Gametes must be haploid because they play a crucial role in sexual reproduction. In humans and most other organisms, gametes are the reproductive cells—sperm in males and eggs in females. Each gamete carries only one set of chromosomes, which is essential for maintaining the correct chromosome number in the offspring.
During fertilization, when a sperm cell fuses with an egg cell, the two haploid gametes combine to form a diploid zygote. This zygote then undergoes multiple cell divisions and eventually develops into a new organism. If gametes were diploid instead of haploid, the resulting zygote would have an incorrect number of chromosomes, leading to developmental issues or failure to develop altogether.
Moreover, being haploid allows for genetic diversity through the process of meiosis, where homologous chromosomes are separated and recombined. This genetic variation is vital for evolution and adaptation in changing environments. In summary, haploid gametes ensure that organisms maintain the correct chromosome number across generations and contribute to genetic diversity.