Meiosis & Genetic Segregation

Haploid (1n)

A cell containing only one set of chromosomes (e.g., gametes/sperm/egg).

Homologous Chromosomes

A pair of chromosomes (one maternal, one paternal) containing the same genes but potentially different alleles.

Synapsis / Tetrad

The pairing of homologous chromosomes during Prophase I, forming a structure with four total chromatids.

Crossing Over (Recombination)

The exchange of genetic material between homologous chromosomes during Prophase I. Increases genetic diversity.

Mendel's Law of Independent Assortment

States that alleles for different traits segregate independently. (Violated by Genetic Linkage).

Genetic Linkage

Genes located very close together on the same chromosome are highly unlikely to be separated by a crossing-over event.

Nondisjunction

The failure of homologous chromosomes (Anaphase I) or sister chromatids (Anaphase II) to separate properly.

Aneuploidy

An abnormal number of chromosomes in a cell (e.g., 45 or 47 instead of 46), usually resulting from nondisjunction.

Trisomy 21 (Down Syndrome)

A prominent example of aneuploidy where a gamete receives an extra copy of chromosome 21 due to nondisjunction.

Meiosis I (Reductional Division)

Reduces ploidy from 2n to 1n by separating homologous chromosomes.

Meiosis II (Equational Division)

Does not change ploidy (stays 1n). Separates sister chromatids, exactly like mitosis.

Diploid (2n)

A cell containing two complete sets of chromosomes (one maternal, one paternal).