Genetic Diversity

Meiosis and Genetic Diversity

Mitosis

  • Definition: Produces identical daughter cells (clones)

  • DNA: Same amount of DNA, genetic information, and number of chromosomes

Reproduction in Multicellular Organisms

  • Process: Joining of egg (X, X) and sperm (X, Y)

  • Homologous Chromosomes: Paired chromosomes carrying matching genes, controlling the same inherited characters

  • Chromosome Reduction:

    • Must reduce from 46 (diploid) to 23 (haploid)

    • Special division for sexual reproduction called Reduction Division

    • Meiosis evolved from mitosis

Stages of Meiosis

  • Two Divisions:

    • Meiosis 1: Separates homologous pairs

    • Meiosis 2: Separates sister chromatids

Meiosis 1

  • Crossing Over (Prophase 1):

    • Homologous chromosomes loosely pair and swap pieces of chromosome (synapsis)

    • Sister chromatids intertwine and exchange segments

Meiosis 2

  • Function: Separates sister chromatids resulting in genetically diverse daughter cells

Differences Between Meiosis and Mitosis

  • Meiosis:

    • 2 divisions, produces 4 genetically different cells

    • Gametes: haploid cells (2n → n)

    • Involves crossing over

  • Mitosis:

    • 1 division, produces 2 genetically identical cells

    • Somatic cells: diploid cells (2n → 2n)

    • No crossing over

Importance of Sexual Reproduction

  • Introduces genetic variation essential for evolution:

    • Independent Assortment: Variation in gamete combinations

    • Crossing Over: Leads to new genetic combinations

    • Random Fertilization: Increases genetic diversity

Genetic Variation from Independent Assortment

  • Humans can produce 2^23 different combinations of gametes due to random assortment

Nondisjunction

  • Definition: Failure of homologous chromosomes to separate during meiosis

  • Consequence: 50% of conceptions are aneuploid (abnormal number of chromosomes)

Disorders due to Nondisjunction

  • Trisomy 13: Lethal condition (1 in 10,000 births)

  • Trisomy 18: Edwards syndrome, mostly females with extra chromosomes

  • Trisomy 21: Down Syndrome, extra 21 chromosome

  • Turner Syndrome: Monosomy of X chromosome results in female sterility

  • Klinefelter Syndrome: 47 XXY, fertility issues with few symptoms

  • XYY Syndrome: 47 XYY, taller individuals with potential personality disorders

Gregor Mendel and Genetics

Mendel’s Experiments

  • Austrian Monk: Developed the experimental approach to genetics

  • Pea Plants: Chosen for controlled mating (self vs. cross-pollination), many varieties, and short generation time

  • Generations:

    • P generation: true breeding plants

    • F1 generation: offspring of P

    • F2 generation: offspring of F1

Mendel’s Principles

  • Alleles: Alternate versions of a gene that separate during meiosis 1

  • Law of Segregation: Alleles for each trait separate during gamete formation

  • Genotypes:

    • Homozygous (PP or pp)

    • Heterozygous (Pp)

  • Phenotype: Expressed physical traits

Punnett Square

  • Tool for predicting offspring traits

  • Example: Pp x Pp yields:

    • Genotypic ratio: 1 PP: 2 Pp: 1 pp

    • Phenotypic ratio: 3 purple: 1 white (75% purple, 25% white)

Genetic Probability and Ratios

  • Testcross: Determines if the dominant trait is homozygous or heterozygous

  • Law of Independent Assortment: Each allele segregates independently during gamete formation (e.g., color is separate from shape)

  • Monohybrid Cross: Study of one character

  • Dihybrid Cross: Study of two traits, yields a 9:3:3:1 ratio for double heterozygous crosses

Rules of Probability

  • Rule of Multiplication: Multiply probabilities of independent events (e.g., throwing 2 sixes)

  • Rule of Addition: Add probabilities of mutually exclusive events

Mendelian Genetics: Key Concepts

Patterns of Inheritance

  • Complete Dominance: Heterozygous and homozygous dominant are indistinguishable

  • Incomplete Dominance: F1 hybrids show intermediate traits

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

Blood Transfusions and Rh Factor

  • Must match blood type to prevent clumping and death

  • Rh Factor: Protein found on red blood cells

Polygenic Inheritance

  • Multiple genes affect a single phenotype (e.g., skin color)

  • Both genetic and environmental factors play a role in phenotypes

Pedigrees

  • Diagram showing relationships between parents/offspring across generations

Genetic Disorders

Autosomal Recessive Disorders

  • Cystic Fibrosis: Genetic disorder affecting respiratory system

  • Tay-Sachs Disease: Breakdown of the nervous system

  • Sickle-Cell Disease: Co-dominant inheritance affecting red blood cells

  • Phenylketonuria (PKU): Can damage the nervous system

Autosomal Dominant Disorders

  • Huntington’s Disease: Lethal dominant allele affecting nerve cells

Epistasis and Polygenic Inheritance

  • Epistasis: One gene's expression affects that of another

  • Chromosome Theory of Inheritance: Genes have specific locations on chromosomes

Thomas Hunt Morgan

  • Studied fruit flies, fast breeding, 4 pairs of chromosomes

  • Sex-Linked Genes: Genes located on X or Y chromosome

  • Sex-Linked Disorders: Include color blindness, Duchenne muscular dystrophy, hemophilia

X Chromosome Inactivation

  • Barr Body: Inactive X chromosome regulating gene dosage in females

  • Tortishell or calico fur in female cats results from random X inactivation

Human Development and Genetic Factors

Role of Y Chromosome

  • Required for testes development during embryonic development

  • SRY Gene: Codes for protein regulating other genes

Genetic Recombination and Linkage

  • Recombination: New gene combinations in offspring

  • If offspring resemble parents, they are parental types; if not, they are recombinants

  • Linked genes are inherited together; higher distance increases recombination frequency

Genetic Mapping

Linkage Map

  • Based on % of crossover events

  • 50% recombination indicates genes are far apart or on different chromosomes

Genomic Imprinting

  • Gene expression depends on whether it comes from the mother or father

Epigenetics and Mitochondrial Inheritance

  • Methylation: Silences genes by adding methyl groups to DNA

  • Non-Nuclear DNA: Genes located in organelles; mtDNA is maternally inherited

Nondisjunction and Genetic Disorders

Chromosomal Abnormalities

  • Nondisjunction: Chromosomes fail to separate properly during meiosis

  • Karyotyping: Can detect abnormalities like Klinefelter syndrome (47 XXY) and Turner syndrome (45 XO)

Chromosomal Mutations

  • Types:

    • Deletion: Removal of a chromosomal segment

    • Duplication: segment is reproduced

    • Inversion: segment is reversed

    • Translocation: segment moves to another chromosome

  • Aneuploidy: Incorrect number of chromosomes (monosomy or trisomy)

  • Polyploidy: More than two complete sets of chromosomes, common in plants, rare in animals.