Mendelian Genetics: Principles, Patterns, and Exceptions (Chapter 14: 14.1–14.4)

Topic 10: Mendelian Genetics

Overview

This topic covers the foundational principles of Mendelian genetics, including Mendel's laws, patterns of inheritance, and notable exceptions. It also explores how these principles apply to human traits and genetic disorders.

Key Learning Objectives

• Define the F1, F2, and parental (P) generations in genetic crosses.

• Describe Mendel's four concepts of inheritance.

• Diagram the law of segregation in diploid cells.

• Explain and interpret the 3:1 ratio in monohybrid crosses and the 9:3:3:1 ratio in dihybrid crosses.

• Predict offspring probabilities using known genotypes.

• Explain the effects of penetrance and expressivity.

• Interpret pedigrees to determine genotypes.

• Identify non-Mendelian inheritance patterns and lethal alleles.

Mendel's Laws and Concepts

Mendel's Four Concepts

• Concept 1: Alternative Versions of Genes - Genes exist in different forms called alleles.

• Concept 2: Inheritance of Two Alleles - Each organism inherits two alleles, one from each parent.

• Concept 3: Dominance - If two alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

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

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 phenotypic ratio, illustrating independent assortment.

Equation:

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

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

Exceptions to Mendel's Laws

Polygenic Inheritance

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

• Examples: Height, skin color in humans.

• Quantitative Traits: These traits show a range of phenotypes and typically follow a normal distribution in the population.

• Environmental Influence: Traits like skin color can be affected by factors such as sunlight exposure.

Additional info: Human height is influenced by approximately 180 genes.

Incomplete Dominance and Codominance

• Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are expressed equally (e.g., AB blood type).

Pedigree Analysis and Human Traits

Pedigree Analysis

Pedigree analysis traces the inheritance of traits through family generations to determine dominant and recessive patterns.

• Dominant Traits: Appear more often and every affected individual has a parent with the trait.

• Recessive Traits: May skip generations; affected individuals can have parents without the trait.

Examples of Dominant and Recessive Traits

Dominant	Recessive
Baldness	No hair loss
Brown eyes	Blue eyes
Dimples	No dimples
Detached earlobes	Attached earlobes
Tongue-rolling	Can't roll tongue

Non-Visible 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

• A recessive allele often results in a protein malfunction or absence.

• Carriers: Heterozygotes carry one recessive allele but do not show the phenotype.

• Only homozygous recessive individuals express the disorder.

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

Dominantly Inherited Disorders

• Some harmful alleles are dominant (less common than recessive).

• Example: Achondroplasia (dwarfism) - heterozygotes show the phenotype.

• Other examples: Huntington disease, Marfan syndrome.

Sex-Linked Inherited Disorders

• Disorders linked to sex chromosomes (X or Y).

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

• Pedigrees can be used to trace inheritance in families.

Lethal Alleles

Dominant Lethal Alleles

• Rare; cause disease in both homozygotes and heterozygotes.

• Often not passed on if they cause death before reproductive age (e.g., Huntington disease).

• Delayed age of onset allows some dominant lethal alleles to be inherited.

Recessive Lethal Alleles

• Lethal only in homozygous recessive individuals.

• Can cause miscarriage or disease later in life.

• Example: Phenylketonuria (PKU) - lack of enzyme to break down phenylalanine; can be managed with diet and supplements.

Summary Table: Types of Inheritance Patterns

Pattern	Description	Examples
Mendelian (Dominant/Recessive)	Single gene, clear dominant/recessive relationship	Widow's peak, attached earlobes
Polygenic	Multiple genes contribute to phenotype	Height, skin color
Sex-linked	Gene located on sex chromosome	Haemophilia, color blindness
Lethal alleles	Alleles causing death in homozygous or heterozygous state	Huntington disease, PKU

Key Terms

• Allele – Alternative form of a gene.

• Genotype – Genetic makeup of an organism.

• Phenotype – Observable traits of an organism.

• Carrier – Heterozygote for a recessive disorder.

• Pedigree – Diagram showing family relationships and inheritance patterns.

• Penetrance – Proportion of individuals with a genotype who show the expected phenotype.

• Expressivity – Degree to which a genotype is expressed in an individual.