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Question Video: Calculating the Probability of an Individual Inheriting Fragile X Syndrome Biology

Fragile X syndrome is an X-linked dominant condition in humans that causes developmental problems. The allele that causes fragile X syndrome is dominant (R), and the allele that does not is recessive (r). A female with the genotype X^r X^r reproduces with a male with the genotype X^R Y. What is the probability, in percent, that the offspring will be male with fragile X syndrome?

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Video Transcript

Fragile X syndrome is an X-linked dominant condition in humans that causes developmental problems. The allele that causes fragile X syndrome is dominant, uppercase R, and the allele that does not is recessive, lowercase r. A female with the genotype X lowercase r X lowercase r reproduces with a male with the genotype X uppercase R Y. What is the probability, in percent, that the offspring will be male with fragile X syndrome?

Before we answer this question, let’s take a look at the information that has been provided to us in the question about fragile X syndrome.

Fragile X syndrome is an X-linked dominant condition. This means the allele for fragile X syndrome is carried on the X chromosome. You may recall that the X chromosome is one of the two sex chromosomes that can be found in humans: X and Y. While biological females tend to have two X chromosomes, biological males have one X chromosome and one Y chromosome. As the allele for fragile X syndrome is dominant, a person, male or female, only needs one dominant allele present in their genotype to develop the disease. In fact, a biological female having both dominant alleles is incredibly rare.

When we are asked to predict the probability of an offspring inheriting a particular phenotype, we can use a Punnett square. A Punnett square is an incredibly useful visual tool for modeling inheritance. First, let’s complete the row and column heads with the genotype of the parents. We can see that the father has an uppercase R on his X chromosome, so has the dominant allele that causes fragile X syndrome. The mother, however, doesn’t.

Each cell of the Punnett square is completed by taking one allele from the row head and one from the column head. This shows how a child will inherit one chromosome in each pair from their biological father and one from their biological mother. For this cell, we have two X chromosomes, one with a dominant allele and one with the recessive allele. Let’s fill in the rest of the Punnett Square. Now, we have all the possible genotypes for a single offspring born to these parents, but what do they tell us?

There is a 50 percent chance that the offspring produced by these parents will have the genotype X uppercase R X lowercase r. They have two X chromosomes, so will be biologically female. As they have one dominant allele for fragile X syndrome, they will most likely have this syndrome. Alternatively, there is a 50 percent chance that the offspring will have the genotype X lowercase r Y and so will be biologically male. They have no dominant allele for fragile X syndrome, so they will not inherit it.

Looking back at our Punnett square, we now have enough information to answer our question. The probability that the offspring born to these parents is a biological male with fragile X syndrome is zero percent.

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