SAT Subject Biology Practice Question 394: Answer and Explanation
4. A researcher has a black guinea pig and wishes to determine if it carries a recessive allele for white hair. Both of the guinea pig's parents are black. Which of the following would be the best method for the researcher to use?
A. Mate the guinea pig with another black guinea pig and look for white offspring
B. Look for white hairs on the guinea pig
C. Mate the guinea pig with a white guinea pig and look for white offspring
D. Observe the chromosomes of a hair cell from a black hair
E. See if the guinea pig has any white siblings
Correct Answer: C
C Whenever you're trying to determine the genotype of an organism that has a dominant phenotype, the best thing to do is something called a test cross. A test cross is a mating between the organism with the unknown genotype and an organism with the recessive phenotype. (Organisms that display the recessive phenotype MUST have the homozygous recessive genotype; the genotype of this organism is known.) Then, you just look at the offspring produced. If the organism with the unknown genotype were homozygous dominant, all the offspring would have the dominant phenotype. If, however, the organism with the unknown genotype were heterozygous, half the offspring would have the dominant phenotype and half would have the recessive phenotype. Look at the two possible Punnett squares:
This is the Punnett square that would result if the genotype of the unknown organism were homozygous dominant. Note that all offspring from this test cross display the dominant phenotype.
This is the Punnett square that would result if the genotype of the unknown organism were heterozygous. Note that half the offspring from this test cross display the dominant phenotype and half display the recessive phenotype.
Knowing this, let's take a look at the answer choices. You should be able to eliminate choice D quickly-all chromosomes look physically alike, regardless of the alleles they carry. Choice B should also be eliminated relatively quickly-just because there are a few white hairs on the guinea pig doesn't mean that it carries the recessive allele. There could be other causes for the occasional white hair. Choice E would give us some information, but not enough to determine if our unknown guinea pig were heterozygous or homozygous. In other words, if the guinea pig did have white siblings, it would tell us that the guinea pig's parents were heterozygous (that's the only way they could produce white offspring), and it tells us that the guinea pig has a chance of carrying the recessive allele for white hair, but it doesn't tell us for sure that it is carrying the recessive allele. It comes down to a choice between answer choice A and answer choice C. Choice C describes a test cross and is the correct answer. The problem with choice A is that you'd be mating your unknown guinea pig with another unknown guinea pig-not a great way to figure something out. The idea is to mate your unknown with a known-then you know what to expect and can use those results to determine the unknown genotype. Note that you should be familiar with a test cross and what it can tell you-and now you are.