How do we know whether an organism expressing a dominant trait is homozygous or heterozygous?

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Understand that an organism expressing a dominant trait can have two possible genotypes: homozygous dominant (e.g., \(AA\)) or heterozygous (e.g., \(Aa\)). Both genotypes show the dominant phenotype, so phenotype alone does not distinguish them.

Perform a test cross by mating the organism with a homozygous recessive individual (e.g., \(aa\)), which will only contribute recessive alleles to the offspring.

Analyze the offspring phenotypes: if all offspring show the dominant trait, the tested organism is likely homozygous dominant; if some offspring show the recessive trait, the tested organism is heterozygous.

Use the expected Mendelian ratios to predict offspring genotypes and phenotypes from the test cross: for a heterozygous parent (\(Aa\)) crossed with \(aa\), expect approximately 50% dominant and 50% recessive offspring.

Conclude the genotype of the original organism based on the presence or absence of recessive phenotype offspring in the test cross.

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Dominant and Recessive Alleles

Dominant alleles express their trait even if only one copy is present, while recessive alleles require two copies to show their trait. Understanding this helps distinguish between organisms that show a dominant trait due to one or two dominant alleles.

Genotype vs. Phenotype

Phenotype is the observable trait, while genotype is the genetic makeup. An organism with a dominant phenotype could have either a homozygous dominant or heterozygous genotype, so knowing the difference is key to identifying the underlying genetic composition.

Test Cross

A test cross involves breeding the organism with a homozygous recessive individual to determine its genotype. If any offspring show the recessive trait, the tested organism is heterozygous; if all offspring show the dominant trait, it is likely homozygous dominant.