Mitosis & Meiosis

Chromosome Structure and Terminology

Understanding chromosome structure and terminology is fundamental to genetics. Chromosomes are the carriers of genetic information, and their structure and nomenclature are key to cell division processes.

• Chromosome: A DNA molecule with associated proteins, visible during cell division.

• Chromatin: The complex of DNA and proteins (mainly histones) found in the nucleus; condenses to form chromosomes during mitosis/meiosis.

• Chromatid: One of two identical halves of a replicated chromosome; sister chromatids are joined at the centromere.

• Centromere Placement: Determines chromosome type (metacentric, submetacentric, acrocentric, telocentric).

• p/q Arms: The short arm (p) and long arm (q) of a chromosome, divided by the centromere.

Example: Human chromosome 1 is metacentric, with nearly equal p and q arms.

Chromosome Number, Ploidy, and Homologs

Chromosome number and ploidy are essential concepts in genetics, describing the genetic content of cells.

• Diploid (2n): Cells with two sets of chromosomes (one from each parent).

• Haploid (n): Cells with one set of chromosomes (gametes).

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Locus (plural: loci): Specific location of a gene on a chromosome.

• Allele: Different forms of a gene at a locus.

Example: Humans have 46 chromosomes (23 pairs); gametes have 23 chromosomes.

Cell Cycle and Stages

The cell cycle consists of interphase (G1, S, G2) and mitotic phase (mitosis and cytokinesis). Each stage has distinct events and durations.

• G1: Cell growth and preparation for DNA replication.

• S: DNA synthesis (replication).

• G2: Preparation for mitosis.

• Mitosis: Division of the nucleus; shortest stage.

Example: Most cells spend the majority of their time in interphase.

Stages of Mitosis and Meiosis

Mitosis and meiosis are processes of cell division with distinct purposes and mechanisms.

• Mitosis: Prophase, Metaphase, Anaphase, Telophase; produces two identical diploid cells.

• Meiosis I: Prophase I (with 5 substages), Metaphase I, Anaphase I, Telophase I; reductional division.

• Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II; equational division.

Comparison: Mitosis maintains chromosome number; meiosis halves it.

Purpose and Outcomes of Mitosis and Meiosis

Mitosis and meiosis serve different biological functions.

• Mitosis: Growth, repair, asexual reproduction; produces genetically identical cells.

• Meiosis: Sexual reproduction; produces four genetically unique haploid gametes.

Mechanisms: In mitosis, sister chromatids line up and segregate; in meiosis, homologous chromosomes pair and segregate.

Genetic Material Changes During Meiosis

Meiosis involves changes in genetic content and ploidy.

• Copy Number ("x"): Refers to the number of DNA copies.

• Ploidy Number ("n"): Refers to the number of chromosome sets.

• Transition Points: Reductional division (Meiosis I) reduces ploidy; equational division (Meiosis II) separates chromatids.

Example: After Meiosis I, cells are haploid but have duplicated chromosomes.

Prophase I Substages and Crossing Over

Prophase I of meiosis I is subdivided into five stages, with crossing over occurring during pachytene.

• Leptotene: Chromosomes condense.

• Zygotene: Synapsis begins; synaptonemal complex forms.

• Pachytene: Crossing over (recombination) occurs between non-sister chromatids.

• Diplotene: Chiasmata visible; homologs begin to separate.

• Diakinesis: Chromosomes fully condensed; nuclear envelope breaks down.

Key Terms: Crossing over, chiasma, synapsis, synaptonemal complex, recombination.

Reductional vs. Equational Division

Meiosis consists of two divisions with distinct outcomes.

• Reductional Division: Meiosis I; reduces chromosome number from diploid to haploid.

• Equational Division: Meiosis II; separates sister chromatids, maintaining haploid number.

Significance: Ensures genetic diversity and correct chromosome number in gametes.

Spermatogenesis vs. Oogenesis

Gamete formation differs between males and females.

• Spermatogenesis: Produces four sperm from each precursor cell.

• Oogenesis: Produces one egg and three polar bodies due to unequal cytokinesis.

Example: Only one functional ovum is produced per meiosis in females.

Mendelian Genetics

Basic Mendelian Crosses

Mendelian crosses involve predicting genotypes and phenotypes of offspring from parental crosses.

