Genetics: The Science of Heredity
The Test Cross
When an organism has a trait controlled by a dominant allele, it can either
be a hybrid or a purebred. To find out which, geneticists can use a test cross.
In a test cross, the organism with the trait controlled by a dominant allele is
crossed with an organism with a trait controlled by a recessive allele. If all
offspring have the trait controlled by the dominant allele, then the parent is
probably a purebred. If any offspring has the recessive trait, then the
dominant parent is a hybrid. Study the test cross below, then answer the
H = Short hair
h= Long hair
Answer thefollowing questions on a separate sheet of paper.
1. Is the long-haired cat in the P generation a hybrid or a purebred? Explain
2. Is the short-haired cat in the P generation a hybrid or a purebred? Explain
3. If the short-haired cat in the P generation were purebred, what would you
expect the offspring to look like?
4. Inhorses, the allele for a black coat (B) is dominant over the allele for a
brown coat (b). A cross between a black horse and a brown horse
produces a brown foal. Is the black horse a hybrid or a purebred? Explain.
5. In guinea pigs, the allele for a smooth coat (5) is dominant over the allele
for a rough coat (5). Explain how you could find out whether a guinea pig
with a smooth coat is a hybrid or a purebred.
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