In a normal human zygote, the number of chromosome pairs is:

Concept

In diploid organisms such as humans, the chromosome complement is organised into paired chromosomes, one member inherited from each parent, so the total chromosome number is always twice the pair count. In humans this pair count is 23: 22 pairs are homologous autosome pairs, and the remaining pair is the sex-chromosome pair (XX or XY), giving 46 chromosomes in total.

Working

1. Fertilisation fuses a haploid sperm (23 chromosomes) with a haploid ovum (23 chromosomes).

2. This restores the diploid chromosome number in the resulting zygote: 23 + 23 = 46 chromosomes.

3. Because chromosomes exist as paired chromosomes in a diploid cell, the 46 chromosomes are organised into 46 divided by 2 = 23 pairs.

4. The zygote carries this same diploid set of 23 pairs through every later mitotic division into the somatic cells of the developing individual.

Cross-check

This matches the accepted human karyotype of 46 chromosomes (44 autosomes plus XX or XY), confirming 23 pairs independently of the sperm-plus-ovum addition above.

The number of chromosome pairs in a normal human zygote is 23.