Question

Explain what is test cross?

Answer 1

Test cross = the cross of an organism with an unknown dominant genotype with an organism that is homozygous recessive for that trait

What does it do?

A test cross can determine whether the individual being tested is homozygous dominant (pure bred) or heterozygous dominant (hybrid).

Test crosses are used to test an individual's genotype by crossing it with an individual of a known genotype. Individuals that show the recessive phenotype are known to have a homozygous recessive genotype. Individuals that show the dominant phenotype, however, may either be homozygous dominant or heterozygous. The phenotypically dominant organism is the individual in question in a test cross. The purpose of a test cross is to determine if this individual is homozygous dominant or heterozygous.

Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. Analyzing the proportions of dominant and recessive offspring determines if the individual in question is homozygous dominant or heterozygous. If all offspring from the test cross display the dominant phenotype, the individual in question is homozygous dominant; if half the offspring display dominant phenotypes and half display recessive phenotypes, then the individual is heterozygous. Since the homozygous recessive individual can only pass on recessive alleles, the alleles the individual in question passes on determine the phenotypes of the offspring.

“Key Points” to remember about a TEST CROSS:

1. the organism with the dominant trait is always crossed with an organism with the recessive trait
2. if ANY offspring show the recessive trait, the unknown genotype is heterozygous
3. if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant

4. large numbers of offspring are needed for reliable results

Significance of Test Cross

• Test cross can be used to determine the genotype of the F1 plant.

• The test cross can be used to support the idea that the reappearance of the recessive character in the F2 generation is due to the heterozygous condition of the F1 plant.

• The test can be used to verify whether any given pair of characters can be alleles (contrasting characters).