The Test Cross
In a genetics problem, sometimes you are only given
the phenotype of an organism. If you are only given
the phenotype, how can you determine its
genotype?
If the organism shows the dominant trait, you must
decide if the organism is homozygous for the
dominant trait (TT) or heterozygous (Tt).
To determine which of these two genotypes the
organism has, you perform what is known as a
TEST CROSS.
A test cross is always performed between the
unknown genotype and a homozygous recessive
genotype.
(If the organism’s phenotype is the recessive trait
then you know that the organism must be
homozygous for the recessive trait.)
These results reveal the genotype of the parent:
▪ If all the offspring display the dominant
phenotype, then the individual in question is
homozygous dominant.
▪ If the offspring displays both the dominant and
recessive phenotypes, then the individual is
heterozygous.
Test crosses work well in species that reproduce
quickly and in large numbers.
With the advances in molecular biological
techniques, geneticists can test for specific alleles
directly, rather than having to wait for the production
of offspring.
Example:
In rabbits, short fur (S) is dominant over long fur
(s). A breeder buys a female rabbit with short fur.
He performs a test cross to determine the genotype
of the female, and finds that two of the offspring
rabbits had long fur and two had short fur.
Use Punnett squares to work through all the
genotype possibilities to determine the genotype of
the female.