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Two different genotypes produce the same dominant phenotype. A homozygous dominant organism (PP) and a heterozygous organism (Pp) both display the dominant trait because the dominant allele masks the recessive one. Since you cannot distinguish between them visually, a test cross is needed to reveal the unknown genotype.

You cross the organism with unknown genotype with one displaying the recessive phenotype, which must be homozygous recessive (pp). This parent can only contribute recessive alleles to offspring, making the recessive parent's contribution predictable and allowing the unknown parent's genotype to be revealed through offspring phenotypes.

If all offspring display the dominant phenotype, the unknown parent must be homozygous dominant (PP). All offspring receive one recessive allele from the homozygous recessive parent and one dominant allele from the homozygous dominant parent, resulting in heterozygous (Pp) offspring that all express the dominant trait.

A 1:1 ratio indicates the unknown parent is heterozygous (Pp). Half the offspring inherit the dominant allele and display the dominant phenotype, while the other half inherit two recessive alleles and display the recessive phenotype. This equal split reveals the heterozygous genotype of the unknown parent.

A test cross is a specific type of cross used to determine unknown genotypes, while monohybrid crosses examine inheritance of a single trait. Test crosses use monogenic traits and monohybrid crosses principles to analyze offspring ratios and deduce parental genotypes based on predictable inheritance patterns.

Only one genotype produces a recessive phenotype: homozygous recessive (pp). An organism displaying a recessive trait must have two copies of the recessive allele because the recessive allele is only expressed when no dominant allele is present to mask it.

Test crosses are designed for simple dominant-recessive traits where one allele completely masks the other. Traits with multiple alleles or incomplete dominance require different analytical approaches because the phenotypes do not follow the simple dominant-recessive pattern that test crosses depend on.