Normal human zygotes all have 46 chromosomes organized into 23 pairs, which is the same number that most adult humans have. Together, these 46 chromosomes contain an estimated 25,000 genes, according to the National Human Genome Research Institute. Sometimes, human embryos develop with an extra chromosome that influences the development of Down syndrome.
NHGRI points to a distinction between the first 22 pairs of chromosomes, or autosomes, and the last pair called "sex chromosomes." Every human has a full set of autosomes from conception and therefore carries them throughout the zygote phase of gesticulation. Some people are conceived with one fewer sex chromosome. This condition, known as Turner syndrome, always results in a female zygote, as the missing chromosome is always the male, or "Y," chromosome.
Numerical abnormalities such as these are not present in the majority of developing zygotes, however, and the majority of humans carry the normal complement of 46. Interestingly, this number is two fewer than the complement carried by most apes. The Department of Energy's Office of Science describes the way the human chromosome number two seems to be arranged as if it resulted from the fusion of an ancestral pair of chromosomes, which it identifies as being the most parsimonious explanation for the difference in chromosome count between humans and apes.