Metaphase I

• Tetrads align at metaphase plate
• Random orientation of homologs
• Independent assortment occurs

Anaphase I

• Homologous chromosomes separate
• Sister chromatids stay together
• Different from mitosis!

Telophase I and Cytokinesis

• Two haploid cells form
• Each has one chromosome from each homologous pair
• Sister chromatids still joined

Meiosis II (Equational Division)

Similar to mitosis but with haploid cells

Prophase II

• Chromosomes condense
• Spindle forms
• No crossing over

Metaphase II

• Chromosomes align at metaphase plate
• Single file (not pairs)

Anaphase II

• Sister chromatids separate
• Move to opposite poles

Telophase II and Cytokinesis

• Nuclear envelopes reform
• 4 haploid cells produced
• Each genetically unique

Sources of Genetic Variation

1. Crossing Over (Prophase I)

• Exchange of DNA between homologous chromosomes
• Creates new allele combinations
• Recombinant chromosomes

2. Independent Assortment (Metaphase I)

• Random orientation of homologous pairs
• 2ⁿ possible combinations (n = # of chromosome pairs)
• Humans: 2²³ = ~8 million combinations

3. Random Fertilization

• Any sperm can fertilize any egg
• (2²³)² = ~70 trillion combinations

Mitosis vs. Meiosis

Errors in Meiosis

Nondisjunction: Homologs or sister chromatids fail to separate

Results:

• Aneuploidy: abnormal chromosome number
• Monosomy: 2n - 1 (one chromosome missing)
• Trisomy: 2n + 1 (one extra chromosome)

Examples:

• Down syndrome: Trisomy 21 (3 copies of chromosome 21)
• Turner syndrome: Monosomy X (45, X)
• Klinefelter syndrome: XXY (47, XXY)

Key Concepts

1. Meiosis has two divisions producing 4 haploid cells
2. Meiosis I: homologous chromosomes separate
3. Meiosis II: sister chromatids separate
4. Crossing over (Prophase I) exchanges DNA
5. Independent assortment randomizes chromosome distribution
6. Three sources of variation: crossing over, independent assortment, random fertilization
7. Nondisjunction causes aneuploidy

(b) Sources of Genetic Diversity:

1. Crossing Over (Recombination):

When: Prophase I of meiosis I

Process:

• Homologous chromosomes pair up (synapsis)
• Form tetrads (4 chromatids total, 2 per chromosome)
• Chiasmata form - points where chromatids cross
• Non-sister chromatids exchange DNA segments
• Chromatids break and rejoin at same position

Result:

• Recombinant chromosomes with new allele combinations
• Mix of maternal and paternal genes on same chromosome
• Occurs multiple times per chromosome pair (1-3 crossovers typical)

Example:

Before crossing over:
Maternal: A---B---C
Paternal: a---b---c

After crossing over:
Recombinant 1: A---b---c  (maternal-paternal mix)
Recombinant 2: a---B---C  (paternal-maternal mix)

Frequency:

• Genes far apart → more likely to cross over
• Genes close together → less likely (linked genes)
• Used to create genetic maps

2. Independent Assortment:

When: Metaphase I of meiosis I

Process:

• Homologous pairs line up randomly at metaphase plate
• Maternal and paternal chromosomes orient independently
• Each pair's orientation independent of other pairs
• Random which homolog goes to which pole

Number of combinations:

$\text{Possible combinations} = 2^n$

where n = haploid number

Example (n=3):

• 3 homologous pairs
• Each pair can orient 2 ways
• Total: 2³ = 8 different gamete combinations

Visualization:

Pair 1: M₁ or P₁
Pair 2: M₂ or P₂  
Pair 3: M₃ or P₃

Possible gametes (8 total):
1. M₁M₂M₃  (all maternal)
2. M₁M₂P₃
3. M₁P₂M₃
4. M₁P₂P₃
5. P₁M₂M₃
6. P₁M₂P₃
7. P₁P₂M₃
8. P₁P₂P₃  (all paternal)

3. Random Fertilization:

Additional diversity:

• Any male gamete can fuse with any female gamete
• For humans (n=23):
  • Male: 2²³ = 8.4 million possible sperm
  • Female: 2²³ = 8.4 million possible eggs
  • Combinations: (2²³)² = 70 trillion possible zygotes!

Without crossing over! With crossing over → essentially infinite variation

(c) Calculation for n=3:

From independent assortment alone:

$\text{Gamete types} = 2^n = 2^3 = 8$

$\boxed{8 \text{ different gamete types possible}}$

From one parent: 8 types From both parents (after fertilization):

$\text{Zygote combinations} = 2^n \times 2^n = 2^{2n}$

$\text{Combinations} = 2^6 = 64$

$\boxed{64 \text{ possible genetic combinations in offspring}}$

This is WITHOUT crossing over!

With crossing over:

• Each homologous pair can undergo recombination
• Effectively infinite combinations
• No two siblings identical (except identical twins)

Total genetic diversity formula:

$\text{Diversity} = 2^n \text{ (assortment)} \times \text{(crossovers per pair)}^{\text{number of pairs}} \times 2^n \text{ (mate)}$

Why sexual reproduction evolved:

• Genetic variation → adaptation → evolution
• Different combinations → some survive changing environments
• Eliminates harmful mutations more efficiently (genetic load)

$\boxed{\text{Meiosis creates variation via crossing over + independent assortment}}$