Genetics: Cell Division, Classical Genetics, and Chromosomes

Comprehensive vocabulary flashcards covering the cell cycle, mitosis, meiosis, chromosomal mutations, Mendelian genetics, and sex determination.

Cell cycle

The ordered series of events that allows a eukaryotic cell to grow, replicate its DNA, and divide into two daughter cells, comprised of interphase and the mitotic ($\text{M}$) phase.

$G_0$ phase

A quiescent, non-dividing state of the cell cycle where a cell undergoes terminal differentiation and semi-perpetual arrest of cell division, as seen in nerve and heart muscle cells.

$S$ phase

The synthesis phase of interphase during which DNA replication and chromosome duplication take place.

Chromosome

A cellular DNA-containing structure with a centromere, existing as a single chromatid before the $S$ phase or as two sister chromatids after the $S$ phase.

Chromatid

One of the two individual identical copies of a replicated chromosome.

Sister chromatids

The product of DNA synthesis/replication consisting of two identical copies of a chromosome joined together at a centromere.

Centromere

The specialized DNA sequence where sister chromatids join together and where kinetochore proteins assemble.

Karyokinesis

The process of partitioning duplicated chromosomes equally into two daughter cell nuclei during cell division.

Cytokinesis

The physical partitioning of the cytoplasm between daughter cells, driven in animal cells by a contractile ring composed of actin filaments along which Myosin II walks.

Cohesin

A protein complex that holds sister chromatids together along their length and centromere, resisting microtubule pulling forces to ensure proper alignment at the metaphase plate.

Separase

The enzyme that cleaves cohesin at anaphase, breaking the connection between sister chromatids to allow their separation toward opposite poles.

Disjunction

The normal separation and migration of homologous chromosomes or sister chromatids toward opposite centrosome poles during anaphase.

Proto-oncogenes

Normal cellular genes that stimulate cell cycle progression; when mutated and constitutively active, they become oncogenes that promote tumor formation.

Tumor suppressor genes

Genes that prevent cell cycle progression at checkpoints; loss or deletion of both functional alleles (such as $RB1$) can lead to tumor development.

Synapsis

The process during meiotic prophase I in which homologous chromosomes physically pair up and align side-by-side.

Tetrad

The structure formed during meiotic prophase I consisting of a pair of aligned homologous chromosomes containing four sister chromatids.

Synaptonemal complex

A protein structure that forms between paired homologous chromosomes during meiotic prophase I to facilitate genetic recombination.

Crossing over

The physical exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I, generating recombinant chromosomes.

Nondisjunction

The failure of homologous chromosomes to separate during meiosis I or sister chromatids to separate during meiosis II or mitosis, generating aneuploid cells.

Particulate inheritance

Mendel's theory proposing that phenotypic traits are inherited via discrete, intact physical particles (now called alleles) rather than through continuous blending.

Blending inheritance

The disproven historical hypothesis asserting that offspring phenotypes are an inseparable mathematical average of both parental traits.

Inheritance of acquired traits

The disproven hypothesis that physical or environmental modifications acquired by parents during their lifetime are transmitted to their offspring.

Allele

An alternative sequence variant or form of a specific gene.

Homozygous

A genotype in which an organism possesses two identical copies of an allele for a specific gene (such as $YY$ or $yy$).

Heterozygous

A genotype in which an organism possesses two different alleles for a specific gene (such as $Yy$).

Genotype

The specific allelic composition or genetic makeup of an individual organism.

Phenotype

The observable physical, physiological, or biochemical traits of an organism produced by its genotype.

Law of segregation

Mendel's first law stating that two alleles of a gene separate during gamete formation so that each gamete carries only one allele, corresponding to the physical separation of homologous chromosomes during meiosis.

Law of independent assortment

Mendel's second law stating that the segregation and recombination of alleles for one gene occur independently of the segregation of alleles for other unlinked genes.

