The combination of sex chromosomes with autosomes determines whether the foetus will be a boy or a girl. The foetus develops from the zygote which is formed by the fusion of the two gametes, the male gamete or sperm and the female gamete or egg. Gametes are haploid while the zygote is diploid.
Ova or eggs are of one kind only. These contain 22 autosomes and a single X chromosome. Sperms are of two kinds (i) having 22 autosomes and one X chromosome, or (ii) having 22 autosomes and a Y chromosome.
When X bearing sperm fuses with the egg, a female child results with 44 autosomes and two X chromosomes. If Y bearing sperm fuses with the egg then a male child results with chromosomal constitution of 44 autosomes and one X and one Y chromosome.
At metaphase, chromosomes are clearly seen lying at the equator and may be studied under the microscope or by taking a photograph. You can then easily identify and state that the chromosomes in a male human are 44 autosomes +XY and those in a female are 44 autosomes+ XX.