Meiosis and Sexual Life Cycles

Meiosis is cell division in reproductive organs that reduces chromosomes by half to form haploid gametes ( $n$ ).
Fertilization fuses two gametes to restore the diploid chromosome number ( $2n$ ).
In humans, the haploid number is $n = 23$ (22 autosomes, 1 sex chromosome) and the diploid number is $2n = 46$ (44 autosomes, 2 sex chromosomes).
Homologous chromosomes (homologs) are pairs (one paternal, one maternal) matching in size and gene locations; sex chromosomes are $XX$ in females and $XY$ in males.
Heredity involves the transmission of traits via genes (units of heredity) located at specific loci on chromosomes, with alleles representing different versions of a gene.

Meiosis consists of two divisions: Meiosis I (separates homologous chromosomes) and Meiosis II (separates sister chromatids).
Only Meiosis I is preceded by an interphase ( $G_1, S, G_2$ ) where chromosomes are duplicated into sister chromatids joined by cohesins.
Prophase I: Homologs undergo synapsis (pairing) via the synaptonemal complex and exchange DNA at chiasmata (crossing over).
Metaphase I: Homologous pairs align at the metaphase plate.
Anaphase I: Homologs separate and move to opposite poles; sister chromatids remain attached.
Meiosis II: Similar to mitosis, resulting in four genetically distinct haploid daughter cells.

Crossing Over: Occurs in Prophase I, producing recombinant chromosomes with combined DNA from both parents.
Independent Assortment: In Metaphase I, homologs align randomly, creating different combinations of chromosomes in gametes.
Random Fertilization: Any sperm can fuse with any unfertilized egg (ovum), further increasing unique genetic combinations.
Mutations: The original source of all genetic variation and evolution.

Animals: Diploid dominant; gametes are the only haploid cells and do not divide before fertilization.
Plants and Some Algae: Alternation of generations; involves a multicellular diploid sporophyte (produces spores via meiosis) and a multicellular haploid gametophyte (produces gametes via mitosis).
Fungi and Some Protists: Haploid dominant; the zygote is the only diploid stage, which undergoes meiosis to produce haploid cells.
Asexual Reproduction: Produces genetically identical clones (e.g., Hydra); organisms like the bdelloid rotifer increase diversity by incorporating foreign DNA from the environment.

Karyotyping: A visual technique used to display a complete set of chromosomes to identify abnormalities or species variations.
Mitosis vs. Meiosis: Mitosis conserves chromosome number and produces identical cells; Meiosis reduces chromosome sets and produces genetically unique cells.