Mendel's law of segregation states that the two alleles for a gene separate during gamete formation, so each gamete carries only one allele per gene, and alleles recombine randomly at fertilization.
Mendel's law of segregation is the rule that the two copies of a gene (the alleles) split apart when an organism makes gametes. A diploid parent has two alleles for each gene. When it produces sperm or eggs, those two alleles separate so that each gamete ends up with just one of them. At fertilization, two gametes fuse and the pair is restored.
The physical reason this happens is meiosis. Homologous chromosomes (which carry the matching alleles) pull to opposite poles, so each gamete gets one chromosome of the pair and therefore one allele. That's why a heterozygous parent (Rr) makes half its gametes R and half r. This single principle is what makes a monohybrid cross produce a 3:1 phenotypic ratio in the offspring, and it's the foundation for using probability to predict inheritance (EK 5.3.A.1).
This term lives in Unit 5: Heredity, Topic 5.3 Mendelian Genetics, and it backs learning objective AP Bio 5.3.A: explain the inheritance of genes and traits as described by Mendel's laws. Segregation is the reason a Punnett square works at all. Without it, you couldn't predict offspring genotypes, apply the rules of probability to single-gene traits, or interpret a pedigree. It also feeds straight into Unit 5's bigger theme of genetic variation, because separating and recombining alleles at fertilization is one engine that shuffles allele combinations in a population (EK 5.3.A.2).
Keep studying AP® Biology Unit 5
Independent Assortment (Unit 5)
These are Mendel's two laws and they're easy to mix up. Segregation is about one gene (its two alleles split). Independent assortment is about two or more genes on different chromosomes (their alleles sort separately from each other). A monohybrid cross shows segregation; a dihybrid cross adds independent assortment on top.
Meiosis (Unit 5)
Segregation isn't just a rule, it's a description of what meiosis physically does. Homologous chromosomes separate in meiosis I, which is the actual mechanism that pulls the two alleles apart into different gametes.
Genetic Variation (Unit 5)
Segregation plus random fertilization is a variation generator. Each gamete carries a random one of two alleles, and two random gametes fuse, so the zygote gets a new allele combination (EK 5.3.A.2).
Heterozygous and Homozygous Genotypes (Unit 5)
Segregation only produces two different gamete types when a parent is heterozygous (Rr). A homozygous parent (RR or rr) segregates too, but every gamete ends up identical, which is why homozygous crosses don't give you ratio variety.
Expect this as a definition-style multiple-choice question ("Which of the following best describes Mendel's law of segregation?" or "According to Mendel's law of segregation, what happens to alleles during gamete formation?"). The answer is that allele pairs separate so each gamete gets one allele. You'll also see it embedded in cross problems: a pink x pink snapdragon cross giving a 1:2:1 ratio demonstrates segregation even though the trait shows incomplete dominance. On free response, you won't define it in isolation; you'll use it to justify gamete types, set up a Punnett square, and apply probability to predict offspring ratios.
Segregation is a one-gene rule: the two alleles of a single gene separate during gamete formation. Independent assortment is a multi-gene rule: alleles of genes on different chromosomes sort into gametes independently of one another. If a problem involves only one trait, it's testing segregation. If it tracks two or more traits at once (a dihybrid cross), independent assortment is also in play.
Mendel's law of segregation says the two alleles for a gene separate during gamete formation, so each gamete carries exactly one allele.
The physical mechanism is meiosis: homologous chromosomes separate in meiosis I, pulling the paired alleles into different gametes.
A heterozygous parent (Rr) produces two gamete types in equal proportion (half R, half r), which is what makes monohybrid crosses give predictable ratios.
Segregation is the reason a Punnett square and the rules of probability work for single-gene traits.
Even traits with incomplete dominance still follow segregation, shown by a 1:2:1 phenotypic ratio in a heterozygous cross.
It's the principle that the two alleles for a gene separate during gamete formation, so each gamete gets only one allele, and the pair is restored at fertilization. It's tested in Unit 5, Topic 5.3 under learning objective AP Bio 5.3.A.
Segregation deals with one gene (its two alleles split apart). Independent assortment deals with two or more genes on different chromosomes sorting separately from each other. Monohybrid crosses test segregation; dihybrid crosses test both.
Yes. Segregation describes how alleles separate, not how they're expressed. A cross of two pink snapdragons (incomplete dominance) gives a 1:2:1 ratio, which still demonstrates segregation perfectly.
The two alleles of each gene separate so that each gamete receives just one allele. This happens because homologous chromosomes move to opposite poles during meiosis I.
Punnett squares assume each gamete carries one allele per gene, which is exactly what segregation guarantees. That's what lets you list gamete types and use probability to predict offspring genotypes and phenotypes.
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