Study Guide: Meiosis, Sexual Reproduction, and Chromosomal Inheritance

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Sexual reproduction involves the fusion of gametes (sperm and egg), resulting in offspring with genetic variation.
Asexual reproduction produces offspring genetically identical to the parent, without the fusion of gametes (e.g., binary fission, budding).
Purpose of sexual reproduction: To produce gametes (sperm and eggs) that combine to form genetically unique offspring.
Purpose of asexual reproduction: To rapidly produce identical offspring, often in stable environments.
Example: Bacteria reproduce asexually by binary fission; humans reproduce sexually.

Homologous chromosomes: Pairs of chromosomes (one from each parent) with the same genes at the same loci, but possibly different alleles.
Allele: Alternative versions of a gene found at the same locus.
Locus (plural: loci): The specific location of a gene on a chromosome.
Sex chromosomes: Chromosomes that determine biological sex (X and Y in humans).
Autosomes: Non-sex chromosomes (22 pairs in humans).
Diploid (2n): Cells with two sets of chromosomes (46 in humans).
Haploid (n): Cells with one set of chromosomes (23 in humans; gametes).

Meiosis: A type of cell division that reduces chromosome number by half, producing haploid gametes.
Gametes: Reproductive cells (sperm and egg) that are haploid.
Fertilization: The fusion of two gametes, restoring the diploid number and forming a zygote.
Zygote: The fertilized egg, which is diploid and develops into a new organism.
Example: Human sperm (n=23) fertilizes egg (n=23) to form zygote (2n=46).

XX: Female genetic sex (two X chromosomes).
XY: Male genetic sex (one X and one Y chromosome).
Which gamete determines sex? The sperm, as it can carry either an X or a Y chromosome; eggs always carry an X.
Example: Sperm with X chromosome + egg (X) = XX (female); sperm with Y chromosome + egg (X) = XY (male).

Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.
Meiosis II: Sister chromatids separate, similar to mitosis.

Prophase I: Homologous chromosomes pair (synapsis) and exchange genetic material (crossing over) at chiasmata.
Metaphase I: Homologous pairs align at the metaphase plate.
Anaphase I: Homologous chromosomes separate to opposite poles.
Telophase I and Cytokinesis: Two haploid cells form; chromosomes are still duplicated.
Interkinesis: Short interphase-like period; no DNA replication occurs.

Prophase II: Chromosomes condense in two haploid cells.
Metaphase II: Chromosomes align at the metaphase plate.
Anaphase II: Sister chromatids separate.
Telophase II and Cytokinesis: Four genetically unique haploid cells are produced.

Independent assortment: Random orientation of homologous pairs during metaphase I leads to different combinations of maternal and paternal chromosomes in gametes.
Crossing over: Exchange of genetic material between non-sister chromatids during prophase I creates recombinant chromosomes.
Random fertilization: Any sperm can fertilize any egg, increasing genetic combinations.
Example: With 23 chromosome pairs, independent assortment alone allows for possible gamete combinations.

Nondisjunction and Trisomy 21: Not covered in this module, but refers to errors in chromosome separation leading to conditions like Down syndrome (extra chromosome 21).