Test Cross

What is a test cross? Test that is conducted to determine if an organism is HOMOZYGOUS DOMINANT or HETEROZYGOUS for a trait

Example: Having freckles is dominant to not having freckles, write down the possible genotypes below. Let’s use F for freckles and f for no freckles: Genotype for Freckles: FF or Ff Genotypes for No Freckles: ff How do we know if a person with freckles has FF or Ff do a test cross!!!

5 Key Points About Test Crosses: 1. The organism with the dominant trait is always crossed with an organism that is HOMOZYGOUS RECESSIVE 2. If ANY offspring shows the recessive trait than the unknown genotype is heterozygous. 3. If ALL offspring have the dominant trait, the unknown is homozygous dominant. Large numbers of offspring are needed for reliable results Start by making a key and writing down what you know.

Example: If having spots is dominant to not having spots in Dalmatians, how can we determine if Bernice is homozygous dominant or heterozygous?

Let’s Write Down What We Know: Let’s say S= spots and s= no spots. Set up two punnett squares with a homozygous recessive in both of them (ss) Then as the other parent place a SS in one punnett square and a Ss is another Essentially, your two punnett squares will have: SS crossed with ss Ss crossed with ss

Interpretation: If all of the Dalmation puppies have spots that we can assume that Bernice is HOMOZYGOUS DOMINANT If any of the Dalmations DO NOT have spots than Bernice is HETEROZYGOUS