Human Zygote Chromosome Number
A human zygote contains 46 chromosomes (Option C). This represents the diploid number resulting from the fusion of two haploid gametes—23 chromosomes from the egg and 23 from the sperm 1, 2.
Fundamental Chromosomal Biology
- Normal human cells contain 46 chromosomes, a discovery made in 1956 by Tjio and Levan that established the foundation for modern human cytogenetics 1, 2
- The zygote forms when two gametes, each carrying 23 chromosomes (haploid number), fuse to create a diploid organism with 46 chromosomes 3
- Females typically present with two X chromosomes (46,XX), while males typically present with one X and one Y chromosome (46,XY) 4
Chromosomal Composition Details
- The 46 chromosomes consist of 22 pairs of autosomes plus 2 sex chromosomes 4
- This diploid state (2n = 46) is established at fertilization and maintained throughout somatic cell divisions 3
- Paternal chromosomes (from sperm) and maternal chromosomes (from egg) remain distinct initially, with RanGTP and F-actin dynamics keeping them separated during the completion of maternal meiosis 5
Clinical Context and Variations
- Chromosomal abnormalities are common in early human embryos, with approximately 52% of Day 3 embryos showing aneuploidy (abnormal chromosome numbers) 3
- Karyotype abnormalities represent the most common known genetic abnormalities causing developmental issues, including numerical anomalies like Klinefelter syndrome (47,XXY) or structural abnormalities 4
- Variation from the normal 46-chromosome complement includes:
Important Caveats
- While 46 is the normal diploid number, human embryos display high chromosomal complexity, with many early embryos showing mosaicism (mixture of normal and abnormal cells) that may resolve during development 3
- The correct chromosome number was misidentified as 48 for over 30 years before the accurate determination in 1955-1956, highlighting how technological limitations can perpetuate errors 2