Sexual reproduction involves the fusion of two gametes (sex cells), usually from two different parents, to form a single cell called a zygote. The zygote then divides by mitosis to develop into a new individual. The fusion of gametes is called fertilisation.Gametes are haploid (n): they contain one set of chromosomes. Body cells are diploid (2n), with two sets, one inherited from each parent. When two haploid gametes fuse, the diploid number is restored:n + n = 2n In humans, n = 23 and 2n = 46. Gametes are made by meiosis, the reduction division that halves the chromosome number. Without meiosis, the chromosome number would double in every generation.
Sexual reproduction produces genetic variation, because the offspring inherit a new combination of alleles. Variation comes from meiosis (crossing over and independent assortment) and from random fertilisation, since any gamete from one parent can fuse with any gamete from the other.Advantages: variation means some individuals may survive a change in the environment, such as a new disease, so the species can adapt through natural selection.
Disadvantages: it is slower than asexual reproduction, it needs time and energy to find a mate, and in species with separate sexes only the females can produce offspring.Worked example: a dog has 78 chromosomes in its body cells. Its gametes contain 78 ÷ 2 = 39 chromosomes, and fertilisation restores 39 + 39 = 78.
Sexual reproduction produces genetic variation, because the offspring inherit a new combination of alleles. Variation comes from meiosis (crossing over and independent assortment) and from random fertilisation, since any gamete from one parent can fuse with any gamete from the other.Advantages: variation means some individuals may survive a change in the environment, such as a new disease, so the species can adapt through natural selection.Disadvantages: it is slower than asexual reproduction, it needs time and energy to find a mate, and in species with separate sexes only the females can produce offspring.Worked example: a dog has 78 chromosomes in its body cells. Its gametes contain 78 ÷ 2 = 39 chromosomes, and fertilisation restores 39 + 39 = 78.