Crossing over during meiosis increases genetic diversity by swapping DNA between chromosomes. This creates new gene combinations in sperm and eggs, making each one unique and leading to variation in offspring.

Male and female gametes are made differently: males produce many tiny, fast sperm, while females produce fewer, larger eggs. Sperm are designed to reach the egg; eggs are built to support early development.

Gametes must be haploid so that when they join, they form a zygote with the correct chromosome number. If both gametes were diploid, the resulting zygote would have too many chromosomes, causing major problems.

To ensure only one sperm enters the egg, the egg’s outer layer changes as soon as the first sperm gets in. This blocks other sperm, preventing extra genetic material from entering and harming development.

External fertilization happens more often in water because sperm can swim easily and eggs stay moist. On land, eggs would dry out and sperm wouldn’t be able to reach them without special systems.

Plants don’t move, so they rely on wind, water, or animals to carry pollen to other flowers. Once pollen lands, a tube grows to carry sperm to the egg deep inside the plant.

Internal fertilization protects the embryo inside the body but produces fewer offspring. External fertilization creates many offspring outside the body but leaves them more exposed to danger.