Meiosis matters for two main reasons: it keeps the chromosome number constant from one generation to the next, and it produces genetic variation.1. Keeping the chromosome number constant. Meiosis halves the chromosome number, producing haploid gametes. At fertilisation two haploid gametes fuse, restoring the diploid number. Without meiosis, the chromosome number would double every generation: human gametes would have 46 chromosomes, zygotes 92, and the next generation's zygotes 184.2. Producing genetic variation. Meiosis makes gametes that are genetically different from each other in two ways:
• Crossing over in prophase I exchanges sections between non-sister chromatids of homologous chromosomes, producing new combinations of alleles on each chromosome.
• Independent assortment in metaphase I: each homologous pair lines up randomly, so each gamete receives a random mix of maternal and paternal chromosomes. In humans this alone gives 223 (over 8 million) combinations.Variation is increased further by random fertilisation: any of the male's gametes can fuse with any of the female's. Ignoring crossing over, two human parents could produce 223 × 223, about 7 × 1013, different combinations of chromosomes in a zygote.Why variation matters: in a genetically varied population, some individuals are likely to have alleles that help them survive changes such as new diseases, predators or a changing climate. They survive and reproduce, passing on those alleles. Genetic variation is therefore the raw material for natural selection and evolution.
• Crossing over in prophase I exchanges sections between non-sister chromatids of homologous chromosomes, producing new combinations of alleles on each chromosome.
• Independent assortment in metaphase I: each homologous pair lines up randomly, so each gamete receives a random mix of maternal and paternal chromosomes. In humans this alone gives 223 (over 8 million) combinations.Variation is increased further by random fertilisation: any of the male's gametes can fuse with any of the female's. Ignoring crossing over, two human parents could produce 223 × 223, about 7 × 1013, different combinations of chromosomes in a zygote.Why variation matters: in a genetically varied population, some individuals are likely to have alleles that help them survive changes such as new diseases, predators or a changing climate. They survive and reproduce, passing on those alleles. Genetic variation is therefore the raw material for natural selection and evolution.