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Test Cross

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:

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Test Cross

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  1. Test Cross

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

  3. 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!!!

  4. 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.

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

  6. 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

  7. 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

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