Mendel hypothesized that each gamete (egg or sperm cell) receives only one copy of each hereditary factor from a pair, and that this separation happens randomly during reproductive cell formation. In modern terms: the two alleles for any given gene split apart so that each gamete carries just one allele instead of two. This idea became known as the Law of Segregation, and it is the core Mendel hypothesis that describes what happens to hereditary information when gametes form.
The Law of Segregation Explained
Before Mendel, most people assumed offspring traits were a blend of both parents, like mixing two colors of paint. Mendel’s pea plant experiments showed something different. When he crossed plants with wrinkled seeds and plants with smooth seeds, the offspring didn’t have semi-wrinkled seeds. They had smooth seeds. The wrinkled trait seemed to vanish entirely in the first generation, only to reappear in roughly one quarter of the next generation.
To explain this pattern, Mendel proposed that hereditary traits are controlled by paired “factors” (what we now call alleles). During the formation of sex cells, each gamete receives only one factor from the pair. The law of segregation states that during sex cell formation, each sex cell will receive one factor out of a pair of factors. This means a parent with two different alleles for a trait, say one for smooth seeds and one for wrinkled seeds, will produce gametes that carry one or the other, never both.
Why Gametes Are “Pure”
A key part of Mendel’s reasoning is sometimes called the “purity of gametes.” Even though an organism can carry two different alleles (one dominant, one recessive) and look identical to a parent with two dominant alleles, its gametes tell a different story. Each gamete carries only a single allele, uncontaminated by the other. The gamete is “pure” for one version of the trait.
This is why a plant that looks smooth-seeded can still produce some wrinkled-seeded offspring. The recessive allele doesn’t blend away or weaken. It travels intact through the gamete, waiting for the chance to pair up with another recessive allele in the next generation. Mendel confirmed this by tracking traits across multiple generations of self-fertilizing plants, verifying that pure-breeding lines always produced offspring identical to themselves.
Equal Probability of Either Allele
Mendel’s hypothesis also specifies that the separation is random. A heterozygous parent (carrying one dominant allele and one recessive allele) has an equal probability of passing on either one. If a pea plant carries one allele for yellow seeds (Y) and one for green seeds (y), roughly half its gametes will carry Y and the other half will carry y. No allele is preferentially shuttled into reproductive cells.
This equal split is what produces the predictable ratios Mendel observed. When two heterozygous plants are crossed, four equally likely allele combinations result, giving the famous 3:1 ratio of dominant to recessive traits in the offspring. The math only works if each gamete has a 50/50 chance of receiving either allele.
How Independent Assortment Builds on Segregation
Mendel’s second major hypothesis, the Law of Independent Assortment, extends the gamete story to multiple traits at once. Segregation describes what happens with alleles of a single gene: the pair splits so each gamete gets one. Independent assortment describes what happens when you track two or more genes simultaneously: the alleles of different genes sort into gametes independently of each other.
For example, a plant heterozygous for both seed shape (Rr) and seed color (Yy) can produce four equally likely gamete types: RY, Ry, rY, and ry. Whether a gamete receives R or r has no influence on whether it also receives Y or y. This independence holds true as long as the genes sit on different chromosomes or are far enough apart on the same chromosome. Segregation is the foundation; independent assortment is the extension to more complex inheritance patterns.
Mendel’s “Factors” in Modern Terms
Mendel never used the word “allele” or “gene.” He referred to hereditary units as “factors” (Factoren in his original German), and he used that word sparingly. He wrote that in pure-breeding individuals, “completely identical factors act together” in the male and female gametes. In organisms that don’t breed true (heterozygotes, in modern language), those factors differ from one another and separate during gamete formation.
Mendel also had no knowledge of meiosis, the cell division process that physically separates chromosomes into reproductive cells. That mechanism wasn’t discovered until decades after his 1866 paper. Yet his hypothesis predicted exactly what meiosis would later explain at the cellular level: homologous chromosomes (and the alleles they carry) are pulled apart during cell division, so each resulting gamete ends up with just one copy of each gene. The biological machinery validated what Mendel had inferred purely from counting peas.
The Statement That Fits
If you’re answering a textbook or exam question asking which statement describes Mendel’s hypothesis regarding gametes, look for wording along these lines: each gamete carries only one allele for each inherited trait, and the two alleles of a pair segregate from each other during gamete formation so that offspring receive one allele from each parent. Any answer that includes the concept of paired factors separating equally into sex cells captures the essence of what Mendel proposed. Answers describing blending inheritance, gametes carrying both alleles of a pair, or alleles influencing each other within a single gamete contradict his hypothesis.