Biologists define male and female by one thing: the size of sex cells
In biology, male and female are defined not by chromosomes or anatomy but by gamete size. By convention, organisms producing small sex cells, sperm, are male; those making large ones, eggs, are female. Produce both and you are a hermaphrodite. Many fungi make gametes of equal size, so they have mating types instead of sexes.
Sexual reproduction is found only in eukaryotes. A diploid organism, carrying paired chromosome sets from each parent, makes haploid gametes through meiosis. Along the way, matching chromosomes swap segments in a process called crossover, so every gamete holds a fresh mix of genes. Fertilisation then joins two single sets into a new diploid zygote with half its genetic material from each parent, but a combination that neither parent had.
Gametes need not differ. The green alga Ulva produces look-alike gametes, a condition called isogamy. Most animals show anisogamy, and its extreme form, oogamy, pairs a small swimming cell with a much larger stationary one. A sperm is stripped down for movement, driven by a single long flagellum and lacking much of the machinery an embryo needs. An egg is large, immobile and stocked with the nutrients and components for early development.
How an individual ends up one sex or the other varies across life. Most mammals use the XY system, with males usually XY and females XX. Birds use a ZW system and some insects an XO system, while many reptiles and crustaceans have their sex set by the temperature at which they develop. Some species look nearly identical between the sexes; others, including most birds and mammals, are strongly dimorphic, and mate choice can speed up the divergence.
Mating logistics follow from habitat. Many aquatic animals release eggs and sperm into the water, where fertilisation happens outside the body. On land, most animals fertilise internally so the gametes do not dry out. Most birds have a single rear opening, the cloaca, and transfer sperm by pressing these together, which is called cloacal kissing. In mammals, the embryo develops inside the uterus, fed directly by its mother.
Source: Biological sex
