11.1 Monohybrid Inheritance
Monohybrid inheritance is the study of how a single characteristic with contrasting traits is passed from parents to offspring. Gregor Mendel, known as the father of modern genetics, pioneered this field by conducting breeding experiments on garden pea plants (*Pisum sativum*). Through controlled crosses, Mendel discovered that traits are determined by discrete inheritance factors (now called genes) that separate during gamete formation and recombine during fertilisation.
Learning Objectives
- Define monohybrid inheritance and monohybrid cross.
- Identify the seven characteristics and contrasting traits of *Pisum sativum* studied by Mendel.
- Explain the reasons why Mendel chose pea plants for his inheritance experiments.
- Define and distinguish key genetic terms: gene, allele, locus, phenotype, genotype, dominant allele, recessive allele, dominant trait, recessive trait, homozygote, heterozygote, parental generation, filial generation, purebreed, and hybrid.
- Construct and interpret schematic diagrams and Punnett squares for monohybrid crosses to determine genotypic and phenotypic ratios.
- State and explain Mendel's First Law (Law of Segregation).
Schematic Diagram of a Monohybrid Cross and Punnett Square
Alleles segregate during meiosis, gametes carry one allele each, and random fertilisation produces the \(F_1\) and \(F_2\) generations with predictable genotypic and phenotypic ratios.
Parental Cross
Purebreed tall \(TT\) × purebreed dwarf \(tt\) produces all heterozygous \(Tt\) offspring.
\(F_1\) Self-cross
\(Tt\) × \(Tt\) produces \(1\ TT : 2\ Tt : 1\ tt\) genotypically.
Phenotypic Ratio
\(3\ \text{tall} : 1\ \text{dwarf}\), or \(75\%\) tall : \(25\%\) dwarf.
Short Explanation
Definition
Monohybrid inheritance involves the inheritance of one characteristic and contrasting traits controlled by a single gene. Genetics is the study of heredity and the variation of inherited characteristics. Gregor Mendel is known as the father of modern genetics for scientifically explaining the principles of inheritance.
Why Mendel chose pea plants
Mendel chose the pea plant (*Pisum sativum*) because it can be easily grown and bears many seedlings, possesses both male and female reproductive structures, has a short generation interval, and possesses contrasting traits that are obvious and easy to identify.
Seven characteristics studied
Seed shape, seed colour, pod shape, pod colour, flower colour, flower position, and plant height. Each characteristic has a clear dominant trait and a contrasting recessive trait.
Monohybrid cross process
Meiosis separates homologous chromosomes, producing gametes with single alleles. Random fertilisation produces the \(F_1\) generation, and self-crossing the \(F_1\) generation produces the \(F_2\) generation with predictable ratios.
Mendel's First Law
A characteristic of an organism is controlled by a pair of alleles, and only one of the allelic pair is inherited in a gamete.
Seven Characteristics Studied by Mendel
Key genetic terms
Gene, allele, phenotype, genotype, dominant allele, recessive allele, homozygote, heterozygote, purebreed, and hybrid.
Probability and sample size
Mendel obtained ratios close to \(3:1\), for example \(5474\ \text{round} : 1850\ \text{constricted} = 2.96:1 \approx 3:1\). The actual ratio may differ from the expected ratio due to chance, probability, and sample size; a larger sample size produces results closer to the theoretical \(3:1\) ratio.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is meant by a purebreed?
In a fertilisation between two rats, \(B\) represents the dominant allele for black fur, whereas \(b\) represents the recessive allele for white fur. If the outcome of fertilisation is \(\frac{1}{4}\ BB : \frac{1}{2}\ Bb : \frac{1}{4}\ bb\), what are the genotypes and fur colours of their parents?
State Mendel's First Law.
Common Mistakes
- Confusing phenotype with genotype: writing genetic letters (like \(TT\) or \(Tt\)) when asked for phenotype, or writing descriptive words (like “tall”) when asked for genotype.
- Writing pairs of alleles inside gametes, for example writing “\(Tt\)” in a single gamete circle instead of showing separate gametes carrying “\(T\)” and “\(t\)”.
- Using different letters for contrasting traits of the same characteristic, for example using \(T\) for tall and \(d\) for dwarf, instead of \(T\) and \(t\).
- Forgetting that the \(3:1\) phenotypic ratio is a statistical probability that requires large sample numbers to be accurately observed.
Concept Misunderstandings
Recessive alleles are modified, weakened, or destroyed when present alongside a dominant allele in a heterozygote.
The recessive allele remains intact and unchanged; its expression is merely suppressed or masked by the dominant allele, allowing it to reappear unchanged in future generations when paired with another recessive allele.
The dominant trait is always more common or “better” in a natural population.
Dominance describes only how an allele is expressed phenotypically in a heterozygote; it does not indicate evolutionary superiority, fitness, or frequency in a population.
Summary
- Monohybrid inheritance is the inheritance of one characteristic controlled by a single gene with contrasting traits.
- Mendel studied 7 distinct characteristics in *Pisum sativum*, each having a clear dominant and recessive trait.
- A gene occupies a specific locus on a chromosome; alternative forms of a gene are alleles.
- A cross between two purebreed individuals with contrasting traits produces \(100\%\) heterozygous offspring (\(F_1\)) showing the dominant phenotype.
- Self-crossing of the \(F_1\) generation yields an \(F_2\) generation with a genotypic ratio of \(1 : 2 : 1\) (homozygous dominant : heterozygous : homozygous recessive) and a phenotypic ratio of \(3 : 1\) (dominant : recessive).
- Mendel's First Law (Law of Segregation) explains that alleles separate during gamete formation (meiosis) so that each gamete carries only one allele for each characteristic.
Short Activity
Objective: test your understanding of monohybrid inheritance, Mendel's First Law, and genetic terminology.
A. Objective Quiz
In pea plants, purple flower colour (\(B\)) is dominant over white flower colour (\(b\)). What is the expected phenotypic ratio in the \(F_2\) generation when two heterozygous purple-flowered plants (\(Bb\)) are crossed?
Which statement correctly describes Mendel's First Law (Law of Segregation)?
B. Fill in the Blanks
A gene is the basic unit of inheritance consisting of a DNA segment located on a specific of a chromosome.
An individual with two identical alleles at a particular gene locus is described as a .
C. Matching / Drag and Drop
Drag each genetic term to its correct textbook description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Genetic Term to Its Description
Write the correct genetic term next to each description.
| Description | Genetic Term |
|---|---|
| 1. The genetic composition of an organism that cannot be seen | |
| 2. The observable physical or physiological characteristic of an organism | |
| 3. An alternative form of a gene for a specific trait located on the same locus | |
| 4. An individual carrying two identical alleles that breeds true across generations |
Answer choices: Genotype • Phenotype • Allele • Purebreed
