Summary
Crossing an unknown dominant phenotype with a homozygous recessive to reveal its genotype.
In depth
The unknown dominant individual is crossed with a homozygous recessive. All dominant offspring suggest a homozygote, while a 1:1 ratio shows a heterozygote. It is used in breeding animals and plants.
A test cross uses a homozygous recessive because it contributes only recessive alleles, so every offspring phenotype reveals the gamete of the unknown parent. A large sample is needed: with 10 offspring all dominant, the chance that a heterozygote was mistaken for a homozygote is about 0.1 percent (0.5 to the tenth power).
Back-crosses to a parent are related. In the dihybrid test cross, results can also show linkage: more parental than recombinant types means the genes are on the same chromosome, which Morgan exploited in fruit flies.
Example: a breeder wants to know whether a black dog (dominant) carries a hidden brown allele, so crosses it to a brown dog.
Frequently asked questions
What is Test Cross?
Crossing an unknown dominant phenotype with a homozygous recessive to reveal its genotype.
Which area of biology covers Test Cross, and how advanced is it?
Test Cross is one of the 54 topics in Genetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Monohybrid Cross, Linked Genes, Law of Segregation.
Can you give an example of Test Cross?
A black dog (B_) is test crossed to a chocolate (bb) one to find if it carries the recessive allele.
What should I learn before Test Cross?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Test Cross. Intermediate topics assume the Beginner vocabulary.
What topics are related to Test Cross?
The closest topics to Test Cross are Monohybrid Cross, Linked Genes, Law of Segregation, Homozygous and Heterozygous. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Test Cross?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Test Cross. To practise the whole of Genetics, use the Genetics quiz.
How is Test Cross different from Monohybrid Cross?
Test Cross: Crossing an unknown dominant phenotype with a homozygous recessive to reveal its genotype. Monohybrid Cross: A cross following one gene with two alleles, giving a 3:1 phenotype ratio in the F2 generation. Read both side by side and note what each one explains.
How should I study Test Cross step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Monohybrid Cross and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.