➗ Lecture 2

Overall Theme

• Survivors of 4 B years of evolution are organisms that constantly diversify genetically but are also programmed to be vulnerable to constant fitness selection

  • So diversify but select

Zygote Mitosis

• Diploid zygote undergoes rounds of mitosis to form diploid gamete progenitor cells

  • Mutations can occur during these mitotic divisions

• In higher animals, germline cells (cells that form gametes, sex cells) are set aside early in embryogenesis to reduce DNA damage from mitosis

• Eggs are formed early and stored for decades → vulnerable to environmental mutagens

• Sperm is formed continuously throughout life via mitosis

Plant Zygote Mitosis

• Diploid zygote undergoes mitosis to form diploid gamete progenitor cells, mutations can occur

• In plants, embryonic cells are at shoot apical meristem (SAM) → produces leaves, stem, branches

• Quiescent center (QC) in SAM = low-rate mitotic stem cells

• QC later converts SAM into floral meristem

• Floral meristem forms carpel (female gonad which produces egg nuclei) and stamen (male gonad which produces sperm nuclei)

• Plant germline is not set aside early → Many mitotic divisions occur before gametes form

• Allows more chances for mutations → Some can be beneficial for adaptation

Gamete Formation

• In animals: Eggs are formed during embryonic development (gestation), then stored until needed; sperm formed continuously after puberty

• In plants: Gametes are not set aside early; instead, they are formed later from the floral meristem after many mitotic divisions

Gametes in Animals vs. Plants

• Higher plants need more rounds of DNA-damaging mitosis to make gametes

• Higher animals need fewer rounds of mitosis

• Reflects difference in gamete formation process, not life strategy

  • Mechanistic difference, not strategy/survival difference

  • More mutations in plants potentially allows them to evolve at faster rate and adapt more

Meiosis

• Meiosis creates haploid gametes (sperm/eggs)

• Recombination (crossing over) occurs within homologous chromosomes from parents → Diversifies gametes

  • Chromosomes go on top of each other to “match up” areas

• Independent assortment of whole homologous chromosomes from parents → Further diversifies gametes

• Both processes increase genetic variation in offspring

Functions of Stages of Meiosis

• Task 1: Reduce homologous chromosome number from diploid → haploid

• Task 2: Chromosome recombination (crossing over) → Major driver of genetic diversity

• Task 3: Random assortment of chromosomes → arguably most important driver of genetic diversity in eukaryotes

Meiosis Process

• Original State: Diploid progenitor cell with 2 copies (homologs) of each chromosome

• Chromosome Replication: Produces sister chromatids (46 chromosomes, 92 chromatids)

• Pairing & Recombination: Maternal and paternal chromosomes pair and cross over (increases diversity)

• Meiosis I:

  • Random assortment of homologs → Massive diversity

  • Each daughter cell gets one homologous chromosome from each pair → Ensures gametes have 50% of chromosomes (23 chromosomes, but 46 chromatids)

• Meiosis II:

  • Sister chromatids separate → Each gamete gets half of each chromosome (23 chromosomes, 23 chromatids)

  • Final result: independent assortment + recombination → each sperm/egg is genetically unique genetic differences among siblings (except identical twins)

Outcome of Recombination

• Maternal and paternal alleles from adjacent loci mix and form new combinations

• Increases genetic diversity in gametes

Crossing Over

• Crossovers are relatively rare

• Involve two double-strand DNA breaks → Accurate fusion needed

• Mistakes can cause loss of entire chromosome arm

• On average: 1 crossover per chromosome per meiosis

DNA Double-Strand Breaks (DSBs) in Meiosis

• Occur in maternal and paternal chromosomes

• Repair required to restore DNA integrity

• Enables crossing over and recombination

• Tightly regulated in terms of their timing, location, and frequency to ensure accurate chromosome separation and prevent genome instability

Crossing-Over and Genetic Diversity

• Can occur nearly anywhere along a chromosome

• Happens in all autosomal chromosomes → Enormous diversification potential

  • e.g. Each human chromosome has ~1000 genes so there are >1000 loci combinations can mix maternal/paternal alleles

  • Total possible combinations across autosomes: ~1000^22 → Millions of trillions of possibilities

Sex Chromosome Recombination

• Large regions of X and Y chromosomes do not recombine

• X-X chromosomes can recombine normally

• In X-Y, non-recombination is likely because X and Y evolved from a homologous autosomal pair → Now partially non-homologous

  • It’s hypothesized that originally, X and Y were fully homologous autosomes

  • Over evolution, Y lost many genes → No longer matches X along most of its length

  • Only small regions remain homologous → Allow limited recombination

  • Rest of X-Y pair is non-homologous → Cannot undergo crossing over safely

    • Y chromosome’s job is to repress X (female) for male trait manifestation?

