Development from a diploid zygote proceeds by mitosis, which conserves the full chromosome complement (2N) in every daughter cell, and by differentiation, which partitions those genetically identical cells into two lineages: somatic (body) cells, which build the tissues and organs of the individual, and germ cells, which populate the gonads and undergo meiosis to produce haploid gametes. The distinction is defined by heritability rather than by function — because only the germ line contributes DNA to the next generation, a mutation arising in a somatic cell can affect the individual's own viability but cannot be transmitted to offspring, whereas a mutation in a germ-line cell can be, subject to the further filter of whether the affected gamete succeeds in fertilization. This concept belongs to human genetics and developmental biology and supplies the causal link between cell division, inheritance, and the level at which natural selection operates.
Development from a diploid zygote proceeds by mitosis, which conserves the full chromosome complement (2N) in every daughter cell, and by differentiation, which partitions those genetically identical…