BIO 311D Exam 1 Study Guide: Genetics, Inheritance, Evolution, and Speciation

Transmission of Genetic Information

Mitosis and Meiosis

Genetic information is passed from one generation to the next through the processes of mitosis, meiosis, and fertilization. Understanding these processes is fundamental to genetics and cell biology.

• Mitosis: A type of cell division that results in two daughter cells, each genetically identical to the parent cell. Occurs in somatic (body) cells for growth and repair.

• Meiosis: A specialized form of cell division that reduces the chromosome number by half, producing four genetically unique gametes (sperm or egg cells). Occurs in germ cells for sexual reproduction.

• Chromosome Arrangement: In mitosis, chromosomes line up singly; in meiosis I, homologous chromosomes pair and separate, while in meiosis II, sister chromatids separate.

• Chromosome Number: Mitosis maintains the chromosome number (diploid to diploid), while meiosis reduces it (diploid to haploid).

• Genetic Variation: Meiosis introduces genetic variation through independent assortment and recombination (crossing over).

• Asexual vs. Sexual Reproduction: Asexual reproduction produces genetically identical offspring; sexual reproduction increases genetic diversity, which can be advantageous in changing environments.

Example: Human somatic cells (body cells) have 46 chromosomes (diploid), while gametes (sperm/egg) have 23 chromosomes (haploid).

The Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• G1 Phase: Cell grows and carries out normal functions.

• S Phase: DNA is replicated; each chromosome now consists of two sister chromatids.

• G2 Phase: Cell prepares for division.

• Mitosis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Key Structures: Chromosome (single DNA molecule), sister chromatids (identical copies), homologous pairs (chromosomes with the same genes, one from each parent).

Ploidy and Chromosome Number

• Diploid (2n): Two sets of chromosomes (e.g., human somatic cells, 2n = 46).

• Haploid (n): One set of chromosomes (e.g., human gametes, n = 23).

• Reduction in Meiosis: Meiosis reduces chromosome number from diploid to haploid.

Key Terms and Relationships

• Chromosome: DNA molecule with associated proteins.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Allele: Different versions of a gene.

• Gene: Segment of DNA coding for a protein.

• Locus: Physical location of a gene on a chromosome.

Stages of Mitosis and Meiosis

• Mitosis Stages: Prophase, Metaphase, Anaphase, Telophase.

• Meiosis Stages: Meiosis I (homologous chromosomes separate), Meiosis II (sister chromatids separate).

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I of meiosis, increasing genetic diversity.

Genetic Variation Mechanisms

• Independent Assortment: Random distribution of homologous chromosomes during meiosis I. Number of possible combinations: $2^n$, where n = number of homologous pairs.

• Recombination: Crossing over creates new allele combinations.

Mendelian Inheritance and Chromosome Linkage

Basic Principles

Mendelian genetics explains how traits are inherited through discrete units called genes. Chromosome linkage and recombination further influence inheritance patterns.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes on different chromosomes assort independently during gamete formation.

• Punnett Squares: Tools to predict offspring genotypes and phenotypes from parental crosses.

• Monohybrid Cross: Involves one gene; Dihybrid Cross: Involves two genes.

Dominance and Molecular Basis

• Dominant Traits: Often due to gain of function mutations.

• Recessive Traits: Often due to loss of function mutations.

• Dominance ≠ Prevalence: A dominant trait is not necessarily more common in a population.

Genetic Linkage and Recombination

• Linked Genes: Genes located close together on the same chromosome tend to be inherited together.

• Crossing Over: Can break linkage and create new recombinant genotypes.

• Map Unit (centimorgan): A unit of measure for genetic linkage; 1 map unit = 1% recombination frequency.

Non-Mendelian Inheritance

• Incomplete Dominance: Heterozygotes show an intermediate phenotype.

• Codominance: Both alleles are fully expressed (e.g., ABO blood types).

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

• Epistasis: One gene affects the expression of another gene (e.g., coat color in mice).

