Mendelian Genetics: Principles, Patterns, and Human Applications (Ch. 14.1–14.4)

Topic 10: Mendelian Genetics

Introduction

Mendelian genetics explores how traits are inherited from one generation to the next, based on the pioneering work of Gregor Mendel. This topic covers the foundational laws of inheritance, the distinction between dominant and recessive traits, and the complexities of human genetic disorders.

Core Concepts in Mendelian Genetics

Key Generations in Genetic Crosses

• P Generation: The parental generation in a genetic cross.

• F1 Generation: The first filial generation, offspring of the P generation.

• F2 Generation: The second filial generation, offspring of the F1 generation.

Mendel's Four Concepts

• 1. Alternative versions of genes (alleles) account for variations in inherited characters.

• 2. For each character, an organism inherits two alleles, one from each parent.

• 3. If the two alleles differ, the dominant allele determines the organism’s appearance; the recessive allele has no noticeable effect.

• 4. Law of Segregation: The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Law of Segregation in Diploid Cells

• In a cell with diploid number = 2, alleles separate during meiosis, ensuring each gamete receives only one allele for each gene.

Monohybrid and Dihybrid Crosses

• Monohybrid Cross: Involves one gene; F2 generation shows a 3:1 ratio of dominant to recessive phenotypes.

• Dihybrid Cross: Involves two genes; F2 generation shows a 9:3:3:1 ratio, illustrating independent assortment.

Equation:

$\text{Phenotypic Ratio (Monohybrid)} = 3:1$

$\text{Phenotypic Ratio (Dihybrid)} = 9:3:3:1$

Test Cross

• Used to determine the genotype of an individual with a dominant phenotype by crossing with a homozygous recessive individual.

Penetrance and Expressivity

• Penetrance: The proportion of individuals with a particular genotype that actually displays the associated phenotype.

• Expressivity: The degree to which a genotype is expressed in an individual.

Non-Mendelian Inheritance Patterns

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single phenotype, resulting in continuous variation.

• Examples: Height, skin color in humans.

• Traits are quantitative and often follow a normal distribution in populations.

• Human height is influenced by ~180 genes.

• Environmental factors (e.g., nutrition, sunlight) can also affect polygenic traits.

Skin Color Example

• Skin color is determined by three genes encoding melanin-producing enzymes.

• Genes show incomplete dominance; each dominant allele adds to skin darkness.

Number of Dark Skin Alleles	Phenotype
0	Lightest
1	Very light
2	Light
3	Medium-light
4	Medium
5	Medium-dark
6	Darkest

Human Mendelian Traits and Pedigree Analysis

Pedigree Analysis

• Used to trace inheritance patterns of traits through family generations.

• Helps determine if a trait is dominant or recessive.

Dominant Traits

• Appear more often in progeny.

• Offspring with the trait always have a parent with the trait.

Recessive Traits

• Appear less often in progeny.

• Offspring with the trait may have parents who do not display the trait.

Examples of Dominant and Recessive Traits

Dominant	Recessive
Baldness	No hair loss
Right-handedness	Left-handedness
Astigmatism	Normal vision
Brown eyes	Blue eyes
Dimples	No dimples
Detached earlobes	Attached earlobes
Tongue-rolling	Can't roll tongue

Non-Physical Dominant and Recessive Traits

Dominant	Recessive
Immunity to poison ivy	Rash with poison ivy
High blood pressure	Normal/low blood pressure
A & B blood type	O blood type
Susceptible to migraines	No migraines
Tone deafness	Normal hearing
Tasting PTC	Cannot taste PTC

Genetic Disorders

Recessively Inherited Disorders

• Caused by homozygous recessive genotypes.

• Heterozygotes are carriers (have one copy of the recessive allele but do not show the phenotype).

• Examples: Cystic fibrosis, Sickle-cell disease, Tay Sachs disease.

Dominantly Inherited Disorders

• Caused by dominant alleles; heterozygotes express the disorder.

• Example: Achondroplasia (dwarfism).

• Other examples: Huntington disease, Marfan syndrome.

Sex-Linked Inherited Disorders

• Linked to genes on sex chromosomes (X or Y).

• Examples: Haemophilia (X-linked recessive), Rett syndrome (X-linked dominant).

Lethal Alleles

• Some alleles are lethal in the homozygous state.

• Dominant lethal alleles are rare; often not passed on due to early death (unless onset is after reproductive age).

• Recessive lethal alleles cause death if homozygous; can result in miscarriage or later-life disease.

• Example: Phenylketonuria (PKU)—homozygous recessive individuals lack an enzyme to break down phenylalanine.

Summary Table: Types of Inheritance and Disorders

Inheritance Type	Example Trait/Disorder	Key Features
Mendelian (single gene)	Tongue rolling, PTC tasting	Clear dominant/recessive patterns
Polygenic	Height, skin color	Continuous variation, multiple genes
Sex-linked	Haemophilia	Linked to X or Y chromosome
Dominant disorder	Achondroplasia	One copy of allele causes disorder
Recessive disorder	Cystic fibrosis	Two copies needed for disorder
Lethal allele	PKU	Homozygous state is fatal

Additional info: Polygenic traits and environmental effects are not strictly Mendelian but are important for understanding real-world inheritance patterns. Pedigree analysis is a key tool in human genetics due to ethical constraints on experimental crosses.