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.