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Question Video: Completing a Punnett Square to Show Inheritance of a Dominant Genetic Disease Biology

Polydactyly is an inherited disorder, and in most cases, it is caused by a dominant allele (D). Use this partially completed Punnett square, which shows the genotypes of a male and female, to predict the probability, in percent, that a child born to these parents would inherit polydactyly.

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Video Transcript

Polydactyly is an inherited disorder, and in most cases, it is caused by a dominant allele, uppercase D. Use this partially completed Punnett square, which shows the genotypes of a male and female, to predict the probability, in percent, that a child born to these parents would inherit polydactyly.

Polydactyly is a condition where an individual will have more fingers or toes than the typical five. This is the result of a dominant allele, which is indicated as uppercase D in our question. Since we are not provided information otherwise, we can assume that this is a simple dominant trait, meaning the other version of the trait, that is, the typical hand, is the result of a recessive, or lowercase d, allele for this gene.

A dominant allele is an allele that is always expressed in the phenotype if it is present in the genotype. Therefore, the genotypes that could produce polydactyly are uppercase D uppercase D or uppercase D lowercase d. Basically, an individual only needs to have one dominant allele in order to express polydactyly. The genotype for the typical number of fingers or toes would be lowercase d lowercase d. The individual could only express this phenotype with two recessive alleles.

We can see by the father’s genotype in the Punnett square, uppercase D lowercase d, that he must have polydactyly, since he has a dominant allele. The mother, on the other hand, does not have a dominant allele, so she must have the typical number of fingers and toes.

Now, let’s see what characteristics their offspring might have by filling out the Punnett square. To fill out a Punnett square, we should combine the allele from the row header with the allele from the column header. Together, these make a possible genotype that may be found in the offspring of these parents. If we do the same for the remaining squares in the grid, we can complete the Punnett square.

We can now see that about two out of four or half of the offspring will have the genotype uppercase D lowercase d. We can tell this because two out of the four squares in the Punnett square have this genotype. The other two out of four or about half of the offspring will have the genotype lowercase d lowercase d. The offspring with two recessive alleles will express the typical number of fingers and toes. However, the offspring with the uppercase D lowercase d genotype will express polydactyly.

The question asks us about the probability that a child of these parents will inherit polydactyly. The proportion of offspring that we see in a Punnett square with a particular genotype corresponds to the probability of each child of those parents having that particular genotype. We found that half of the squares in the Punnett square produced a genotype that would express polydactyly. Therefore, the probability of a child of these parents inheriting a genotype that would express polydactyly is about half.

However, we are asked for the probability in percent rather than in a fraction. So, we can do this quick calculation to convert fraction to percentage, or we can simply recognize that half of 100 percent is 50 percent. The probability in percent that a child born to these parents would inherit polydactyly is 50 percent.

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