A monohybrid cross is a genetic cross between two organisms tracking a single trait (one gene). On the AP Bio exam it's used to figure out which allele is dominant and to predict offspring genotype and phenotype ratios using probability.
A monohybrid cross follows one gene at a time. You take two parents that differ in a single trait, like tall versus short pea plants, mate them, and watch what the offspring look like. "Mono" means one, and that one gene is the whole story here.
This is Mendel's original setup. By tracking a single trait across generations, you can apply Mendel's law of segregation (EK 5.3.A.1): the two alleles for a gene separate during gamete formation, so each gamete carries just one copy. Cross two heterozygous parents (Aa x Aa) and you get the classic 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. That predictable outcome is why the monohybrid cross is the foundation of single-gene inheritance, and why the rules of probability (EK 5.3.A.2.i) work so cleanly on it.
This lives in Unit 5: Heredity, topic 5.3 Mendelian Genetics, and it directly supports learning objective AP Bio 5.3.A: explain the inheritance of genes and traits as described by Mendel's laws. The monohybrid cross is where you prove dominance and recessiveness (EK 5.3.A.2.ii) and where you first apply probability to predict offspring (EK 5.3.A.2.i). Master this and the dihybrid cross, the test cross, and pedigree analysis all become extensions of the same logic instead of separate things to memorize.
Keep studying AP® Biology Unit 5
Dihybrid Cross (Unit 5)
A dihybrid cross is just a monohybrid cross doubled. You track two genes instead of one, which lets you test Mendel's law of independent assortment. If you can run a monohybrid Punnett square, a dihybrid is the same move applied to two traits at once (yielding the 9:3:3:1 ratio).
Mendel's Law of Segregation (Unit 5)
The monohybrid cross is the experiment that demonstrates segregation. Because the two alleles separate into different gametes, an Aa parent passes A to half its gametes and a to the other half, which is exactly what produces the 3:1 ratio.
Genetic Variation (Unit 5)
Fertilization fuses two haploid gametes back into a diploid zygote (EK 5.3.A.2), creating new allele combinations. The monohybrid cross shows this in miniature: two heterozygous parents can produce a recessive offspring neither parent visibly carried.
Pedigree Analysis (Unit 5)
Pedigrees are monohybrid-cross logic applied to real families across generations. The same dominant/recessive rules that predict pea-plant offspring let you deduce whether a human trait is autosomal recessive from a family tree.
Expect monohybrid crosses in multiple-choice as ratio and dominance questions. A classic stem gives you two heterozygous parents and asks for the phenotypic ratio (answer: 3:1). Another crosses a homozygous dominant with a homozygous recessive and asks which allele is dominant (the one all the offspring show). You'll need to set up a Punnett square fast, read off genotype and phenotype ratios, and use probability to predict the chance of a specific offspring. Free-response questions can ask you to justify a dominance relationship or predict a cross outcome, so practice explaining WHY the ratio comes out the way it does, not just stating it.
A monohybrid cross tracks ONE gene; a dihybrid cross tracks TWO. The giveaway is how many traits the parents differ in. One trait (flower color) is monohybrid and gives a 3:1 ratio; two traits (seed color AND seed shape) is dihybrid and gives a 9:3:3:1 ratio.
A monohybrid cross follows a single gene between two parents to determine dominance and inheritance.
Crossing two heterozygotes (Aa x Aa) gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
It's the experimental basis for Mendel's law of segregation, since the two alleles separate into different gametes.
The number of traits tells you the cross type: one trait is monohybrid, two traits is dihybrid.
A test cross (against a homozygous recessive) is a special monohybrid cross used to find an unknown genotype.
Probability rules apply directly, so you can predict the chance of any single offspring genotype or phenotype.
It's a cross between two organisms that differ in one trait, used to study how a single gene is inherited. It demonstrates Mendel's law of segregation and lets you predict offspring ratios, like the 3:1 phenotypic ratio from two heterozygous parents.
No. A monohybrid cross tracks one gene and a dihybrid cross tracks two. The clue is how many traits the parents differ in: one trait is monohybrid (3:1 ratio), and two traits is dihybrid (9:3:3:1 ratio).
An Aa x Aa cross gives a 3:1 phenotypic ratio (three dominant to one recessive) and a 1:2:1 genotypic ratio (one homozygous dominant, two heterozygous, one homozygous recessive).
Yes. A test cross is a monohybrid cross against a homozygous recessive individual, done to figure out whether a dominant-looking organism is homozygous or heterozygous. If any recessive offspring appear, the unknown parent must be heterozygous.
Cross two homozygous parents (like tall x short). The trait that shows up in all the F1 offspring is controlled by the dominant allele, and the masked trait is recessive.
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