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Unit 6C Test Crosses People often want to know what genes an organism has: Genes in horses or dogs for breeding Genes that they may have (for diseases) Genes in plants for breeding successful crop plants. So…how do you know what genes an organism has if you do not know what traits the parent had? Test crosses are used: A test cross is the crossing (mating) of the organism with another organism that you know the genotype of. You then see what percentages of offspring you have. Ex: I cross a black cat with a cat that I know is homozygous black (BB). x Example WW or Ww Ww I have a plant with White Flowers. White is dominant (W). Red is recessive (w). I want to know what the genotype of my plant is. = What two genotypes could it be? WW or Ww I will cross the flower with one that I know the genotype of: Ww If all of the offspring are White, what was the genotype of the original flower? If any offspring are Red (ww), then I know that the original flower was heterozygous. WW WW Ww Ww W w W WW Ww w Ww ww