AP Syllabus focus:
‘Fusion of two haploid gametes restores the diploid chromosome number and creates new combinations of alleles in zygotes.’
Fertilization is the key genetic “reset” between generations: it returns cells to a diploid state and, by combining two independently produced gametes, generates offspring with new allele combinations across the genome.
Core idea: fertilization reshuffles alleles while restoring diploidy
Sexually reproducing organisms alternate between haploid (n) gametes and diploid (2n) body cells.
This diagram summarizes the sexual life cycle by tracking chromosome number through meiosis (2n → n) and fertilization (n + n → 2n). It visually anchors fertilization as the event that restores diploidy and initiates the next diploid stage (zygote) in the cycle. Source
Practice Questions
FAQ
Polyspermy prevention ensures only one sperm nucleus contributes chromosomes to the egg.
If multiple sperm fused, the zygote would become polyploid/aneuploid, often disrupting development and obscuring typical diploid allele pairings.
In angiosperms, one sperm nucleus fuses with the egg nucleus to form the diploid zygote.
A second sperm nucleus fuses with the central cell to form endosperm; genetically, this is separate from zygote allele combination.
No. Fertilisation combines existing alleles into a single nucleus; it does not mutate or edit DNA.
Any new DNA sequence changes would come from replication errors or mutagens, not the fusion event itself.
Sex chromosomes can change which alleles are paired because an egg typically contributes one sex chromosome, while sperm may contribute different sex chromosomes.
This affects whether certain loci are present in one copy or two in the zygote, depending on the species’ sex determination system.
Each zygote forms from a different pairing of gametes.
Because each parent produces many genetically distinct gametes, random fusion means each sibling can inherit a different set of parental alleles across the genome.
