As in monohybrid crosses, you can’t tell by looking at the dominant trait whether it is homozygous or heterozygous. 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. - 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 - 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. Genotype: Gametes: AaBb x aabb Genotype: AaBb x Gametes: AB Ab aB ab aabb ab Genotype: AaBb x Gametes: AB Ab aB ab Punnett Square: aabb ab Genotype: AaBb x Gametes: AB Ab aB ab Punnett Square: ab AB AaBb Ab Aabb aB aaBb ab aabb aabb ab Now, you have an organism that is homozygous dominant for one trait, but heterozygous for the other – the test cross/back cross still works. - 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 - - 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 Genotype: Gametes: AABb x aabb Genotype: Gametes: AABb x AB AB Ab Ab aabb ab ab ab ab Genotype: Gametes: Punnett Sq: AABb x AB AB Ab Ab aabb ab ab ab ab Genotype: Gametes: AABb x AB AB Ab Ab Punnett Sq: ab AB AaBb Ab Aabb aabb ab ab ab ab Genotype: Gametes: AABb AB AB Ab Ab x aabb ab ab ab ab Punnett Sq: ab AB AaBb Ab Aabb 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