USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Germline mosaicism refers to a situation where a person has two or more genetically different cell lines in their germ cells (sperm or eggs) as a result of spontaneous mutations that occur during early embryonic development. These mutations may arise in the germline cells of the embryo and can lead to a mix of normal and mutated alleles.

Spontaneous mutations are changes in the DNA sequence that occur naturally without any external influence. They can result from errors in DNA replication or repair processes, as well as environmental factors. In the context of germline mosaicism, these spontaneous mutations can create a scenario where only some of the gametes (egg or sperm) carry the mutation while others do not. This can lead to offspring being affected by a genetic condition even if one parent does not have the condition or show symptoms.

Understanding germline mosaicism is crucial because it impacts inheritance patterns and the likelihood of offspring inheriting a genetic disorder. This phenomenon also highlights the complexity of genetic transmission and the importance of thorough family histories in genetic counseling.