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
Fertilization links these stages by uniting genetic material from two parents into one new individual.
What fertilization accomplishes
Restores diploid chromosome number: two haploid nuclei combine to re-form 2n.
Creates new combinations of alleles: maternal and paternal versions of genes come together in the same nucleus, forming a genotype not identical to either parent.
Key terms for AP Biology
A clear understanding of chromosome sets and allele combinations helps you predict genotypes across generations.
Fertilization: The fusion of two haploid gametes (or their nuclei) to form a diploid cell, combining genetic material from two parents.
Fertilization is often described as the union of sperm and egg in animals, but the same genetic principle applies in plants (sperm nuclei delivered by pollen to an egg in the ovule).
This labeled schematic shows key stages of sperm–egg interaction culminating in a zygote. It helps connect the term “fusion of two haploid gametes” to a concrete cellular sequence, emphasizing that fertilization produces the first diploid cell of the new organism. Source
Zygote: The diploid cell produced by fertilization; it is the first cell of a new organism and contains one complete set of chromosomes from each parent.
A zygote’s genome includes homologous chromosome pairs, with one homolog inherited from each parent.
How fertilization restores the diploid chromosome number
In general, gametes are produced by meiosis and contain one chromosome from each homologous pair. During fertilization, the two haploid chromosome sets are brought together.
This life-cycle diagram traces how meiosis produces haploid gametes and how fertilization restores the diploid (2n) condition in the zygote. It also connects fertilization to inheritance across generations by showing the zygote undergoing mitosis to form a multicellular diploid organism. Source
Chromosome-level view
Each gamete contains n chromosomes (one set).
After fertilization, the zygote contains 2n chromosomes (two sets).
For each chromosome type (e.g., chromosome 1), the zygote has:
one maternal homolog (from the egg)
one paternal homolog (from the sperm)
This restoration is essential for maintaining a species’ chromosome number across generations; without it, chromosome number would halve each generation.
How fertilization creates new allele combinations
Fertilization does more than restore 2n; it produces a new genetic combination because each parent contributes one allele per gene (for genes on autosomes and for many genes on sex chromosomes, depending on sex and species).
Alleles combine at the same loci
For a gene with two possible alleles (e.g., A and a):
A haploid gamete carries one allele at that gene’s locus.
The zygote carries two alleles at that locus (one from each parent), producing:
homozygous combinations (AA or aa), or
heterozygous combinations (Aa)
These combinations matter because allele pairs influence phenotype through dominance relationships and gene expression patterns.
Allele: An alternative version of a gene found at the same locus on homologous chromosomes.
Random fertilization multiplies genetic possibilities
Even if you ignore how gametes became genetically distinct, fertilization itself is a random event: any one sperm (or pollen sperm nucleus) can fuse with any one egg. This randomness increases the number of possible allele combinations in offspring because:
each parent produces many genetically different gametes
gametes meet and fuse by chance
In other words, the zygote genotype reflects which two gametes happened to fuse, not a predetermined matching of maternal and paternal alleles.
Genetic consequences in the zygote
Once formed, the zygote’s genotype becomes the genetic template for development.
What is fixed at fertilization
The full diploid set of chromosomes for the new organism
The organism’s unique combination of alleles across loci
In species with sex chromosomes, the sex chromosome combination contributed by the two gametes (mechanism varies by organism)
What fertilization does not do
It does not “blend” alleles into an average; alleles remain discrete units at loci on chromosomes.
It does not selectively choose “best” alleles; allele pairing is a consequence of which gametes fuse.
Connecting fertilization to inheritance patterns
Fertilization is the step that makes Mendelian inheritance observable in offspring because it:
pairs alleles at each locus (one from each parent)
produces predictable genotype frequencies when combined with probability models
generates offspring variation even within the same set of parents, because each zygote can receive a different allele combination
In diploid organisms, subsequent mitotic divisions copy the zygote’s genome into all somatic cells, so the allele combinations formed at fertilization are propagated throughout the organism’s body cells.
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.
Practice Questions
Explain how fertilisation maintains the chromosome number of a species from one generation to the next. (2 marks)
States that gametes are haploid (n) and contain one set of chromosomes. (1)
States that fusion of two haploid gametes produces a diploid zygote (2n), restoring the chromosome number. (1)
Describe how fertilisation leads to new combinations of alleles in a zygote. Your answer should refer to haploid gametes and the formation of a diploid cell. (5 marks)
States that each gamete is haploid and carries one allele for each gene/locus. (1)
States that fertilisation involves fusion of gamete nuclei to form a zygote. (1)
States that the zygote is diploid and therefore has two alleles at each locus, one from each parent. (1)
Explains that which sperm/pollen nucleus fuses with which egg is random, so allele pairings vary between zygotes. (1)
Links this to offspring having genotypes not identical to either parent (new allele combinations). (1)
