of the Y chromosome in every cell, e.g., XYYY) are considerably more rare. The extra genetic material in these cases can lead to skeletal abnormalities, dental abnormalities, decreased IQ, delayed development, and respiratory issues, but the severity features of these conditions are variable. XX male syndrome XX male syndrome occurs when there has been a recombination in the formation of the male gametes, causing the SRY portion of the Y chromosome to move to the X chromosome. When such an X chromosome contributes to the child, the development will lead to a male, because of the SRY gene. Genetic genealogy In human genetic genealogy (the application of genetics to traditional genealogy), use of the information contained in the Y chromosome is of particular interest because, unlike other chromosomes, the Y chromosome is passed exclusively from father to son, on the patrilineal line. Mitochondrial DNA, maternally inherited to both sons and daughters, is used in an analogous way to trace the matrilineal line. Brain function Research is currently investigating whether male-pattern neural development is a direct consequence of Y-chromosome-related gene expression or an indirect result of Y-chromosome-related androgenic hormone production. Microchimerism The presence of male chromosomes in fetal cells in the blood circulation of women was discovered in 1974. In 1996, it was found that male fetal progenitor cells could persist postpartum in the maternal blood stream for as long as 27 years. A 2004 study at the Fred Hutchinson Cancer Research Center, Seattle, investigated the origin of male chromosomes found in the peripheral blood of women who had not had male progeny. A total of 120 subjects (women who had never had sons) were investigated, and it was found that 21% of them had male DNA. The subjects were categorised into four groups based on their case histories: Group A (8%) had had only female progeny. Patients in Group B (22%) had a history of one or more miscarriages. Patients Group C (57%) had their pregnancies medically terminated. Group D (10%) had never been pregnant before. The study noted that 10% of the women had never been pregnant before, raising the question of where the Y chromosomes in their blood could have come from. The study suggests that possible reasons for occurrence of male chromosome microchimerism could be one of the following: miscarriages, pregnancies, vanished male twin, possibly from sexual intercourse. A 2012 study at the same institute has detected