Video Transcript
The allele for freckles in humans is dominant and represented by the letter F. What is the probability that a couple who are both heterozygous for the allele will have a child with freckles? Use a Punnett square to help you.
This question asks us to cross two parents, each heterozygous, and determine the probability that their offspring will express the dominant freckle trait. First, it’s helpful to review some of the details presented in the question. The allele for freckles is dominant and represented by the capital letter F. The fact that this trait is dominant means that even if an offspring only inherits one allele, they will express the freckles trait because a dominant allele is always expressed. The set of alleles a person possesses for one characteristic is called their genotype.
The question has told us that both parents in this couple are heterozygous for the freckles characteristic. This means that they have one dominant allele and one recessive allele that they inherited from their parents. So their genotype will be represented by one uppercase F for the dominant allele and one lowercase f for the recessive. This also means that both these heterozygous parents have freckles since they both possess one dominant allele. In total, there are three different genotypes possible for the freckles characteristic.
A person could be homozygous for the dominant trait. Homozygous means that they have two alleles that are the same. In this case, their genotype would be represented by two uppercase Fs. A person with this genotype would also have freckles. Alternatively, a person could be homozygous for the recessive trait. In this case, their genotype would be represented with two lowercase f’s. A person with this genotype would not have freckles. Now, we’re ready to solve the problem.
The question has instructed us to use a Punnett square to help calculate our answer. A Punnett square is a tool used to predict the genotypes of the offspring from a particular cross, in this case, two parents who are heterozygous for freckles. We’ll write one parent’s alleles at the top of the square and the other parent’s alleles on the side. Remember that both parents are heterozygous.
Next, we fill in the cells with all the possible genotypes of the offspring for this cross. The offspring can only inherit one allele from each parent. So you can see in the first cell that offspring inherited one dominant allele from one parent and a second dominant allele from the other. But in the cell beneath it, they inherited a dominant allele from one parent and a recessive allele from the second. The same is true in each of the cells. Each offspring inherits one allele from each of their parents.
The question asks about the probability of a child being born with freckles. Since we already know which genotypes will produce offspring with freckles, we can see that three of the four potential offspring have a genotype that would produce freckles. So there’s a 75 percent probability that a couple who are both heterozygous for the allele for freckles will have a child who has freckles.
