Video Transcript
Phenylketonuria, PKU, is an inherited disorder caused by a recessive allele, lowercase r, located on chromosome 12 in humans. A male and a female who are both heterozygous for the disorder with the genotype capital R lowercase r want to conceive a child. What is the probability a child of theirs will inherit the disorder PKU? (A) Zero percent, (B) 25 percent, (C) 50 percent, (D) 75 percent, or (E) 100 percent.
Let’s first clarify some of the key terms we see in this question. First, we’re told this disease is caused by a recessive allele. A recessive allele is only expressed in the phenotype of an individual if a dominant allele is not present. Since humans are diploid and will have two copies of every gene located on autosomes like chromosome 12, that means an individual with PKU must have two recessive alleles for this gene in order to show the recessive phenotype. So the genotype of an individual with PKU must be lowercase r lowercase r.
We’re told that the potential parents in this situation are heterozygous for the R gene, meaning that they have the genotype capital R lowercase r. Because their genotype contains the capital R or dominant allele, that means neither of these parents have PKU. However, they both carry it via the recessive allele in their genotype.
In the question, we’re asked about the probability that a child will inherit a particular trait. The tool that we use to determine the probabilities of different genotypes and phenotypes in the offspring is a Punnett square. So let’s create a Punnett square for this situation. Since both parents have the same genotype, we can write the same genotype across the top and down the side of the Punnett square. Now our Punnett square is set up and ready to fill out. We fill out a Punnett square by combining the letter of the allele in the column header with the letter of the allele in the row header. So the genotype in the first square of our Punnett square will be capital R capital R. And we can likewise fill out the remaining boxes in the Punnett square.
The convention when writing out genotypes that contain both a dominant and a recessive allele is to always put the dominant allele first. So these are the genotypes of the possible offspring of this couple. Now we need to look specifically for the PKU genotype, lowercase r lowercase r, in our Punnett square. We can see that only one out of the four boxes in our Punnett square contains this genotype. That means that there is a one out of four, or one-quarter, chance that a child of this couple will inherit the disorder PKU.
All of our answer choices, however, are given in percentage form rather than fraction. To convert from a fraction to a percentage, we can do this quick calculation, or we can simply recognize that a quarter is 25 percent. Either way, we should come to the conclusion that the probability that a child of this couple will inherit the disorder PKU is (B) 25 percent.
