Question

You are studying Alexander disease. All previous pedigree data indicates that this disease is an autosomal dominant trait. However you notice that in one instance the trait skips a generation. If Alexander disease is autosomal dominant, what is a likely explanation as to why it skipped a generation this one time?

Answer 1

If Alexander disease, an autosomal dominant trait, appears to skip a generation, reduced penetrance or a new mutation could explain this anomaly. It indicates that an individual may carry the dominant allele without expressing the trait or it could be a late onset, making it undiagnosed in the skipped generation.

Step-by-step explanation:

If Alexander disease is an autosomal dominant trait but appears to skip a generation in a pedigree, a likely explanation is reduced penetrance of the allele responsible for the disease. This means that not all individuals carrying the dominant allele will express the trait.

So, in this instance, it's possible that a person inherited the gene but did not express the disease, and then passed it on to the next generation where it became apparent. Another explanation might be a new mutation that arose in an individual of the later generation, or the disease was not diagnosed in the skipped generation due to its late onset nature.

Autosomal dominant traits usually show a vertical pattern of inheritance, with each affected individual having an affected parent, and having a 50% chance of passing on the trait to offspring if they are heterozygous. If a person is homozygous dominant for the trait, all offspring will inherit the disease.