Mendelian Genetics and the Modern Evolutionary Synthesis

Introduction

This study guide covers the foundational concepts of inheritance, from early theories to Mendel's discoveries and their integration into modern evolutionary biology. It is essential for understanding how genetic information is transmitted and how populations evolve over time.

Theories of Inheritance

Blending vs. Particulate Inheritance

Early theories of inheritance attempted to explain how traits are passed from parents to offspring. Two main models were proposed: blending inheritance and particulate inheritance.

• Blending Inheritance: Traits in offspring are an average (blend) of parental traits. Darwin's pangenesis suggested that all parts of the body contribute hereditary information ("gemmules") to the offspring.

• Problems with Blending: Blending would dilute variation over generations, making natural selection ineffective. Experiments by Francis Galton contradicted this model.

• Particulate Inheritance: Traits are inherited as discrete units (later called genes), preserving variation across generations.

Weismann's Germ Plasm Theory

August Weismann proposed a distinction between germ cells (reproductive cells) and somatic cells (body cells), arguing that only germ cells transmit hereditary information.

• Germ Plasm: The hereditary material is confined to germ cells in the gonads; somatic cells do not transmit genetic information to offspring.

• Somatic Cells: Carry out bodily functions but are not involved in heredity.

• Implications: Acquired characteristics in somatic cells are not inherited.

Diagram: Germ plasm is (potentially) immortal, while somatic cells are disposable and regenerated each generation.

Exceptions and Extensions

• In some organisms (e.g., plants, corals, sponges), germ cells are not segregated early, and both germ and somatic cells can arise from stem cells.

• Mutations in these cells can be transmitted to offspring.

Mendelian Inheritance of Simple Traits

Gregor Mendel and His Experiments

Gregor Mendel, considered the founder of modern genetics, conducted experiments with pea plants to uncover the principles of heredity.

• Experimental Design: Used true-breeding plants with discrete traits, controlled crosses, and statistical analysis.

• Key Observations: Traits did not blend but appeared in predictable ratios in offspring.

Mendel's Laws

• Law of Dominance: In heterozygotes, one allele (dominant) masks the effect of the other (recessive).

• Law of Segregation: Alleles for a trait separate during gamete formation, so each gamete carries only one allele for each gene.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes.

Modern Genetic Terminology

• Gene: A sequence of DNA that codes for a product (RNA or protein).

• Locus: The specific location of a gene on a chromosome.

• Allele: Different versions of a gene at a given locus.

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

• Phenotype: The observable traits of an organism.

• Homozygote: An individual with two identical alleles at a locus.

• Heterozygote: An individual with two different alleles at a locus.

Punnett Squares and Probability

Punnett squares are used to predict the genotypic and phenotypic ratios of offspring from genetic crosses.

• Product Rule: Probability of independent events occurring together is the product of their individual probabilities.

• Sum Rule: Probability of mutually exclusive events is the sum of their individual probabilities.

Example: For a monohybrid cross (Aa x Aa), the probability of AA offspring is .

Complex Patterns of Inheritance

Beyond Simple Dominance

• Incomplete Dominance: Heterozygotes show an intermediate phenotype.

• Codominance: Both alleles are fully expressed in the phenotype.

• Pleiotropy: One gene affects multiple traits.

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood types).

• Physical Linkage: Genes close together on a chromosome tend to be inherited together.

• Recombination: Crossing over during meiosis can separate linked genes, increasing genetic diversity.

• Epistasis: One gene affects the expression of another gene.

• Polygenic Traits: Traits controlled by multiple genes, often showing continuous variation (e.g., height, skin color).

• Environmental Effects: The environment can influence the expression of genetic traits.

Population Genetics and the Hardy-Weinberg Principle

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model to study genetic variation in populations under ideal conditions.

• Equation:

• p: Frequency of one allele (e.g., A)

• q: Frequency of the other allele (e.g., a)

• p + q = 1$ (sum of allele frequencies)

• Genotype Frequencies:

  • = frequency of homozygous dominant (AA)

  • = frequency of heterozygotes (Aa)

  • = frequency of homozygous recessive (aa)

Assumptions of Hardy-Weinberg Equilibrium

• Random mating

• No mutation

• No migration (gene flow)

• Large population size (no genetic drift)

• No natural selection

If observed genotype frequencies deviate from Hardy-Weinberg expectations, it suggests that one or more assumptions are not met, indicating evolutionary processes at work.

Calculating Allele and Genotype Frequencies

Given observed numbers of genotypes in a population, allele frequencies can be calculated as follows:

• Allele Frequency (A):

• Allele Frequency (a):

Example Table: Calculating Genotype and Allele Frequencies

Allele Frequencies:

• A:

• a:

Applications and Reverse Calculations

• Hardy-Weinberg can be used to estimate carrier frequencies for recessive diseases.

• If the frequency of a recessive phenotype is known, the allele frequency can be estimated as .

Example: If 1 in 12,000 babies is born with a recessive disorder, .

Summary Table: Key Terms and Concepts

Conclusion

Understanding Mendelian genetics and the Hardy-Weinberg principle is fundamental to the study of inheritance and evolution. These concepts provide the basis for analyzing genetic variation within populations and predicting how traits are transmitted across generations.