XXY is a rare condition, but it happens. The story says the child chooses. How can an infant child choose?
Granted, it’s wiki but...
Klinefelter syndrome or Klinefelter’s syndrome, is the symptoms of a disease caused by additional X genetic material in males. Also known as 47,XXY or XXY , is a genetic disorder in which there is at least one extra X chromosome to a standard human male karyotype, for a total of 47 chromosomes rather than the 46 found in genetically normal humans.[1] While females have an XX chromosomal makeup, and males an XY, individuals with Klinefelter syndrome have at least two X chromosomes and at least one Y chromosome.[2] Because of the extra chromosome, individuals with the condition are usually referred to as “XXY males”, or “47,XXY males”.[3]
Still base male.
The choice is deferred. I don't know to what age.