A monohybrid cross is a breeding experiment that tracks just one trait, such as flower color or seed shape, as it passes from parents to offspring. Following one trait at a time is how the basic rules of inheritance were first worked out, in garden peas grown by the monk Gregor Mendel. Its most surprising lesson is that a trait can vanish completely in one generation and then come back in the next.
The Vocabulary You Need Before Drawing a Cross
Genes come in different versions called alleles, and each plant or animal carries two alleles for a gene, one from each parent. A pea plant might carry two alleles for purple flowers, two for white, or one of each. The pair of alleles is the genotype, while the trait you can actually see, like purple flowers, is the phenotype. An individual with two matching alleles is called homozygous, and one with two different alleles is heterozygous.
Dominance explains why some alleles stay hidden. A dominant allele can mask a recessive one, so a plant with one of each looks just like a plant with two dominant alleles.1EBSCO. Monohybrid inheritance Geneticists write the dominant allele with a capital letter (P) and the recessive with the same lowercase letter (p). That means PP and Pp plants both have purple flowers, and only pp plants have white flowers.2Global Change Biology (or similar open textbook platform, using LibreTexts equivalent based on text). What Is a Monohybrid Cross? Definition & Example
The letter shorthand in homework problems follows a few habits. Use one letter per gene, chosen from the dominant trait, and always write the capital first: Pp, never pP. Read pp as two recessive copies, which is the only way the recessive trait shows.
When a problem gives you a genotype like Tt, read it as two separate alleles, one inherited from each parent, and not as a single code. Before you start, write out what each letter stands for, for example P means purple and p means white. That keeps the later steps from getting muddled.
“Dominant” only describes which version shows when both are present. It does not mean common or stronger. A dominant allele can be rare in a population, and dominance says nothing about whether a trait is healthier or better.
Mendel’s Pea Example Step by Step
Mendel bred garden peas in his monastery garden from 1858 to 1866 and counted the traits of the offspring. He began with true-breeding parents. A true-breeding plant gives the same trait every time it self-pollinates, which means it has two matching versions, PP or pp. Pea plants self-pollinate easily, so pure lines were simple to keep, and starting with known genotypes made the later results predictable.
Crossing a pure purple-flowered plant (PP) with a pure white-flowered plant (pp) gives a first offspring generation, called F1, in which every plant looks like one parent.3OpenStax Biology. What Is a Monohybrid Cross? Definition & Example Every F1 plant gets P from one parent and p from the other, so all are Pp and purple.
When F1 plants are crossed with each other, the second generation, F2, shows a 3:1 ratio of the dominant trait to the recessive trait.4OpenStax Biology. What Is a Monohybrid Cross? Definition & Example For flower color, about three purple plants appear for every white one.5Introduction to Genetics. 1.5 Punnett Squares – 3:1 Ratio Mendel saw that alleles stay intact rather than blending, and pass on separately.6Biology LibreTexts. What Is a Monohybrid Cross? Definition & Example7Biology LibreTexts. What Is a Monohybrid Cross? Definition & Example
The counting is what made this work. Mendel tallied large numbers of plants for each cross instead of just noting what he saw, and a ratio that kept appearing across many crosses pointed to a rule rather than random variation. The numbers came out close to 3:1 but not exact, which shows the ratio is a tendency of chance rather than a fixed count.
Other pea traits follow the same pattern. Round versus wrinkled seeds behaves the same way, with round dominant, and yellow versus green seed color also gives a 3:1 result in F2.
How to Draw the Punnett Square
A Punnett square, devised by British geneticist Reginald Punnett, predicts all possible offspring of a cross and how often each should appear.8OpenStax Biology. What Is a Monohybrid Cross? Definition & Example Given both parents’ genotypes and which allele is dominant, you can predict every genotype and look the offspring could have.9Introduction to Genetics. 1.5 Punnett Squares – 3:1 Ratio10Introduction to Genetics. 1.5 Punnett Squares – 3:1 Ratio
Draw a square split into four boxes. Each allele on the outside stands for one gamete, meaning one egg or one sperm (pollen in plants), so you list each parent’s two different alleles once, not the whole genotype. Here is the Pp by Pp cross:
- Step 1: Write one parent’s alleles, P and p, across the top.
- Step 2: Write the other parent’s P and p down the side.
- Step 3: Fill each box with one letter from the top and one from the side, giving PP, Pp, Pp and pp.
- Step 4: Count genotypes: 1 PP, 2 Pp, 1 pp, a 1:2:1 genotype ratio.
