. 2:37
• —This 50/50 ratio indicates that the heterozygous parent passed on both its dominant and recessive alleles to its offspring. 2:41
• —If the unknown parent is heterozygous, the test cross will yield offspring with a 50% dominant phenotype and 50% recessive phenotype (e.g., half yellow and half green peas). 2:52