Sexual Reproduction
Sexual reproduction is reproduction in which two parents contribute gametes that fuse, producing offspring with a new combination of genes.
Definition
Sexual reproduction is when two parents make a new living thing together, each giving half of the instructions, so the offspring is a mix of both and not an exact copy of either. The two parents produce haploid gametes by meiosis, and a male gamete (sperm) fuses with a female gamete (egg) in fertilization to form a diploid zygote that inherits one set of chromosomes from each parent and is genetically unique; most animals and plants reproduce this way, and in flowering plants pollen carries the sperm to the egg inside the flower. The cycle alternates meiosis, which halves the chromosome number and recombines alleles, with syngamy, which restores diploidy by fusing gametes from two individuals. Variation arises from crossing over, independent assortment, and random fertilization, producing offspring with novel genotypes on which selection can act. Explanations for why sex is so common despite the twofold cost of producing males include faster adaptation, the Red Queen hypothesis (staying ahead of coevolving parasites), and the ability to purge harmful mutations by recombining them out of the lineage.
Example
A puppy gets half its DNA from its mother and half from its father, which is why it might have its mother's coat color and its father's ears. A human egg with $23$ chromosomes joins a sperm with $23$ to form a zygote with $46$, and because each parent can produce millions of genetically different gametes, the chance of two siblings other than identical twins sharing the same DNA is essentially zero. Combining $2^{23}$ possible chromosome assortments from each parent gives about $7 \times 10^{13}$ possible zygote combinations from a single couple before crossing over is even counted, and anisogamy, the difference in size between a large egg and a tiny sperm, is the defining basis of what biologists call male and female.
Key Insight
Sexual reproduction makes every offspring different, which gives a group a better chance that some members will survive if the environment changes. It costs more time and energy than asexual reproduction, but the variation it creates is the raw material for adaptation and evolution. Sex does not create new alleles, since mutation does that; it creates new combinations, which matters because the fitness of an allele often depends on the alleles it is paired with.