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Germline mosaicism, also known as gonadal mosaicism, is a condition in which the precursor (germline) cells to ova and spermatazoa are a mixture (mosaic) of two or more genetically different cell lines. This mosaicism originates within the host, as it’s not introduced by an external source (such as transplantation). It occurs when a germline cell develops mutations in its genome, differentiates from its precursor cell and propagates to create a new cell line. Depending on when the mosaicism occurred will dictate the degree of how many cells will be affected. Germline mosacism is thought to occur more frequently than anticipated, as it is difficult to detect.

Mosaicism in gametes
After fertilization, germline cells will differentiate separately from somatic cells in the developing embryo. As the embryo matures into a fetus, the germline cells will continue to replicate. Females are born with all of their eggs yet part of egg development occurs during puberty. Males develop sperm cells in adolescence, during puberty. Regardless of these time lines, mutations affecting germline cells can arise at any moment. Once these mutated cells replicate, mosaicism can occur in the gametes. The time when this cell differentiation occurred will affect the amount of mosaicism in the gametes. The earlier the differentiation, the more that will be affected.

Attributed diseases
Children born from a mosaic germline cell will acquire different DNA than the parents. The new mutations acquired, also known as de novo mutations, could lead to disease in the children. Blood screening in the parents will not produce any genetic link, as mutations in the germline do not affect the rest of the body. Germline mosaicism is an explanation why parents without autosomal dominant diseases can give rise to children with them.

Duchenne and Becker muscular dystrophy
These two X-linked diseases are known to be linked to germline mosaicism. In a study of non-carrier families conducted by Helderman-van den Enden et al, they describe a predicted 14-20% reoccurrence risk for females to transmit these disorders again, along with the disease haplotype. Most of these germline mutations arise in females.

Down Syndrome
In a study by Delhanty, JD, germline mosaicism was linked to 5% of Down Syndrome cases. In addition to Down Syndrome's trisomy 21, trisomy 13 can also be linked to germline mosaicism. These resultant aneuploidies are more connect to younger women.

Infertility in males
A study of semen samples from 100 men found low-level germline mosaicism in one third of infertile men, with increased incidence with advancing paternal age.