In AP Bio, a zygote is the single diploid (2n) cell formed when a haploid sperm and a haploid egg fuse during fertilization. It carries one complete set of chromosomes from each parent and is the starting point for a new organism's development.
A zygote is what you get the instant fertilization happens. Two haploid (1n) gametes, a sperm and an egg, fuse, and their chromosomes combine into one diploid (2n) cell. That single cell holds a full chromosome set: one copy of each chromosome from mom, one from dad.
The math here matters. Each gamete is haploid because of meiosis, where homologous chromosomes separate in meiosis I and sister chromatids separate in meiosis II (EK 5.2.A.1). Combine two 1n cells and you're back to 2n. That's the whole point of the haploid-then-fuse cycle: it keeps chromosome number constant generation after generation. The zygote is the moment that diploid number gets restored.
The zygote lives in Unit 5: Heredity, specifically Topic 5.2 (Meiosis and Genetic Diversity), and it's the payoff for everything meiosis sets up. Learning objective AP Bio 5.2.A asks you to explain how meiosis generates genetic diversity, and the zygote is where that diversity finally lands. Crossing over (EK 5.2.A.2) and independent assortment scramble the alleles in each gamete, and random fertilization decides which two gametes meet. The zygote is the unique genetic combination that results. Without the haploid gametes that meiosis produces, fusion would double the chromosome count every generation, so the zygote is also the reason ploidy stays stable.
Keep studying AP Biology Unit 5
Fertilization (Unit 5)
Fertilization is the event; the zygote is the product. The instant a sperm and egg fuse their genetic material, the haploid + haploid = diploid math produces a zygote.
Gametes (Unit 5)
Gametes are the two haploid building blocks. A zygote is literally just two gametes added together, which is why each gamete must be 1n for the zygote to be 2n.
Random Fertilization (Unit 5)
Random fertilization is one of three sources of genetic variation, alongside crossing over and independent assortment. It decides which sperm meets which egg, so each zygote is a fresh, unpredictable combination of parental alleles.
Embryo and Blastocyst (Unit 5)
The zygote doesn't stay a single cell. It divides by mitosis into an embryo and then a blastocyst, so the zygote is the first cell in a lineage that mitosis copies over and over.
Most often this shows up in multiple-choice questions about the haploid-to-diploid cycle. A stem might hand you a haploid egg fusing with a haploid sperm and ask what the resulting cell or process is called, testing whether you can connect gametes, fertilization, and the diploid zygote. On free response, the 2026 Short FRQ Q4 asked about chromosome movement during meiosis I and why gametes carry the chromosome numbers they do. The zygote is the natural endpoint of that reasoning: explain why gametes are haploid, then show that fusing two of them restores the diploid number. If you're asked about genetic variation in offspring, tie the zygote back to random fertilization, crossing over, and independent assortment.
A gamete is a single haploid (1n) sex cell, a sperm or an egg, made by meiosis. A zygote is the diploid (2n) cell formed when two gametes fuse. One gamete + one gamete = one zygote. Don't call the egg a zygote; it only becomes a zygote after fertilization.
A zygote is the single diploid (2n) cell formed when a haploid sperm and haploid egg fuse during fertilization.
Two haploid gametes combine into one diploid zygote, which is how chromosome number stays constant from one generation to the next.
The zygote is the moment all three sources of genetic variation pay off: crossing over, independent assortment, and random fertilization.
After fertilization, the zygote divides by mitosis to become an embryo and then a blastocyst.
A gamete is 1n and a zygote is 2n; the egg only becomes a zygote once a sperm fertilizes it.
A zygote is the single diploid (2n) cell created when a haploid sperm fuses with a haploid egg during fertilization. It carries one full chromosome set from each parent and is the first cell of a new organism.
Diploid (2n). It's formed by fusing two haploid (1n) gametes, so one set of chromosomes comes from the egg and one from the sperm, restoring the full diploid number.
A gamete is a single haploid sex cell (sperm or egg) produced by meiosis. A zygote is the diploid cell you get when two gametes fuse. Gametes are 1n; the zygote is 2n.
Mitosis. Once the zygote forms, it divides by mitosis to grow into an embryo and blastocyst, making identical copies of itself. Meiosis happens earlier, to make the haploid gametes that fused into the zygote.
Because of crossing over and independent assortment during meiosis, plus random fertilization deciding which sperm meets which egg. Together these make the odds of two identical zygotes essentially zero.
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