• Monohybrid Cross: One gene; typical F2 ratio is 3:1 (phenotype).

• Dihybrid Cross: Two genes; typical F2 ratio is 9:3:3:1.

• Trihybrid Cross: Three genes; more complex ratios.

• Punnett Square: Visual tool for predicting offspring.

• Forked Line Method: Useful for multi-gene crosses.

Example: Crossing Aa x Aa yields 1 AA : 2 Aa : 1 aa (genotype).

Backward Problems and Test Crosses

Backward problems require deducing parental genotypes from offspring data. Test crosses are used to determine unknown genotypes.

• Test Cross: Cross with homozygous recessive to reveal genotype.

• Expected Ratios: For Aa x aa, expect 1:1 ratio.

Mendel’s Four Postulates

Mendel’s principles form the foundation of classical genetics.

• Postulate 1: Unit factors exist in pairs.

• Postulate 2: Dominance and recessiveness.

• Postulate 3: Segregation of unit factors during gamete formation.

• Postulate 4: Independent assortment of different genes.

Useful Rules and Probability in Mendelian Crosses

Probability rules simplify genetic calculations.

• Sum Rule: Probability of either of two mutually exclusive events.

• Product Rule: Probability of two independent events both occurring.

• Conditional Probability: Probability of an event given another event.

Example: Probability of Aa offspring from Aa x Aa is .

Binomial Theorem in Genetics

The binomial theorem is used to calculate probabilities of combinations in genetic crosses.

• Equation:

• Variables: n = total trials, x = number of successes, p = probability of success, q = probability of failure.

• Pascal’s Triangle: Used to determine binomial coefficients.

Example: Probability of 2 Aa in 4 offspring:

Pedigree Analysis

Pedigrees are used to track inheritance patterns of traits.

• Dominant Traits: Usually appear in every generation.

• Recessive Traits: May skip generations.

Extensions of Mendelian Genetics

Modifying Mendelian Assumptions

Extensions of Mendelian genetics arise when classical assumptions are altered, leading to modified ratios and inheritance patterns.

• Incomplete Dominance: Heterozygote shows intermediate phenotype.

• Co-dominance: Both alleles are fully expressed in heterozygote.

• Multiple Alleles: More than two alleles exist for a gene.

• Lethal Alleles: Some alleles cause death when present in certain genotypes.

• Pleiotropy: One gene affects multiple traits.

Example: ABO blood group system (multiple alleles, co-dominance).

Sex Linkage and Reciprocal Crosses

Genes located on sex chromosomes exhibit unique inheritance patterns.

• X-linked Genes: Notated as XA or Xa; males are hemizygous.

• Reciprocal Crosses: Switching parental sexes to reveal inheritance patterns.

Example: Color blindness in humans is X-linked.

Gene Interactions

Multiple genes can interact to influence a single trait, leading to non-standard ratios.

• Epistasis: One gene masks the effect of another.

• Complementation: Two genes required for a trait; mutation in either disrupts trait.

• Additive: Effects of genes add together.

• Suppression: One gene suppresses the effect of another.

• Redundancy: Multiple genes can compensate for each other.

• Novel/Synthetic: New phenotype arises from gene interaction.

Example: 9:7 ratio in F2 suggests complementation.

Short Cut Methods for Modified Ratios

Modified ratios can be built from 9:3:3:1 blocks if certain assumptions are met.

• Assumptions: Genes assort independently; no linkage.

• Method: Use interaction worksheets to build ratios.

Polygenic/Quantitative Inheritance

Types of Polygenic Inheritance

Polygenic traits are governed by multiple genes, leading to continuous variation.

• Quantitative Traits: Measurable, continuous (e.g., height).

• Meristic Traits: Countable, discrete (e.g., number of seeds).

• Threshold Traits: Expressed only when a certain threshold is reached (e.g., disease susceptibility).

Discontinuous vs. Continuous Traits

Traits can be classified based on their variation.

• Discontinuous: Distinct categories (e.g., blood type).

• Continuous: Range of values (e.g., height).

Multiple Gene Hypothesis and Additive Alleles

The multiple gene hypothesis explains quantitative inheritance as the result of additive effects of multiple genes.

