Unit 5 Heredity Study Notes: How Cells Make Gametes and Generate Variation

Meiosis

A specialized cell division that produces haploid gametes by reducing chromosome number from diploid (2n) to haploid (n) and generating new allele combinations.

Gamete

A haploid sex cell (e.g., sperm or egg in animals) produced by meiosis; fuses with another gamete at fertilization to restore diploidy.

Diploid (2n)

Having two homologous sets of chromosomes (one set from each parent).

Haploid (n)

Having one set of chromosomes (one homolog from each pair), as in gametes.

Chromosome

A DNA molecule with associated proteins; after DNA replication, each chromosome consists of two identical sister chromatids.

Sister chromatids

Two identical copies of a replicated chromosome produced during S phase, attached at the centromere until they separate (in Meiosis II).

Centromere

The region where sister chromatids are attached; it does not split in Meiosis I but does split in Meiosis II.

Homologous chromosomes

A maternal and paternal chromosome pair with the same genes in the same order but potentially different alleles; not identical copies of each other.

Locus

The physical location of a gene on a chromosome.

Interphase

The cell-cycle period before meiosis that includes DNA replication; meiosis begins after interphase is complete.

S phase

The part of interphase when DNA is replicated, producing sister chromatids; there is no DNA replication between Meiosis I and Meiosis II.

Meiosis I (reductional division)

The first meiotic division that separates homologous chromosomes, reducing chromosome number from 2n to n.

Meiosis II (equational division)

The second meiotic division that separates sister chromatids; it resembles mitosis but occurs in haploid cells.

Prophase I

The longest meiotic phase in which homologous chromosomes pair (synapsis) into tetrads and crossing over occurs, increasing genetic variation.

Synapsis

The tight pairing of homologous chromosomes along their lengths during Prophase I, forming a tetrad (bivalent).

Tetrad (bivalent)

The paired structure of two homologous chromosomes during Prophase I; contains four chromatids total (two sister chromatids per homolog).

Crossing over (recombination)

Exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes in Prophase I, creating new allele combinations.

Chiasma (chiasmata)

The visible site(s) where non-sister chromatids have exchanged segments during crossing over.

Recombinant chromatid

A chromatid produced by crossing over that carries allele combinations not found together on either parental homolog.

Metaphase I

Stage in which tetrads (homologous pairs) line up at the metaphase plate with random orientation, setting up independent assortment.

Independent assortment

The random distribution of maternal and paternal homologs into gametes due to random tetrad orientation in Metaphase I; yields 2^n possible chromosome combinations (ignoring crossing over).

Nondisjunction

Failure of chromosomes to separate properly (homologs in Meiosis I or sister chromatids in Meiosis II), producing abnormal gametes.

Aneuploidy

An abnormal number of specific chromosomes (e.g., extra or missing a chromosome) often caused by nondisjunction; can lead to trisomy or monosomy after fertilization.

Polyploidy

Having extra whole sets of chromosomes; especially common and sometimes viable in plants.

Random fertilization

Variation generated when one of many possible sperm fertilizes one of many possible eggs, combining two independently produced gametes.