2.  As in monohybrid crosses, you can’t tell by looking at the dominant trait whether it is homozygous or heterozygous.

3.  So in order to find out what the genotype is – do a test cross with an organism that has both recessive characteristics.

4.  As in monohybrid crosses, you can’t tell by looking at the dominant trait whether it is homozygous or heterozygous.  So in order to find out what the genotype is – do a test cross with an organism that has both recessive characteristics. - If the parent is heterozygous you get a ratio of 1:1:1:1

5.  As in monohybrid crosses, you can’t tell by looking at the dominant trait whether it is homozygous or heterozygous.  So in order to find out what the genotype is – do a test cross with an organism that has both recessive characteristics. - If the parent is heterozygous you get a ratio of 1:1:1:1 - If it is homozygous, all the offspring will show the dominant phenotype for both characteristics.

6. Genotype:AaBbxaabb Gametes:

7. Genotype:AaBbxaabb Gametes: AB Ab aB ab ab

8. Genotype:AaBbxaabb Gametes: AB Ab aB ab ab Punnett Square:

9. Genotype:AaBbxaabb Gametes: AB Ab aB ab ab Punnett Square: ab ABAaBb AbAabb aBaaBb abaabb

10.  Now, you have an organism that is homozygous dominant for one trait, but heterozygous for the other – the test cross/back cross still works.

11.  In the cross: - For the homozygous dominant characteristic, no recessive trait will show up in the offspring

12.  Now, you have an organism that is homozygous dominant for one trait, but heterozygous for the other – the test cross/back cross still works.  In the cross: - For the homozygous dominant characteristic, no recessive trait will show up in the offspring - For the heterozygous characteristic the recessive trait will show up

13. Genotype:AABbxaabb Gametes:

14. Genotype:AABbxaabb Gametes: AB AB Ab Abab ab ab ab

15. Genotype:AABbxaabb Gametes: AB AB Ab Abab ab ab ab Punnett Sq:

16. Genotype:AABbxaabb Gametes: AB AB Ab Abab ab ab ab Punnett Sq: ab ABAaBb AbAabb

17. Genotype:AABbxaabb Gametes: AB AB Ab Abab ab ab ab Punnett Sq:  ALL show the dominant trait for A and HALF show the recessive trait for B. From this, you can deduce the genotype as AA for trait A and Bb for trait B ab ABAaBb AbAabb