A testcross is the mating of an organism with an unknown genotype with an organism whose genetic makeup for a trait is known, to determine the alleles (versions of genes ) controlling the trait carried by the former. An organism displaying the dominant phenotype is mated with a purebred (homozygous) recessive individual of the same species, and the phenotype of the progeny is checked to determine the genotype of the dominant parent.

Testcross

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Testcross

Q: What is the purpose of a testcross?

The purpose of a testcross is to determine which alleles —versions of a target gene—are carried by an organism with a dominant phenotype but unknown genotype . It reveals if the organism is homozygous (carrying two copies of the dominant allele) or heterozygous (carrying one dominant allele and one recessive allele).

Q: What is a monohybrid testcross?

A monohybrid testcross is used to determine an organism’s genotype for a single trait. It involves crossing an organism with an unknown genotype of the trait under investigation and another organism that is homozygous recessive for that trait. An example would be a cross between a dog with a red coat (a recessive phenotype controlled by a single gene) and a dog with a black coat, the dominant phenotype. Because the dominant phenotype can be seen in dogs that are either homozygous for the dominant allele or heterozygous, a testcross is necessary to determine the genotype of the black-coated dogs.

Q: What is a dihybrid testcross?

A dihybrid testcross is used to determine an organism’s genotype for two traits. It involves crossing an organism with an unknown genotype for the two traits under investigation with an organism that is homozygous recessive for both traits. In addition to the organism’s genotype, this type of cross reveals gene linkages, if any. If the dominant organism is heterozygous for the genes of interest and the genes assort independently, then the resulting offspring display a phenotypic ratio of 1:1:1:1.

Q: How can a testcross determine if a dominant phenotype is homozygous or heterozygous?

If all offspring from a testcross have the dominant phenotype , the dominant parent is homozygous—carrying two copies of the dominant allele of the target gene controlling the trait. If some offspring exhibit the recessive phenotype, the parent is heterozygous—carrying one dominant allele and one recessive allele of the target gene.

genetics

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Nov. 4, 2026 •History

What is a testcross?

What is the purpose of a testcross?

What is a monohybrid testcross?

What is a dihybrid testcross?

How can a testcross determine if a dominant phenotype is homozygous or heterozygous?

Testcross, the mating of an organism whose genetic constitution, or genotype, is unknown with an organism whose entire genetic makeup for a trait is known, to determine which versions of the genes (alleles) controlling the trait are carried by the former. In such experiments, an organism with a dominant phenotype (observable trait produced by a dominant allele) but unknown genotype is crossed with an organism that is homozygous recessive (having two recessive alleles) for that trait. The phenotypes of the resulting offspring reveal the genotype of the dominant parent—if at least one offspring displays the recessive phenotype, then the dominant parent must be hybrid, or heterozygous (carrying one dominant and one recessive allele), for the given trait.

Types and examples

The simplest testcross is the monohybrid testcross, which involves an organism with an unknown genotype for a single trait and a homozygous recessive organism for that same trait. In a breed of dog, for example, in which the gene for black coat color is dominant over (suppresses the effect of) the gene for red coat color, a dog with a black-colored coat may be either pure breeding (homozygous), with two alleles for black coat color, or heterozygous, with one allele for black coat color and one for red coat color. To determine which genotype the dog has, it is mated to a red-coated dog, which invariably has two alleles for red coat color and is therefore homozygous recessive for the coat-color trait. If all the offspring are black, the black-coated parent must be homozygous and have both alleles for black coat color; if some of the young are red, the parent must be heterozygous, carrying one gene for black coat color and one for red coat color.

A testcross can be performed between organisms that differ in more than one trait. When the cross is between an organism with an unknown genotype for two different traits and an organism that is homozygous recessive for both traits, or double homozygous recessive, then it is called a dihybrid testcross. In addition to determining the genotype of an organism, this cross can also reveal whether the two genes under investigation are linked (likely to be inherited together) or whether they separate from each other during gamete formation. If the parent with the unknown genotype is hybrid for both the traits (a double heterozygote), then at least one of the progeny will exhibit the phenotype of the recessive parent.