CHAPTER: NON-MENDELIAN INHERITANCE
Patterns Beyond Mendel’s Laws
Introduction: Understanding Genetic Variation Beyond Mendelian Inheritance
This lesson discusses Non-Mendelian Inheritance, an extension of classical genetics that explains inheritance
patterns that do not follow the simple dominant and recessive rules discovered by Gregor Mendel. While
Mendelian inheritance explains many traits, real-life genetics often involve more complex patterns.
The study of genetics is the branch of biology concerned with genes, heredity, and genetic variation. Understanding
genetics helps explain how traits are passed from parents to offspring and why organisms differ from one another.
Key terms in genetics include:
• Inheritance – the process by which genetic information is passed from parents to offspring
• Variation – differences among organisms
• Gene – a segment of DNA that controls a trait
• Genotype – the genetic makeup of an organism
• Phenotype – the observable characteristics or traits of an organism
• Homozygous – having two identical alleles
• Heterozygous – having two different alleles
These concepts provide the foundation for understanding non-Mendelian inheritance patterns.
Section 1: Foundations of Mendelian Genetics and the Transition to Non-Mendelian
Patterns
Gregor Mendel studied pea plants and discovered the basic principles of heredity. His experiments focused on traits
such as plant height and flower color. Mendel observed that traits were inherited according to dominant and recessive
alleles.
One important tool used in genetics is the Punnett square, which predicts the possible genotypes and phenotypes of
offspring.
However, scientists later discovered inheritance patterns that did not follow Mendel’s simple ratios. These patterns
became known as non-Mendelian inheritance.
The major typesdominance
• Incomplete discussed in this lesson are:
1
, • Codominance
• Multiple alleles
Section 2: Incomplete Dominance – The Emergence of Intermediate Phenotypes
Incomplete dominance occurs when neither allele completely dominates the other. As a result, the offspring show
an intermediate phenotype.
EXAMPLE:
A red snapdragon flower (FrFr) crossed with a white snapdragon flower (FwFw) produces pink flowers
(FrFw).
FrFr × FwFw → FF
r w
The offspring are 100% pink because the alleles blend to create a new intermediate phenotype.
Another example:
A white flower (FwFw) crossed with a pink flower (FrFw) produces:
• 50% pink flowers
• 50% white flowers
This pattern demonstrates predictable inheritance even in incomplete dominance.
The lesson emphasizes the importance of identifying the correct genotype to understand how intermediate
phenotypes are formed.
Section 3: Codominance – Equal Expression of Alleles
Codominance occurs when both alleles are fully and equally expressed in the phenotype.
For example, crossing red and white flowers may produce offspring with both red and white patches instead of a
blended pink color.
FrFr × FwFw → FF
r w
In codominance, the heterozygous offspring express both traits simultaneously.
2
Patterns Beyond Mendel’s Laws
Introduction: Understanding Genetic Variation Beyond Mendelian Inheritance
This lesson discusses Non-Mendelian Inheritance, an extension of classical genetics that explains inheritance
patterns that do not follow the simple dominant and recessive rules discovered by Gregor Mendel. While
Mendelian inheritance explains many traits, real-life genetics often involve more complex patterns.
The study of genetics is the branch of biology concerned with genes, heredity, and genetic variation. Understanding
genetics helps explain how traits are passed from parents to offspring and why organisms differ from one another.
Key terms in genetics include:
• Inheritance – the process by which genetic information is passed from parents to offspring
• Variation – differences among organisms
• Gene – a segment of DNA that controls a trait
• Genotype – the genetic makeup of an organism
• Phenotype – the observable characteristics or traits of an organism
• Homozygous – having two identical alleles
• Heterozygous – having two different alleles
These concepts provide the foundation for understanding non-Mendelian inheritance patterns.
Section 1: Foundations of Mendelian Genetics and the Transition to Non-Mendelian
Patterns
Gregor Mendel studied pea plants and discovered the basic principles of heredity. His experiments focused on traits
such as plant height and flower color. Mendel observed that traits were inherited according to dominant and recessive
alleles.
One important tool used in genetics is the Punnett square, which predicts the possible genotypes and phenotypes of
offspring.
However, scientists later discovered inheritance patterns that did not follow Mendel’s simple ratios. These patterns
became known as non-Mendelian inheritance.
The major typesdominance
• Incomplete discussed in this lesson are:
1
, • Codominance
• Multiple alleles
Section 2: Incomplete Dominance – The Emergence of Intermediate Phenotypes
Incomplete dominance occurs when neither allele completely dominates the other. As a result, the offspring show
an intermediate phenotype.
EXAMPLE:
A red snapdragon flower (FrFr) crossed with a white snapdragon flower (FwFw) produces pink flowers
(FrFw).
FrFr × FwFw → FF
r w
The offspring are 100% pink because the alleles blend to create a new intermediate phenotype.
Another example:
A white flower (FwFw) crossed with a pink flower (FrFw) produces:
• 50% pink flowers
• 50% white flowers
This pattern demonstrates predictable inheritance even in incomplete dominance.
The lesson emphasizes the importance of identifying the correct genotype to understand how intermediate
phenotypes are formed.
Section 3: Codominance – Equal Expression of Alleles
Codominance occurs when both alleles are fully and equally expressed in the phenotype.
For example, crossing red and white flowers may produce offspring with both red and white patches instead of a
blended pink color.
FrFr × FwFw → FF
r w
In codominance, the heterozygous offspring express both traits simultaneously.
2