- Step 5: Count looks: three boxes purple, one white, a 3:1 phenotype ratio.11Introduction to Genetics. 1.5 Punnett Squares – 3:1 Ratio
Each box stands for a 1 in 4 chance, so there is a 3 in 4 chance of the dominant trait and a 1 in 4 chance of the recessive trait for each offspring.12OpenStax Biology. What Is a Monohybrid Cross? Definition & Example
Check your finished grid. The usual slips are putting the whole genotype Pp in one slot instead of splitting P and p, confusing the 1:2:1 genotype ratio with the 3:1 phenotype ratio, and forgetting that Pp and PP both show the dominant trait.13BIOMA: Jurnal Ilmiah Biologi. IDENTIFICATION OF STUDENTS’ MISCONCEPTIONS USING THE CERTAINTY OF RESPONSE INDEX (CRI) METHOD ON MONOHYBRID CROSSING MATERIAL When a question asks for a “ratio,” check whether it wants genotypes (letters) or phenotypes (looks), because the answers differ for the same cross.
Other crosses you may be asked to solve use the same grid. PP by pp gives all Pp, so all show the dominant trait. Pp by pp gives half Pp and half pp, a 1:1 ratio. PP by Pp gives all dominant-looking offspring, half PP and half Pp.
Why the 3:1 Ratio Happens
When eggs or sperm form, the two alleles of a gene separate, and each egg or sperm has an equal chance of carrying either one.14Biology LibreTexts. What Is a Monohybrid Cross? Definition & Example A Pp parent has one P and one p in each body cell, so it makes half P and half p reproductive cells. Each egg or sperm ends up with just one allele, never both, and which one it gets is a matter of chance. This separation happens during meiosis, the cell division that makes eggs and sperm, when paired chromosomes carrying different gene versions move into different cells.15Biology LibreTexts. What Is a Monohybrid Cross? Definition & Example16Lumen Learning. What Is a Monohybrid Cross? Definition & Example
Fertilization then joins one egg with one sperm, so the offspring’s pair is made of one allele from each parent. With P and p equally likely from each side, the four possible pairings are PP, Pp, Pp and pp, each equally likely. Three of those four contain at least one P, which is how three purple to one white arises even though the white version is hidden in two of the three.17Biology LibreTexts. What Is a Monohybrid Cross? Definition & Example Because it is a probability, four offspring will not always split exactly 3 and 1, just as four coin flips vary, so small families can easily stray from the ratio.
The Test Cross for a Hidden Genotype
A purple pea plant might be PP or Pp, and looking at it cannot tell you which. That matters to a breeder who wants a line that always produces purple flowers, because a Pp plant will also produce some white offspring. To find the genotype of an individual showing the dominant trait, breeders use a test cross.18Nature Education. Test Crosses The unknown individual is crossed with one that has two recessive alleles.19Global Change Biology (or similar open textbook platform, using LibreTexts equivalent based on text). What Is a Monohybrid Cross? Definition & Example If the unknown is PP, all offspring show the dominant trait. If it is Pp, about half the offspring show the recessive trait.
The squares make the difference plain. PP by pp fills all four boxes with Pp. Pp by pp gives two Pp and two pp boxes, a 1:1 result that reveals a Pp parent.
The recessive partner works because it adds only p, so the unknown plant’s alleles show directly in the offspring. A single white offspring is enough to show the parent is Pp. A few all-purple offspring do not fully prove PP, since chance can hide p, but more offspring give a more reliable answer.
When Results Do Not Match 3:1
You can count offspring after a cross and compare them with the Punnett square prediction.20Biology OER. What Is a Monohybrid Cross? Definition & Example Biology classes often use a chi-square test, a calculation that asks whether a gap from the expected ratio is likely just chance.21PubMed. Using zebrafish to learn statistical analysis and Mendelian genetics No real cross gives exactly 3:1, so the test judges whether your counts are close enough. You compare them to the numbers the grid predicts, such as 75 purple and 25 white out of 100 offspring. A small gap is ordinary chance, while a large one suggests a different pattern of inheritance.
Sometimes neither allele fully masks the other, and the heterozygous offspring look intermediate.22OpenStax Biology. What Is a Monohybrid Cross? Definition & Example In snapdragons, crossing red-flowered and white-flowered plants gives pink offspring.23OpenStax Biology. What Is a Monohybrid Cross? Definition & Example Crossing two pink plants gives a 1 red: 2 pink: 1 white result, since each genotype looks different. Here the phenotype ratio matches the genotype ratio, unlike the pea example, where the genotype ratio is 1:2:1 but the phenotype ratio is 3:1. Crossing pinks brings back pure red and white plants, and that return shows the alleles stayed intact, as Mendel found.
Some alleles are deadly when an individual carries two copies.24PubMed. Lethal allele Crossing two yellow mice gave a 1:2 ratio of non-yellow to yellow rather than 1:2:1, because mice with two yellow alleles never survived.25PubMed. Lethal allele The grid still predicts 1:2:1, but those mice die before they can be counted. A 2:1 result with a missing group is a clue to a lethal allele, though confirming it means finding evidence that those offspring died.