TEST CROSS

TEST CROSS 

It is a cross between an organism of an unknown genotype and a homozygous recessive organism.

Purpose of test cross

An organism showing the recessive phenotype must have a homozygous genotype for the recessive allele. For example, a white Guinea has bb genotype and a dwarf pea 🫛 plant has tt genotype. However, an organism displaying a dominant phenotype may have a homozygous genotype or a heterozygous. For instance, a black guinea pig may have BB genotype or Bb genotype and a tall pea plant may have TT or Tt genotype. It may interested to a breeder to know the genotype of such an organism.

Result of a Test Cross

1) If the test cross yield offspring of which 50%show the dominant character and 50% show the recessive character,i.e , F1 ratio is 1:1, the individual test is heterozygous. This is so because the individual showing the recessive trait must have received one recessive allele from each parent.

Example1

    

2)  If all the offspring of the test cross show the dominant trait, the individual being tested in homozygous dominant with genotype BB for guinea pig and TT for pea plant.

Example 2

Use of Test Cross

It reveals the genotype of an organism displaying the dominant phenotype. It was devised by Mendel and continues to be an important tool of geneticists.

Types of Test Cross

It is of two types:-

a) Monohybrid test cross - It deals with a single character and gives F1 ratio of 1:1.

b) Dihybrid test cross - It deals with the two traits at a time and gives F1 ratio of 1:1:1:1 for a cross between double heterozygote and double recessive homozygote, BbSs* bbss guinea pig, RrYy*rryy pea plants .