Question

Dentinogenesis imperfecta is a rare, autosomal, dominantly inherited disease of the teeth that occurs in about one in 8000 people (Witkop 1957 ). The teeth are somewhat brown in color, and the crowns wear down rapidly. Assume that a male with dentinogenesis imperfecta and no family history of the disease marries a woman with normal teeth. What is the probability that (a) their first child will have dentinogenesis imperfecta? (b) their first two children will have dentinogenesis imperfecta? (c) their first child will be a girl with dentinogenesis imperfecta?

Short answer

Answer: The probabilities are as follows: (a) 50%, (b) 25%, and (c) 25%.

Step-by-step solution

Determine the genotypes of the parents

The father has dentinogenesis imperfecta, so his genotype is Dd. The mother does not have dentinogenesis imperfecta, so her genotype is dd.

Create a Punnett Square

We will create a 2x2 Punnett square to determine the possible genotypes of their children. ``` D d _________ d| Dd | dd | _________ d| Dd | dd | _________ ```

Calculate the probability for (a) their first child will have dentinogenesis imperfecta?

From the Punnett square, we can see that there are 4 possible outcomes, and 2 of them (the two Dd boxes) represent children that will have dentinogenesis imperfecta. So, the probability that their first child will have dentinogenesis imperfecta is 2/4, which simplifies to 1/2 or 50%.

Calculate the probability for (b) their first two children will have dentinogenesis imperfecta?

Since the probability of each child having dentinogenesis imperfecta is independent, we only need to multiply the probability of the first child having the disease (1/2) by the probability of the second child having the disease (1/2). Doing so, we get a probability of (1/2) * (1/2) = 1/4 or 25%.

Calculate the probability for (c) their first child will be a girl with dentinogenesis imperfecta?

In order to answer this question, we need to take into account the sex of the child. The probability of having a girl or a boy is 1/2. So, the probability of having a child with dentinogenesis imperfecta (1/2) must be multiplied by the probability of having a girl (1/2). This yields a probability of (1/2) * (1/2) = 1/4 or 25%.

Key concepts

Dentinogenesis Imperfecta

Dentinogenesis imperfecta is a genetic disorder affecting the development of dentin, the hard tissue beneath the enamel in our teeth. This condition causes teeth to appear discolored, often taking on a brownish hue, and makes them prone to wear and breakage.
What sets dentinogenesis imperfecta apart is its mode of inheritance: it is inherited in an autosomal dominant pattern. This means only one parent needs to pass on the faulty gene associated with this disorder for the child to be affected.
For individuals with dentinogenesis imperfecta, everyday tasks like chewing can become challenging. Regular dental care is crucial for managing the symptoms and preventing further damage to their teeth.

Punnett Square

A Punnett square is a simple graphical tool used in genetics to predict the possible combinations of alleles, or gene variants, during reproduction. It helps visualize the genetic scenarios possible when crossing two individuals.
In the scenario where one parent has dentinogenesis imperfecta (genotype "Dd") and the other has normal teeth (genotype "dd"), a Punnett square can display the probability of their offspring inheriting the condition.
A typical 2x2 Punnett square may look like this, where the rows and columns represent the gametes from each parent:

• Top row with symbols D and d for the affected parent
• Left column with symbol d, as both contributions in the column are from the unaffected parent

From this, you can fill out the grid and find out the potential genotypic outcomes for the children.

Autosomal Dominant Inheritance

Autosomal dominant inheritance is one of the key ways genetic traits and disorders can be passed down through families. In this type of inheritance, the gene responsible is located on one of the 22 non-sex chromosomes, called autosomes.
The "dominant" part means that inheriting just one copy of the altered gene from either parent is enough for the child to express the trait or develop the condition. In our example, dentinogenesis imperfecta is an autosomal dominant disorder, so anyone with at least one "D" allele will exhibit symptoms.
This type of inheritance pattern often leads to the condition appearing in every generation of a family line, as affected individuals have a 50% chance of passing on the disorder to each child. Understanding these patterns helps both in predicting family risk and in providing genetic counseling.