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.
One red and 1-3 green genes on the X, in tandem array. Blue on an autosome. Unequal crossing over can lead to loss of red or green. Sometimes a C-O between a red and a green leads to a hybrid gene with anomalous color vision. Border of PAR1 occurs within the Xg blood group gene.