Test Cross vs Hybrid Cross
What is the difference between test cross and dihybrid cross?
When you look at someone with a dominant trait, you can't tell whether they are homozygous
or heterozygous. For instance, if someone has brown eyes, they could be BB or Bb. Remember
that Mendel always started his crosses with a purebred (homozygous) P generation. How did
he know his dominant parent was homozygous? He developed what is known as a test cross. He
took a recessive (in our case blue eyes, or bb) and mated it with a pea plant showing the
dominant trait. If in the F1 generation the offspring were all dominant, then the dominant
parent was probably BB. If there were any recessives in the F1, then the dominant parent
had to be Bb. A dihybrid cross is following the inheritance of two traits simultaneously.
For example, Mendel wanted to know what the probability was that peas could be both yellow
and smooth or green and wrinkled. So those genotypes would be written YYSS and yyss.
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Update: June 2012