• Equation for Extreme Phenotypes: (n = number of genes)

• Maximum Contributing Alleles:

• Binomial Coefficient: Used to calculate frequency of phenotypic categories.

Example: If 3 genes control a trait, there are extreme phenotypes.

Heritability

Heritability measures the proportion of phenotypic variation due to genetic factors.

• Broad-sense Heritability (H2):

• VG: Genetic variance

• VP: Total phenotypic variance

Interpretation: Higher H2 means more variation is genetic.

Linkage and Mapping

Outcomes of Meiosis and Linkage

Linkage affects the assortment of genes during meiosis.

• Independent Assortment: Genes on different chromosomes assort independently.

• Complete Linkage: Genes close together on same chromosome inherited together.

• Linkage with Crossing Over: Recombinant gametes produced by crossing over.

• Linkage Group: Set of genes inherited together.

Genetic Mapping and Recombination Frequency

Genetic mapping uses recombination frequencies to estimate distances between genes.

• Recombination Frequency (RF):

• Map Units/CentiMorgans (cM): 1 cM = 1% recombination.

Example: RF of 20% = 20 cM apart.

Crossing Over and Double Crossovers

Crossing over produces recombinant chromosomes; double crossovers can occur between three genes.

• NCO: Non-crossover (parental type)

• SCO: Single crossover

• DCO: Double crossover

Example: In a three-point cross, DCOs help determine gene order.

Mapping Three Genes

Three-point crosses allow determination of gene order and distances.

• Parental Haplotypes: Original combinations of alleles.

• Gene Order: Determined by comparing DCOs to parental types.

• Distance Calculation: Use RF between pairs of genes.

Interference and Coefficient of Coincidence

Interference measures the effect of one crossover on the likelihood of another nearby crossover.

• Coefficient of Coincidence (C):

• Interference (I):

Interpretation: I > 0 means fewer DCOs than expected.

Non-Mendelian/Extra-Nuclear Inheritance

Concepts and Types

Non-Mendelian inheritance involves genetic material outside the nucleus, such as mitochondria and chloroplasts.

• Cytoplasmic Inheritance: Genes in organelles inherited independently of nuclear genes.

• Maternal Effect: Phenotype determined by mother's genotype or phenotype.

Examples and Mechanisms

Different organisms exhibit unique patterns of extra-nuclear inheritance.

• 4 o'clock plants: Chloroplast inheritance; maternal transmission.

• Chlamydomonas: Chloroplast inheritance; biparental or uniparental.

• Neurospora (poky): Mitochondrial inheritance; heterokaryon formation.

• Yeast (petite): Three types of petite mutants; nuclear and mitochondrial genotypes.

• Ephesia larva, Limnaea: Maternal effect inheritance.

Heteroplasmy and Heterokaryon

Heteroplasmy refers to the presence of more than one type of organellar genome in a cell. Heterokaryon is a cell with multiple nuclei.

• Heteroplasmy: Leads to variable phenotypes.

• Heterokaryon: Used in Neurospora studies.

Uni- and Bi-parental Inheritance

Inheritance of organellar genes can be uniparental (usually maternal) or biparental.

• Maternal Inheritance: Most common due to egg cytoplasm.

• Biparental Inheritance: Both parents contribute organelles.

Molecular Aspects of Mitochondrial and Chloroplastic Genomes

Mitochondria and chloroplasts have their own genomes, distinct from nuclear DNA.

• Mitochondrial Genome: Circular DNA; codes for essential proteins.

• Chloroplastic Genome: Circular DNA; codes for photosynthetic proteins.

Yeast Petite Mutants

Yeast petite mutants illustrate different types of mitochondrial inheritance.

• Nuclear Petite: Mutation in nuclear gene.

• Cytoplasmic Petite: Mutation in mitochondrial DNA.

• Segregational Petite: Both nuclear and mitochondrial involvement.

Maternal Effect

Maternal effect occurs when the mother's genotype or phenotype determines the offspring's phenotype.

• Determining Individual: Usually the mother.

• Aspect Responsible: Mother's genotype or phenotype.

Inheritance Patterns in Ephesia Larva and Limnaea

Both examples illustrate maternal effect inheritance.

• Ephesia Larva: Larval phenotype determined by mother's genotype.

• Limnaea: Shell coiling direction determined by mother's genotype.