Single gene inheritance and meiosis

Mendel's First Law

The principle of equal segregation stating that members of a gene pair separate equally into gametes during meiosis, resulting in each haploid gamete carrying only one allele.

Allele

Alternative functional variants of a specific gene that arise via mutation and can produce differing phenotypic traits.

$F_2$ monohybrid phenotypic and genotypic ratios

The physical trait distribution of $3:1$ dominant to recessive among offspring, which reflects an underlying genetic distribution of $1:2:1$ ($1$ homozygous dominant, $2$ heterozygous, $1$ homozygous recessive).

Testcross

The mating of an organism expressing a dominant phenotype with a homozygous recessive individual to determine the dominant parent's genotype.

Leptonema

The initial sub-stage of prophase I in which chromatin condenses, tips anchor to the nuclear membrane, and chromosomes cluster into the bouquet stage to facilitate homolog pairing.

Zygonema

The prophase I sub-stage characterized by the initiation of pairing (synapsis) between homologous chromosomes to form bivalents.

Pachynema

The prophase I sub-stage where chromosomes thicken, the synaptonemal complex fully develops, and genetic recombination occurs between non-sister chromatids.

Diplonema

The prophase I sub-stage in which the synaptonemal complex breaks down, leaving homologs held together specifically at chiasmata where crossing over occurred.

Diakinesis

The final sub-stage of prophase I characterized by chromosome terminalization, nuclear envelope disintegration, and attachment of spindle fibers to kinetochores.

Reductional division

The separation of homologous chromosome pairs during anaphase I, which reduces the chromosome count from $2n$ (diploid) to $1n$ (haploid).

Centrosome

A non-membrane-bound cytosolic microtubule-organizing center containing cylindrical centrioles that anchors spindle fiber minus ($-$) ends.

Microtubule motor mechanism in anaphase

Kinetochore-associated proteins that hydrolyze ATP to move chromosomes along depolymerizing spindle fibers toward the minus ($-$) end anchored at the centrosome.

Meiosis II

An equational division occurring without prior DNA replication in which sister chromatids segregate to opposite poles, yielding $4$ haploid gametes.