Pedigrees and Human Genetics

• Pedigree Analysis: Used to infer inheritance patterns (autosomal dominant, autosomal recessive, sex-linked).

• Genetic Disorders: Examples include Down syndrome (trisomy 21), caused by nondisjunction.

• Sex Determination: In mammals, XX = female, XY = male.

• Sex-Linked Genes: Genes located on sex chromosomes, often X-linked; use specific notation (e.g., XAXa).

Microevolution and Natural Selection

Gene Pool and Hardy-Weinberg Equilibrium

Microevolution refers to changes in allele frequencies within a population over time. The gene pool is the total collection of alleles in a population.

• Allele Frequency: Proportion of a specific allele among all alleles for a gene in a population.

• Genotype Frequency: Proportion of a specific genotype among all individuals in a population.

• Hardy-Weinberg Equation: Predicts genotype frequencies under no evolution:

$p^2 + 2pq + q^2 = 1$

$p + q = 1$

• Assumptions: No mutation, random mating, no gene flow, infinite population size, no selection.

Mechanisms of Microevolution

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Gene Flow: Movement of alleles between populations.

• Mutation: Source of new genetic variation.

• Nonrandom Mating: Alters genotype frequencies.

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype.

• Disruptive Selection: Favors both extreme phenotypes.

• Stabilizing Selection: Favors intermediate phenotypes.

Balancing Selection

• Heterozygote Advantage: Heterozygotes have higher fitness than either homozygote.

• Frequency-Dependent Selection: Fitness depends on how common a phenotype is.

Sexual Selection

• Intrasexual Selection: Competition among individuals of one sex.

• Intersexual Selection: Mate choice by the opposite sex.

Adaptive Radiation

• Rapid evolution of diversely adapted species from a common ancestor.

Speciation and Macroevolution

Species Concepts and Reproductive Isolation

Speciation is the process by which new species arise. The biological species concept defines species as groups of interbreeding natural populations that are reproductively isolated from other such groups.

• Limitations: Not applicable to asexual organisms or fossils; morphological and genetic differences may not always indicate reproductive isolation.

Mechanisms of Reproductive Isolation

• Prezygotic Barriers: Prevent mating or fertilization.

  • Habitat isolation

  • Temporal isolation

  • Behavioral isolation

  • Mechanical isolation

  • Gametic isolation

• Postzygotic Barriers: Prevent hybrid offspring from developing into viable, fertile adults.

  • Reduced hybrid viability

  • Reduced hybrid fertility

  • Hybrid breakdown

Modes of Speciation

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often via polyploidy, habitat differentiation, or sexual selection.

Polyploidy

• Definition: Condition in which an organism has extra sets of chromosomes.

• Role in Evolution: Common in plants, rare in animals; can lead to instant speciation.

Evidence for Evolution

• Homologous Structures: Similar structures due to shared ancestry.

• Analogous Structures: Similar function, different ancestry.

• Vestigial Structures: Remnants of features that served a function in ancestors.

Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Chromosome Number in Daughter Cells	Diploid (2n)	Haploid (n)
Genetic Identity	Identical to parent	Genetically unique
Role	Growth, repair	Sexual reproduction

Table: Prezygotic and Postzygotic Barriers

Barrier Type	Mechanism	Example
Prezygotic	Habitat isolation	Two species occupy different habitats
Prezygotic	Temporal isolation	Species breed at different times
Prezygotic	Behavioral isolation	Different courtship rituals
Prezygotic	Mechanical isolation	Incompatible reproductive structures
Prezygotic	Gametic isolation	Gametes cannot fuse
Postzygotic	Reduced hybrid viability	Hybrid offspring do not develop properly
Postzygotic	Reduced hybrid fertility	Hybrids are sterile
Postzygotic	Hybrid breakdown	Offspring of hybrids are weak or sterile

Additional info: These notes synthesize and expand upon the study guide's learning objectives, providing definitions, examples, and tables for clarity. For practice, students should apply these concepts to case studies and problem-solving exercises as suggested in the original guide.