Chapter 14: Mendelian Genetics and Extensions

Genetics: Foundations and Terminology

Key Terms in Genetics

Genetics is the study of heredity and the variation of inherited characteristics. Understanding genetics requires mastery of specific terminology, including gene, allele, genotype, phenotype, homozygous, and heterozygous.

• Gene: A segment of DNA that encodes information for a specific trait.

• Allele: Different forms of a gene found at the same locus on homologous chromosomes.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, or aa).

• Phenotype: The observable physical or physiological traits of an organism.

• Homozygous: Having two identical alleles for a gene (e.g., AA or aa).

• Heterozygous: Having two different alleles for a gene (e.g., Aa).

Theories of Inheritance: Historical Perspectives

Early Theories and Observations

Before Mendel, several theories attempted to explain inheritance. Early microscopists like Antoni van Leeuwenhoek described sperm, and Hartsoeker illustrated the homunculus theory, which posited a miniature human inside sperm.

• Blending Inheritance: Suggested offspring are a blend of parental traits.

• Pangenesis (Darwin): Proposed that all parts of the body contribute to gametes.

Gregor Mendel: The Father of Modern Genetics

Mendel's Approach and Model Organism

Gregor Mendel, an Austrian monk, revolutionized genetics by applying mathematics and controlled experiments using garden pea plants (Pisum sativum).

• Used true-breeding plants to ensure consistent traits across generations.

• Selected peas for their ease of growth, short generation time, and distinct traits.

Mendel's Experiments and Discoveries

Monohybrid Crosses and the Principle of Segregation

Mendel performed controlled crosses between plants with contrasting traits and analyzed the resulting generations.

• Experiment 1: Crossed true-breeding purple and white flowers. All F1 offspring were purple.

• Experiment 2: Self-fertilized F1 plants. The F2 generation showed a 3:1 ratio of purple to white flowers.

• Conclusion: The white trait was masked (recessive), not lost.

Dominant and Recessive Traits

Traits can be dominant or recessive. Dominant alleles mask the expression of recessive alleles in heterozygotes.

• Dominant (A): Expressed in both homozygous (AA) and heterozygous (Aa) individuals.

• Recessive (a): Expressed only in homozygous recessive (aa) individuals.

Particulate Inheritance and Principle of Segregation

Mendel proposed that hereditary "factors" (now called genes) are discrete and maintain their integrity across generations. Each individual has two alleles per gene, which segregate during gamete formation.

• Each gamete receives only one allele for each gene.

• Fertilization restores the diploid state.

Genotype and Phenotype

The genotype determines the phenotype. For a single gene with two alleles:

Genotype	Phenotype
AA	Dominant trait
Aa	Dominant trait
aa	Recessive trait

Allelic Variation and Mutation

Alleles differ due to mutations affecting gene expression or protein structure. Dominant alleles often produce functional proteins, while recessive alleles may result from loss-of-function mutations.

• Example: In Mendel's peas, white flowers (pp) cannot produce purple pigment, while heterozygotes (Pp) can.

Punnett Squares and Genetic Predictions

Using Punnett Squares

Punnett squares are tools to predict the genotypes and phenotypes of offspring from parental crosses.

• List all possible gametes from each parent.

• Combine gametes to determine offspring genotypes.

Dihybrid Crosses and Independent Assortment

Principle of Independent Assortment

Mendel extended his analysis to two traits at once (dihybrid crosses). He found that alleles of different genes assort independently during gamete formation, leading to new combinations of traits.

• Example: Crossing YYRR (yellow, round) with yyrr (green, wrinkled) yields F1 heterozygotes (YyRr).

• F2 generation shows a 9:3:3:1 phenotypic ratio.

Phenotype	Fraction
Yellow round (Y_R_)	9/16
Yellow wrinkled (Y_rr)	3/16
Green round (yyR_)	3/16
Green wrinkled (yyrr)	1/16

Testcrosses

Determining Unknown Genotypes

A testcross involves crossing an individual with a dominant phenotype (unknown genotype) with a homozygous recessive individual to determine the unknown genotype.

Chromosomal Basis of Mendel's Principles

Meiosis and Segregation

Mendel's principles are explained by the behavior of chromosomes during meiosis. Homologous chromosomes (and thus alleles) segregate during Anaphase I, and different chromosome pairs assort independently.

Extensions to Mendelian Genetics

Incomplete Dominance

In incomplete dominance, heterozygotes display an intermediate phenotype between the two homozygotes.

Multiple Allelism and Codominance

Many genes have more than two alleles in a population. Codominance occurs when both alleles are expressed equally in the phenotype, as seen in human ABO blood types.

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. For example, the sickle cell allele affects hemoglobin structure and malaria resistance.

Gene Interactions and Polygenic Traits

Epistasis describes interactions where one gene's expression depends on another gene. Polygenic traits are controlled by multiple genes, resulting in continuous variation (e.g., human skin color).

Human Genetics and Pedigree Analysis

Studying Human Inheritance

Human genetic studies use pedigrees to trace inheritance patterns, as controlled breeding is unethical and impractical in humans.

Inheritance Pattern	Description	Examples
Autosomal Dominant	One mutant allele causes disorder; appears in every generation	Huntington Disease
Autosomal Recessive	Two mutant alleles required; may skip generations	Cystic Fibrosis
X-linked Recessive	More common in males; females need two mutant alleles	Hemophilia A
X-linked Dominant	Rare; both sexes affected, but inheritance patterns differ	Hypophosphatemia

Pedigree Symbols

• Circle: Female

• Square: Male

• Shaded: Affected individual

• Half-shaded: Carrier (for recessive traits)

Summary Table: Mendelian vs. Non-Mendelian Inheritance

Type	Key Features	Example
Mendelian (Simple)	One gene, two alleles, dominant/recessive	Purple/white pea flowers
Incomplete Dominance	Heterozygote intermediate phenotype	Pink four-o'clock flowers
Codominance	Both alleles expressed	AB blood type
Pleiotropy	One gene, multiple effects	Sickle cell disease
Polygenic	Multiple genes, continuous variation	Human skin color

Additional info: This guide covers the core principles of Mendelian genetics, their chromosomal basis, and important extensions, providing a foundation for understanding inheritance in plants, animals, and humans.