Monohybrid Cross Explained: Definition, Steps, Examples & Mendelian Comparison

Table of Contents

• Introduction to Monohybrid Cross
• Steps of Monohybrid Cross
• Examples of Monohybrid Cross
• Monohybrid Cross vs Dihybrid Cross (7 Differences)
• References and Sources

Introduction to Monohybrid Cross

• A monohybrid cross is a genetic cross between two individuals with homozygous genotypes for a single trait, typically resulting in offspring with an opposite phenotype.
• These crosses are commonly used to determine the genotypes of the offspring from homozygous parents.
• The hybrid produced from a monohybrid cross helps identify the dominant genotype within the alleles.
• Although monohybrid crosses are often linked to homozygous genotypes, they are also useful in analyzing the genetic composition of individuals with heterozygous genotypes.
• The success of a monohybrid cross is assessed by evaluating the monohybrid ratio observed in the second-generation offspring.
• Monohybrid crosses are carried out to determine the dominant allele associated with a particular genetic trait.
• The cross involves two parents: one homozygous for one allele and the other homozygous for the contrasting allele.
• This cross results in heterozygous hybrid offspring that express the dominant trait.

Steps of Monohybrid Cross

• Monohybrid crosses are conducted to estimate phenotypic and genotypic ratios and to identify the dominant allele.
• A specific character or trait is chosen, and alleles are represented using alphabet characters—dominant alleles with uppercase letters and recessive alleles with lowercase letters.
• A Punnett square is created by noting the phenotype and genotype of the parent organisms involved in the cross.
• The genotype of the gametes is identified, considering that gametes are haploid due to meiotic division.
• All possible combinations of the genotypes are filled into the Punnett square, since fertilization occurs randomly.
• Phenotypic and genotypic ratios of the resulting offspring are calculated and recorded.
• The offspring produced from this cross are referred to as the F1 generation.

Examples of Monohybrid Cross

1. Mendel’s Peas

• George Mendel used monohybrid crosses to study dominant and recessive traits in pea plants.
• One example involved plant height, where some pea plants were tall and others were short.
• The tall pea plant had a homozygous genotype represented as TT, while the short or dwarf plant had a homozygous genotype tt.
• Crossing these two plants resulted in heterozygous offspring with the genotype Tt.
• Phenotypically, the resulting plants were tall, indicating that the tall trait is dominant, and the short trait is recessive.

2. Huntington’s Disease

• Huntington’s disease is a genetic disorder caused by the Huntingtin gene.
• Research was conducted to identify the genotype responsible for the disease.
• When a homozygous dominant individual was crossed with a homozygous recessive individual, all offspring inherited the dominant allele.
• It was concluded that the dominant allele is responsible for the disease, meaning all children from such a cross would express the disorder.

Monohybrid Cross vs Dihybrid Cross (7 Differences)

Definition

• Monohybrid Cross: A genetic cross between two individuals with homozygous genotypes for a single character, often producing opposite phenotypes.
• Dihybrid Cross: A genetic cross between individuals with homozygous or heterozygous genotypes for two characters.

Occurs between

• Monohybrid Cross: Homozygous individuals with different alleles for one trait.
• Dihybrid Cross: Homozygous or heterozygous individuals with different alleles for two traits.

Phenotypic Ratio (F2 Generation)

• Monohybrid: 3:1
• Dihybrid: 9:3:3:1

Genotypic Ratio (F2 Generation)

• Monohybrid: 1:2:1
• Dihybrid: 1:2:2:4:1:2:1:2:1

Test Cross Ratio

• Monohybrid: 1:1:1:1
• Dihybrid: 1:1

Significance

• Monohybrid: Identifies the dominant allele of a character.
• Dihybrid: Analyzes the assortment of traits in offspring.

Examples

• Monohybrid: Tall vs. dwarf pea plants.
• Dihybrid: Yellow round vs. green wrinkled pea seeds.

References and Sources

• Verma, P. S., & Agarwal, V. K. (2005). Cell biology, genetics, molecular biology, evolution, and ecology (Multicolored ed.). S. Chand Publishing.
• AskingLot. (n.d.). How do you use a monohybrid cross in a Punnett square? AskingLot. Retrieved from https://askinglot.com/how-do-you-use-a-monohybrid-cross-in-a-punnett-square
• Pediaa. (2018, October 8). Difference between monohybrid and dihybrid inheritance. Pediaa.com. Retrieved from https://pediaa.com/difference-between-monohybrid-and-dihybrid-inheritance/
• Bailey, R. (2020, August 27). Monohybrid cross: A genetics definition. ThoughtCo. Retrieved from https://www.thoughtco.com/monohybrid-cross-a-genetics-definition-373473
• AskAnyDifference. (n.d.). Difference between dominant and recessive allele. AskAnyDifference.com. Retrieved from https://askanydifference.com/difference-between-dominant-and-recessive-allele/
• Berufsschule Schongau. (n.d.). Monohybrid cross answers [PDF]. Retrieved from https://www.berufsschule-schongau.de/monohybrid_cross_answers.pdf