Two parents are needed in sexual reproduction.
When the male and female gametes combine, they restore the full complement of chromosomes - or diploid number - to produce a human embryo.
The gametes in animals are sperm and eggs.
The gametes in flowering plants are pollen and eggs.
Humans can only reproduce sexually.
The advantages of sexual reproduction are:
Human body cells have 23 pairs of chromosomes in the nucleus.
22 pairs are known as autosomes, and control characteristics only, but one pair carries genes that determine sex - whether offspring are male or female:
These diagrams are known as human karyotypes, and show all the chromosomes, cut out from a micrograph and aligned in pairs.
The blue box shows the two sex chromosomes - these are different sizes, and are an X (larger chromosome) and a Y (smaller one).
The red box shows the two sex chromosomes - these are the same size, both two X, larger chromosomes.
A genetic diagram, like a Punnett square, shows how biological sex is inherited.
The two possible combinations are:
The ratio of female to male offspring is 1:1, ie half of the offspring will be girls and half will be boys ie on average. This can also be converted into a probability of 50% (XX) and 50% (XY).