A guinea pig that is homozygous dominant (BB) for Black coat color looks the same as a guinea pig that is heterozygous (Bb) for Black coat color! So how can we tell if an individual is homozygous dominant (BB) or heterozygous (Bb)? Geneticists can perform a test cross. In a test cross, an individual with an unknown genotype (i.e. BB or Bb) is crossed with an organism that is homozygous recessive (i.e. bb, white coat color) for the trait.
Complete the two Punnett Squares to illustrate a Test Cross.
Homozygous Dominant x Homozygous Recessive Offspring Ratios:
Genotypic Ratio: 4Bb B B b Bb Bb b Bb Bb
Phenotypic Ratio: all black
Heterozygous x Homozygous Recessive
B b b Bb bb b Bb bb
Offspring Ratios:
Genotypic Ratio: 2Bb:2bb
Phenotypic Ratio: 2 black:2 white
What is the Genotype of a Black guinea pig that has at least one white offspring? __ Bb _____
Can you be 100% sure of the Genotype of a Black guinea pig if it has ten offspring and all of them have black coat color? Yes or No
Please Explain your answer.
There is a 50:50 chance that one will be white, but that is still just the odds. We could be pretty sure of its genotype, but we could not be 100% sure of it.