Final answer:
To be genetically identified as a female, a human requires two X chromosomes (XX). The default developmental pathway without the presence of a Y chromosome is female, with conditions like Turner syndrome (XO) showing that even with a single X chromosome, an individual can be phenotypically female.
Step-by-step explanation:
Individuals with a single X chromosome are female, while those with XXY chromosomes are male. In order to be a female, a human requires two X chromosomes (XX). This pairing ensures that females, much like males, have only one functioning copy of the X chromosome in each cell, as one of the X chromosomes in females is inactivated and is known as a Barr body.
Errors in sex chromosome number can lead to conditions such as Turner syndrome, where individuals have a single X chromosome (XO) and still are phenotypically female, although they exhibit certain developmental and health issues. The presence of the SRY gene on the Y chromosome is the key determinant for an embryo to develop into a male. Without it, the default developmental pathway is for the individual to be female.