Explain the pair of chromosomes and Sex Chromosomes?

Morgan also noticed that one pair of chromosomes is different in male and female flies, one of the male chromosomes being shorter and hook-shaped. He called the larger chromosomes X-chromosomes, and the shorter, hooked, homologous chromosomes Y-chromosomes. He suggested that the fly's sex is determined by whether the homologous pair is XX, for females, or XY, for males. Exceptions exist in birds, where the pattern is reversed--females are XY and males are XX. In some insects, where males are XO and females are XX.

In meiosis in most organisms, since males are XY, male gametes receive either an X or a Y chromosome. Since female gametes can receive only X sex chromosomes, when male and female gametes fuse, the probability is that half of the offspring will be male, and half of the offspring will be female.

Autosomes, or non-sex chromosomes, are designated by the letter A, and the letter O is used when the chromosome is missing. Humans have 22 pairs of autosomes and one pair of sex chromosomes.