Chapter 11: Mendel and the Genetics of Inheritance

Learning Objectives

• Describe the inheritance of genes, including dominant and recessive traits, and the law of independent assortment.

• Use Punnett squares for monohybrid and dihybrid crosses to predict genotypic and phenotypic ratios.

• Apply the addition and multiplication rules to predict probabilities of genetic outcomes.

• Explain non-Mendelian inheritance patterns, including incomplete dominance, codominance, epistasis, and polygenic inheritance.

• Interpret pedigree data to determine inheritance patterns.

Pre-Mendel Theories of Heredity

Historical Theories

• Pangenesis: Proposed that "seeds" are produced by all parts of the body and collected in reproductive organs (Ancient Greece, ~400 B.C.E.).

• Preformation: Suggested that sperm or egg cells contained a miniature human (homunculus) that developed into a full organism.

• Blending Inheritance: Hypothesized that heritable traits blend together in offspring and change gradually over generations.

Mendel's Empirical Approach

• Gregor Mendel (1822–1884) used a scientific, empirical approach to study heredity in pea plants during the 1850s and 1860s.

• He performed controlled crosses and analyzed the inheritance of specific traits.

Mendel's Experimental Design

Pea Plant Crosses

• Self-fertilization: Pea plants naturally self-pollinate, allowing for true-breeding lines.

• Cross-fertilization: Mendel transferred pollen from one plant to another to create hybrids.

• Traits Studied: Mendel focused on clear, contrasting traits (e.g., flower color, seed shape, pod color).

Key Terms

• Character: A heritable feature (e.g., flower color).

• Trait: A specific variant of a character (e.g., purple or white flowers).

• True-breeding: Plants that produce offspring of the same trait when self-pollinated.

Monohybrid Crosses

Experimental Results

• Crossed true-breeding plants with contrasting traits (e.g., purple x white flowers).

• F1 generation: All offspring showed only one parental trait (dominant).

• F2 generation: Both traits reappeared in a 3:1 ratio (dominant:recessive).

Key Concepts

• Dominance: The trait that appears in the F1 generation is dominant.

• Recessiveness: The trait that is masked in the F1 but reappears in the F2 is recessive.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.

Punnett Square Example (Monohybrid Cross)

• Parental genotypes: TT (tall) x tt (dwarf)

• F1 genotype: All Tt (heterozygous)

• F2 genotypes: 1 TT : 2 Tt : 1 tt

• F2 phenotypes: 3 tall : 1 dwarf

Dihybrid Crosses and the Law of Independent Assortment

Experimental Results

• Crossed plants differing in two traits (e.g., seed shape and color).

• F2 generation showed a 9:3:3:1 phenotypic ratio, supporting the Law of Independent Assortment: alleles of different genes assort independently during gamete formation.

Punnett Square Example (Dihybrid Cross)

• Parental genotypes: RRYY x rryy

• F1 genotype: All RrYy

• F2 phenotypes: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Probability in Genetics

Rules of Probability

• Addition Rule: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

• Multiplication Rule: The probability of two independent events both occurring is the product of their individual probabilities.

Example Calculations

• Probability of a specific genotype in offspring:

• Probability of at least one outcome: (if mutually exclusive)

Extensions of Mendelian Genetics

Incomplete Dominance

• Neither allele is completely dominant; heterozygotes have an intermediate phenotype.

• Example: Red (RR) x White (rr) snapdragons produce Pink (Rr) offspring.

Codominance

• Both alleles are fully expressed in heterozygotes.

• Example: ABO blood group system—IA and IB alleles are codominant, resulting in AB blood type.

Multiple Alleles

• More than two alleles exist for a gene in a population.

• Example: ABO blood group has three alleles (IA, IB, i).

Epistasis

• One gene affects the expression of another gene.

• Example: Coat color in Labrador retrievers—one gene determines pigment color, another gene determines pigment deposition.

Polygenic Inheritance

• Multiple genes independently affect a single trait, resulting in continuous variation.

• Example: Human skin color is influenced by several genes, each contributing to the overall phenotype.

Pedigree Analysis

Pedigree Symbols and Interpretation

• Pedigrees are diagrams that show the inheritance of traits across generations in families.

• Symbols: Circles represent females, squares represent males, shaded shapes indicate affected individuals.

• Used to determine whether a trait is dominant, recessive, autosomal, or sex-linked.

Table: Pedigree Symbols

Human Genetic Disorders

Autosomal Dominant Disorders

• Achondroplasia: A form of dwarfism caused by a dominant allele. Homozygous dominant genotype is often lethal.

Autosomal Recessive Disorders

• Sickle-cell disease: Caused by a recessive allele affecting hemoglobin structure. Heterozygotes (carriers) are usually healthy but may have some symptoms.

• Hereditary hemochromatosis: An autosomal recessive disorder causing iron overload. Carriers do not show symptoms.

Table: Sickle-cell Genotypes and Phenotypes

Key Terms and Definitions

• Gene: A segment of DNA that encodes a functional product, usually a protein.

• Allele: Different versions of a gene.

• Genotype: The genetic makeup of an organism (the alleles present).

• Phenotype: The observable traits of an organism.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• P generation: Parental generation in a genetic cross.

• F1 generation: First filial generation, offspring of the P generation.

• F2 generation: Second filial generation, offspring of the F1 generation.

• Monohybrid cross: A cross between individuals differing in one trait.

• Dihybrid cross: A cross between individuals differing in two traits.

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard biology curriculum.