Principles of Inheritance and Variation

In a test cross, an organism showing the dominant phenotype (whose genotype is to be found, e.g., the $F_1$ hybrid) is crossed with the homozygous recessive parent.
For one gene: $Tt \times tt$ gives tall and dwarf in a 1 : 1 ratio.
For two genes: $TtRr \times ttrr$ (double recessive) gives four phenotypes in a 1 : 1 : 1 : 1 ratio.
The ratio of the progeny reveals the genotype of the hybrid and the types of gametes it forms.
A cross between two $F_1$ hybrids is selfing, which gives the $F_2$ generation, not a test cross.
Hence a test cross involves crossing the $F_1$ hybrid with a double recessive genotype.

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (2n = 46).
A normal female has the constitution 44 + XX and a normal male has 44 + XY.
The ovum always carries an X; if it is fertilised by an X-bearing sperm the zygote is XX and develops into a female child.
A zygote with one X and one Y develops into a male; only one X (XO) gives Turner's syndrome, a sterile abnormal female; a zygote with only a Y is not viable.
Hence a zygote with two X chromosomes leads to the birth of a normal female child.