Meiosis and Genetic Variation

Vocabulary terms covering the process of meiosis, sources of genetic variation, chromosomal abnormalities, and methods of prenatal karyotype analysis.

Meiosis

The process of producing gametes, such as sperm and eggs, which involves two divisions to produce four daughter nuclei with half the number of chromosomes ($23$).

Meiosis I

The first meiotic division, known as a reduction division ($diploid \rightarrow haploid$), in which homologous chromosomes are separated.

Meiosis II

The second meiotic division which separates sister chromatids that may not be identical due to crossing over in Prophase I.

Prophase I

The stage of meiosis where chromosomes condense, the nuclear membrane dissolves, homologous chromosomes pair up, and crossing over occurs.

Crossing over (Sobrecruzamiento)

The exchange of genetic material between non-sister chromatids at chiasmata during Prophase I, resulting in new combinations of genes (recombination).

Random orientation

A source of genetic variation in Metafase I where the alignment of homologous pairs (bivalents) at the cell equator is independent and random.

Non-disjunction (No-disyunción)

The failure of chromosomes to separate correctly during Anafase I (homologues) or Anafase II (sister chromatids), leading to aneuploidy.

Aneuploidy

A condition where gametes have an extra or missing chromosome due to the failure of chromosomes to separate correctly during meiosis.

Trisomy 21 (Down Syndrome)

A condition where an individual has three copies of chromosome $21$ because one parental gamete had two copies due to non-disjunction.

Karyotyping

The process by which chromosomes are organized and visualized for inspection, generally used to determine gender or detect chromosomal abnormalities.

Amniocentesis

A karyotyping method involving the extraction of amniotic fluid at approximately $16$ weeks of pregnancy, with a miscarriage risk of about $0.5\%$.

Chorionic Villus Sampling (CVS)

A karyotyping method involving the extraction of placental tissue (chorionic villi) at approximately $11$ weeks of pregnancy, with a miscarriage risk of about $1\%$.

Bivalents

Pairs of homologous chromosomes that align randomly at the cell equator during Metafase I.

Haploid

A cell containing a single set of chromosomes ($23$ in humans), which is the final result of the four daughter cells in meiosis.

Diploid

A cell containing two complete sets of chromosomes ($46$ in humans), which is the state of the parental cell before meiosis begins.