Difficulty: Introductory | Prerequisites: Understanding of the theory of evolution by natural selection (see Theories of Evolution notes).
Darwin's theory of natural selection required heritable variation, but Darwin could not explain how inheritance worked. Gregor Mendel's experiments with pea plants provided the answer. This topic covers Mendel's experimental design, his rejection of two earlier hypotheses (blending inheritance and inheritance of acquired characters), and his theory of particulate inheritance. It also introduces monohybrid and dihybrid crosses and the principle of independent assortment. This material is the gateway to all of genetics: gene expression, chromosomal inheritance, and population genetics all build on what Mendel established.
Mendel crossed pure-breeding pea plants and tracked seven traits across generations. His results disproved both blending inheritance and inheritance of acquired characters. He showed that hereditary information comes in discrete particles (what we now call genes), that each individual carries two copies, and that these copies segregate during reproduction. When he tracked two traits at once, they sorted independently.
Blending inheritance
The hypothesis that offspring traits are a blend of the parents' traits, producing intermediates. In simple terms, if you cross a tall plant with a short one, you would expect a medium plant. Mendel's data showed this does not happen.
Inheritance of acquired characters
The hypothesis that changes acquired during an organism's lifetime are passed to offspring. In simple terms, if you cut the flowers off a plant, the next generation should lack flowers. Mendel's crosses showed this is also wrong.
Particulate inheritance
Mendel's model: hereditary information is carried in discrete, indivisible units (particles), not blended fluids. Think of it as: traits are passed along like playing cards, not like mixing paint.
Pure line (true-breeding line)
A population that produces offspring identical to itself for a given trait, generation after generation. Using pure lines was critical because Mendel knew each parent carried only one form of the hereditary factor.
Phenotype
The observable trait of an organism (e.g. tall or short, purple or white flowers). In simple terms, it is what you can see or measure.
Allele
A version of a gene. For a given trait, an organism may carry two copies of the same allele or two different alleles.
Homozygous
Having two identical alleles for a given gene. A pure-breeding line is homozygous for the trait in question.
Heterozygous
Having two different alleles for a given gene. In Mendel's crosses, F1 offspring were heterozygous.
Dominant allele
The allele whose phenotype is expressed when the organism is heterozygous. In simple terms, it "masks" the other allele.
Recessive allele
The allele whose phenotype is only expressed when the organism is homozygous for that allele. It is hidden in a heterozygote.
Monohybrid cross
A cross between two organisms that differ in a single trait. Mendel used these to establish the basic rules of inheritance.
Dihybrid cross
A cross between organisms differing in two traits simultaneously, used to test whether alleles of different genes are transmitted together or independently.
Independent assortment
Mendel's finding that alleles of different genes sort into gametes independently of one another. In simple terms, inheriting one trait does not affect which version of a different trait you inherit.
Mendel studied seven characteristics in pea plants, each with two distinct phenotypes (e.g. tall vs short, purple vs white flowers)
He used pure-breeding (true-breeding) lines: plants that, when crossed with themselves, always produced offspring identical for the trait
The value of pure lines: because they breed true, Mendel knew each parent carried only one hereditary "version" of the trait. This made it possible to trace exactly what was inherited.
Blending inheritance: two different traits combine to produce an intermediate in the offspring
Inheritance of acquired characters: changes acquired during an organism's lifetime are passed to offspring
Mendel crossed pure-breeding tall plants with pure-breeding short plants
F1 generation: all tall. No medium plants appeared.
This directly contradicts blending inheritance, which would predict intermediate (medium) offspring
The trait is either present or absent: it is A or B, never a blend of AB
Mendel removed flowers from plants; the next generation still had flowers
F2 generation showed short plants reappearing, even though F1 plants were all tall
If traits were acquired from the environment, the short phenotype should have been permanently lost once the tall F1 generation was established. It was not.
Consistency across traits: all seven traits Mendel studied gave similar results. The pattern held across multiple characteristics, not just one.
Reciprocal crosses gave the same results: it did not matter whether the male or female parent contributed the dominant trait. Inheritance was not dependent on the sex of the parent.
Important caveat: many traits in nature are controlled by more than one gene and do not show Mendel's clean ratios. Mendel happened to choose traits governed by single genes with clear dominant/recessive relationships.
Mendel's conclusion: hereditary information is carried in discrete particles, not blended. Three key claims emerged from his data:
Inheritance is particulate, not blending: F1 offspring are not intermediates. The trait is either one form or the other (e.g. black or white chickens, never grey).
