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Derived from the Greek meaning ‘yoked’, a zygote is the cell that results from fertilization. It is the union of a spermatozoon and an ovum — the mature germ cells, known also as the male and female gametes (from the Greek for husband and wife). Each of the two gametes is haploid, meaning that the nucleus has half the number of chromosomes of normal body cells. Their union results in the diploid zygote, with a full set of chromosomes, carrying the combination of genes that will determine all the bodily characteristics of the new individual. When, as a result of this union, matched genes (alleles) at particular sites on the newly paired chromosomes are different from each other, the zygote, and hence the resulting individual, is heterozygous with respect to those genes. It is homozygous if the pairs are identical. Since one of a dissimilar pair of genes can dominate the other, whereas identical pairs can act in unison, this is crucial to the suppression or emergence of the relevant inherited trait.

The zygote carries within its single cell continuing threads in the immemorial lifespan of the human race, as well as the mixed-and-matched microscopic material from which will stem the intricacies common to all human bodies, yet with the remarkable uniqueness of a particular person.