Caused by A to G change in a transfer RNA gene. Heteroplasmy with normal mito necessary. X and Y chromosomes. Genes on X usually on X in all other mammals. Each cell inactivates one X at random.
Descendant cells keep same X inactive.
Border of PAR1 occurs within the Xg blood group gene. A gene on the Y that causes embryonic gonad to become testes. In absence of TDF, gonad becomes ovary. First, XY females were found that had Yp only, so TDF must be on Yp. Gene SRY is right next to PAR boundary, has structure of DNA binding protein, expressed in embryonic gonad.