If I handed you a black guinea pig & asked, “What’s its phenotype for fur color?” You would gently hold the guinea pig,  look at it and reply, “Black, you dummy … all you gotta do is look at it”.  And I would say, “Correct, & please don’t call me dummy”.

If I handed you the same guinea pig & asked, “What’s the genotype of this guinea pig with respect to its fur color?”  You wouldn’t be able to tell me, and I wouldn’t be able to tell you either.  The reason we don’t know is because there are two genotypes that BOTH produce a dominant trait phenotype, homozygous dominant (BB) & heterozygous (Bb), & we can’t see the actual alleles (letters) without serious scientific chromosomal-type analysis — and that’s assuming that a Guinea Pig Genome Project has been completed for us to refer to, & I don’t think it has.
So how do we figure it out?  We perform a TEST CROSS !

What is it?

Test cross = the cross of an organism with an unknown dominant genotype with an organism that is homozygous recessive for that trait

What does it do?

A test cross can determine whether the individual being tested is homozygous dominant (pure bred) or heterozygous dominant (hybrid).


To perform an actual test cross with our black guinea pig, we would need a guinea pig (of the opposite sex) that is homozygous recessive (“bb”).  In other words, we would need a white guinea pig to mate with our black guinea pig.  We would give them a little privacy, hope that the female becomes pregnant, wait for however long the gestation period of a guinea pig is, & THEN we would look at the offspring. If any of the offspring from a test cross have the recessive trait, the genotype of the parent with the dominant trait must be heterozygous The reliability of a test cross increases with the number of offspring produced.

“Key Points” to remember about a TEST CROSS:

1. the organism with the dominant trait is always crossed with an organism with the recessive trait
2. if ANY offspring show the recessive trait, the unknown genotype is heterozygous
3. if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant

4. large numbers of offspring are needed for reliable results

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6 Comments on "What is a Test Cross: Why is it used (Biology)"

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its good and will help with my exam!

Madina S.



I understood in a better way..which I never couldnt understand…& today I understood..after reading this page..

Lina S.

Perfect ..much thanks

Asmamaw Amogne

It’s good. But, too short


i will take the test at home