Why We Have Multiple Chromosome Sets

• Having all genes on one chromosome would reduce genetic diversity

• Genes on the same chromosome are genetically linked

• Linked genes have higher chance of being co-inherited after meiosis

• Multiple chromosomes allow independent assortment → Increases genetic variation

  • Sexual organisms divide genes onto multiple chromosomes → Permits independent assortment to increases genetic diversity in sperm/eggs

Generating Genetic Diversity in Meiosis

• Independent, random assortment: Entire chromosomes shuffled → Thousands of alleles mixed

• Chromosome recombination: Exchanges large chromosome chunks → Hundreds of genes mixed

• Meiosis “tinkers” with many genes → Affects enzymes they encode

• Meiosis I:

  • Random assortment of chromosomes → Generates diversity

  • Maternal and paternal chromosomes pair and recombine → Increases diversity b/w offspring

Independent Assortment

• Arguably the greatest driver of genetic diversification in higher eukaryotes

• 23 sets of human chromosomes → 2²³ ≈ 8,388,608 (~8 million) possible combinations

• A single man producing many sperm can generate massive genetic diversity

• Every person is genetically unique → Within population, there is huge diversification potential within each generation

Meiosis and Genetic Processes Across Generations

• Generation 1: DNA comes from parent’s chromosomes

• Generation 2: Processes occur in you as fetus (females) and continue today (males)

  • Females: All oocytes begin meiosis during fetal development → Paused until puberty

  • Males: Spermatogonia are formed during fetal development but meiosis doesn’t start until puberty

• Generation 3: Processes form gametes for your children

Genetic Ancestry and Gamete Fusion

• Every sexual act combines two unique gametes from maternal and paternal lineages

• Human chromosomes are a fusion of many ancestors

• 15 generations ago: DNA comes from 2¹⁵ = 32,768 ancestors (assuming no relatives)

• ~400 generations ago: Humans practiced crop agriculture → Ancestors were from different continents/environments

  • Humans began practicing crop agriculture

  • The ancestors lived in different regions and environments

  • Modern human DNA reflects mixing of these diverse ancestral populations

Genetic Diversification Across Generations

• Diversification is additive and exponential

  • Additive: Occurs independently each generation

  • Exponential: Multiplies as progeny increase over generations

• Each generation:

  • Inherits past diversity

  • Undergoes new recombination + independent assortment

• Organisms with shorter lifespans can diversify faster

• Raises question: Does lifespan variation among species relate to rate of genetic diversification?

High Fecundity/Fertility and Lineage Diversification

• Organisms with more offspring can diversify their lineage faster

• Rapid population growth within a family increases genetic variation

• Each individual has a unique allele combination → Higher chance some survive new diseases

• Natural selection likely favored this process for survival

Multiplication Potential in Poultry Industry

• Poultry have high fecundity → Produce many offspring quickly

• Enables rapid genetic diversification within flocks

• Supports fast population growth for production purposes

To Review

1. Somatic mutations: From zygote to gamete progenitor cells

2. Meiosis – Recombination: Crossing over within homologous chromosomes → Diversifies gametes

3. Meiosis – Independent assortment: Entire homologous chromosomes shuffled → Diversifies gametes

4. Sexual reproduction: Combines genomes from different ancestral lineages

5. Inter-generational diversification: Progeny multiplication increases diversity over generations

Final Outcome: Sources of Individual Uniqueness

• Siblings differ from each other, parents, grandparents, etc.

• Eukaryotic evolution favors those who diversify progeny

• Additional sources of diversity:

  • Microbiome inheritance: From mother and other close contacts → “Social microbiology”

  • Cultural transmission: Knowledge from caregivers affects gene expression and fitness

    • Secondary influences: Parents, grandparents, etc.

  • Evidence: Genetically identical crops, livestock, pets can show phenotypic differences in different environments

Summary

1. Mitosis: Diploid zygote undergoes rounds of mitosis to form diploid gamete progenitor cells → Mutations can occur during this time

2. Meiosis – Independent assortment: Entire homologous chromosomes from grandparents are shuffled → Diversifies gametes

3. Meiosis – Recombination: Rare crossing over within homologous chromosomes from parents → Further diversifies gametes

4. Sexual reproduction: Combines two unique gametes from different ancestral lineages

5. Inter-generational diversification: Additive each generation, exponential as progeny multiply

Reading 1: Clarifying Mendelian vs non-Mendelian inheritance

• Mendel’s Laws:

  • Segregation: 2 alleles of a gene separate into gametes during meiosis

  • Independent assortment: Alleles of unlinked genes segregate independently

  • Dominance affects phenotype, not inheritance

• Common Misconceptions:

  • Traits like incomplete dominance, codominance, multiple alleles, sex-linked traits, and multigene traits are often mislabeled as non-Mendelian

  • Phenotypic ratios differ from Mendel’s classic 3:1 but genotypic ratios still follow Mendel’s laws

  • Examples of Mendelian Inheritance with altered phenotypes:

  • Incomplete dominance: Snapdragon flowers (red, pink, white) – 1:2:1 genotypic and phenotypic ratio

  • Codominance: Human AB blood type – 1:2:1 genotypic and phenotypic ratio

  • Multiple alleles: Mouse agouti gene – 2 alleles per individual segregate Mendelianly; dominance series affects coat color

  • Sex-linked traits: Hemophilia in humans – X-linked alleles segregate per Mendelian rules

  • Two-gene traits & epistasis: Purple pea flowers (P and C genes), Labrador coat color (E and B genes) – unusual phenotypic ratios arise but genotypic ratios remain Mendelian

• Non-Mendelian Inheritance:

  • Organelle genes: Mitochondrial and chloroplast genes, usually maternally inherited

  • Epigenetics: DNA methylation, histone modification, paramutation, imprinting

  • Maternal-effect genetics: Phenotype depends on mother’s genotype, not offspring’s

  • Meiotic drive: Certain alleles transmitted preferentially, violating segregation

• Main Message:

  • Inheritance should be classified based on genotypic ratios (reflecting allele segregation) rather than phenotypic ratios

  • Phenotypic variation does not imply non-Mendelian inheritance

• Importance:

  • Accurate understanding of Mendelian principles is crucial for genetics education, healthcare, breeding programs, and interpretation of complex traits

  • Misleading online resources like Wikipedia or Khan Academy need expert review and clarification