a mature female or male sex cell, an unfertilized ovum or a sperm cell (spermatazoan), with a single set of chromosomes (haploid) ready to be joined with a gamete of the opposite sex (fertilization); the union of the 2 gametes creates a zygote, a fertilized cell with a full set of paired chromosomes (diploid); in birds, the female is heterogametic, with dissimilar ZW sex chromosomes, the male is homogametic, with similar ZZ sex chromosomes, so the female gamete provides either the male or the female chromosome and thus determines the sex of the zygote—the opposite of the case in mammals