Meiosis, Chromosomes, and Genetic Variation

Vocabulary terms and definitions covering the mechanisms of meiosis, chromosome structures, genetic diversity processes, and chromosomal anomalies.

Germ cells (Germline cells)

Primordial reproductive parent cells that undergo meiosis to produce gametes.

Somatic cells

Normal body cells of a sexually reproducing organism that divide by mitosis to produce two clone progeny cells.

Homologous chromosomes

Chromosome pairs carrying the same genes and traits, with one inherited from the mother and one from the father.

Sister chromatids

A duplicated chromosome and its exact identical copy attached together, which count as one in chromosome number.

Diploid ($2n$)

Having two complete sets of chromosomes, comprising half from the mother and half from the father.

Haploid ($1n$ or $n$)

Having half the number of chromosomes found in a normal body cell of a species, characteristic of gametes.

Gametes

General term for sex cells, such as oocytes or spermatozoa, that possess the haploid chromosome number.

Zygote

The first somatic cell formed by the fusion of gametes, containing a diploid chromosome number ($46$ in humans).

Reproductive fitness

The biological definition of an adult organism, characterized by the capability to produce gametes.

S substage of interphase

The stage prior to meiotic division during which DNA replication and duplication take place.

Tetrad

A structure formed during prophase I when duplicated sister chromatids pair up with their duplicated homologous chromosomes.

Crossing over recombination

The exchange of genetic segments between maternal and paternal homologous chromatids during prophase I to create new patterns of genetic diversity.

Law of independent assortment

The principle stating that homologous chromosome pairs align randomly and independently of each other at the metaphase plate during metaphase I.

Anaphase I

The meiotic stage where homologous chromosome pairs separate to opposite poles while sister chromatids remain attached together.

Anaphase II

The meiotic stage where sister chromatids are pulled apart and separate toward opposite poles.

Nondisjunction

An error occurring when chromosomes fail to separate normally during meiosis, resulting in cells with abnormal chromosome counts.

Trisomy 21

A chromosomal anomaly associated with Down syndrome, occurring when an individual has three chromosomes at the 21st pair instead of two.