Each individual carries two copies of each allele: if F1 were homozygous with only one allele, then all F2 would show only that allele. The reappearance of the recessive phenotype in F2 proves two copies exist.
Individuals can be homozygous or heterozygous: F1 must carry a copy of both alleles (one from each parent). The two copies can be the same (homozygous) or different (heterozygous).
Mendel's question: are alleles from different genes transmitted together, or independently?
Hypothesis 1: alleles from different genes are transmitted together (linked)
Hypothesis 2: alleles from different genes transfer independently
Mendel's dihybrid cross results supported Hypothesis 2. The ratios in the F2 generation matched what you would expect if each gene sorted into gametes independently of the other.
This became Mendel's principle of independent assortment.
Mendelian genetics underpins genetic counselling. When two carriers of a recessive allele for a condition such as cystic fibrosis have children, there is a predictable 1-in-4 chance each child will be affected. The same logic applies to breeding programmes in agriculture: crossing pure lines and predicting offspring traits is how plant and animal breeders develop new varieties.
Students often think dominant means "better" or "more common." It does not. Dominant simply means the allele's phenotype shows up in a heterozygote. A dominant allele can be rare and harmful.
Students often think all traits follow simple Mendelian ratios. Most traits are influenced by multiple genes (polygenic), and Mendel's clean ratios are the exception rather than the rule.
Students often confuse genotype and phenotype. Two organisms can look the same (same phenotype) but carry different alleles (different genotype), as with Mendel's tall F1 heterozygotes and tall pure-breeding homozygotes.
Students often think recessive traits skip a generation on purpose. The trait reappears in F2 because heterozygous F1 parents each pass on a recessive allele by chance, not because the trait was "stored" somewhere.
⚠️ Be able to explain why Mendel used pure lines and what advantage they gave his experimental design.
⚠️ Know the specific evidence that disproved blending inheritance (no intermediates in F1) and inheritance of acquired characters (F2 recessive phenotypes reappear, removed flowers still present in next generation).
⚠️ Understand the three claims of particulate inheritance: inheritance is discrete, individuals carry two copies, individuals can be homozygous or heterozygous.
⚠️ Be able to distinguish independent assortment from linked transmission and explain how dihybrid crosses test this.
True or False: Mendel's F1 offspring from a cross of tall and short pure lines were medium height.
Answer: False. All F1 were tall, disproving blending inheritance.
Fill in the blank: An organism with two different alleles for a gene is called ____.
Answer: Heterozygous.
True or False: Independent assortment means alleles of different genes are always inherited together.
Answer: False. Independent assortment means they sort into gametes independently of each other.
Fill in the blank: Mendel used ____ lines to ensure each parent carried only one form of the hereditary factor.
Answer: Pure-breeding (true-breeding).
Q: Why was it important that Mendel used pure-breeding lines in his experiments?
A: Pure-breeding lines are homozygous, so Mendel knew each parent carried only one form of the hereditary factor. This made it possible to trace exactly which allele came from which parent.
Q: How did Mendel's monohybrid cross results disprove blending inheritance?
A: The F1 generation showed only the dominant phenotype (e.g. all tall), with no intermediates. If blending occurred, the F1 should have been medium.
Q: What evidence from Mendel's experiments disproved inheritance of acquired characters?
A: Mendel removed flowers from plants, yet the next generation still had flowers. Also, the recessive phenotype (e.g. short) reappeared in F2 despite being absent in F1, showing traits were not acquired from the environment.
Q: What are the three key claims of the theory of particulate inheritance?
A: (1) Inheritance is particulate, not blending. (2) Each individual has two copies of each hereditary factor (allele). (3) Individuals can be homozygous or heterozygous.
Q: In a dihybrid cross, what result would you expect if alleles of different genes are transmitted independently?
A: You would expect a 9:3:3:1 phenotypic ratio in the F2 generation, which is what Mendel observed.
Mendel's work solved Darwin's inheritance problem: natural selection requires heritable variation, and particulate inheritance explains how variation is maintained (alleles do not blend away). This connects forward to the chromosome theory of inheritance, where genes are located on chromosomes and Mendel's rules (segregation and independent assortment) are explained by the physical behaviour of chromosomes during meiosis.
Mendel, Mendelian genetics, particulate inheritance, blending inheritance, inheritance of acquired characters, monohybrid cross, dihybrid cross, pure line, true breeding, phenotype, genotype, allele, homozygous, heterozygous, dominant, recessive, independent assortment, segregation, F1 generation, F2 generation, Punnett square, 3:1 ratio, 9:3:3:1 ratio, pea plants, Gregor Mendel, BIOL 180, introduction to biology