Monohybrid cross

A cross between parents differing in a single gene or trait, typically yielding a $3:1$ phenotypic ratio and a $1:2:1$ genotypic ratio in the $F_2$ generation under complete dominance.

Dihybrid cross

A mating cross tracking two independently assorting traits simultaneously, yielding a characteristic $9:3:3:1$ phenotypic ratio in the $F_2$ generation.

Test cross

A controlled genetic cross between an individual showing a dominant phenotype of unknown genotype and a homozygous recessive tester to determine the unknown genotype.

Tester

An individual that is homozygous recessive for the gene(s) being analyzed in a test cross, ensuring its gametes contribute only recessive alleles.

Reciprocal cross

A pair of genetic crosses wherein the phenotypes of the male and female parents are inverted to test whether trait transmission is sex-linked.

Multiplicative law of simple probability

The probability principle stating that the combined chance of two or more independent random events occurring together in exact sequence is equal to the product of their individual probabilities.

Additive law of probability

The probability principle stating that when a given outcome can occur in multiple mutually exclusive ways, the overall chance is the sum of their individual probabilities.

Incomplete dominance

An inheritance pattern in which neither allele is fully dominant, producing a heterozygous phenotype that is intermediate between both homozygous phenotypes.

Codominance

An inheritance mode in which two dominant alleles are simultaneously and distinctly expressed in the phenotype of a heterozygote, such as the $MN$ blood group antigens.

Multiple alleles

The existence of three or more alternative allelic variants of a single gene within a population, such as the $I^A$, $I^B$, and $i$ alleles of the human ABO blood group system.

Pleiotropy

The phenomenon in which a single mutated gene or locus influences multiple, distinct, and seemingly unrelated phenotypic traits, such as seen in sickle-cell anemia.

Polygenic inheritance

The additive contribution of two or more independent genes to a single continuous quantitative phenotypic trait, such as human skin color or height.

Chromosomal Theory of Inheritance

The biological principle demonstrating that genes reside at specific physical loci on chromosomes, first evidenced experimentally by Thomas Hunt Morgan using sex-linked traits in Drosophila melanogaster.

Pseudoautosomal regions (PAR)

Homologous segments located at the termini of the mammalian X and Y chromosomes that pair and synapse during meiosis and exhibit an autosomal pattern of inheritance.

Euploidy

The state of possessing a chromosome number that is an exact integer multiple of a basic haploid chromosome set ($n$, $2n$, $3n$, $4n$, etc.).

Monoploid

An individual organism belonging to a normally diploid species that contains only one basic set of chromosomes ($n$).

Autopolyploidy

A form of polyploidy where all additional chromosome sets originate from the same species via mechanisms like polyspermy, mitotic nondisjunction, or meiotic nondisjunction.

Allopolyploidy

A form of polyploidy resulting from the hybridization of two distinct species followed by chromosome doubling to produce fertile, amphidiploid genomes.

Aneuploidy

A condition in which a cell has a chromosome number that differs from the wild type by an incomplete chromosome set, typically involving the gain or loss of a single chromosome.

Monosomy

An aneuploid state defined by the absence of a single chromosome from a normal diploid set, designated as $2n - 1$.

Trisomy

An aneuploid state defined by the presence of an extra individual chromosome in a diploid organism, designated as $2n + 1$.

Nullisomy

An aneuploid condition characterized by the loss of both members of a homologous chromosome pair, designated as $2n - 2$.

Disomy

In haploid or monoploid organisms, the abnormal condition where one chromosome is represented twice ($n + 1$).

Down syndrome

A human autosomal trisomy involving chromosome 21 (trisomy 21) that occurs in approximately $1\text{ in }1500$ live births and is associated with characteristic facial morphology, short stature, and maternal age-related meiotic nondisjunction.

Turner syndrome

A human sex-chromosome aneuploidy designated as $45, \text{XO}$ occurring in $1\text{ in }5000$ females, characterized by absence of secondary sexual characteristics, infertility, short stature, and a webbed neck.

Klinefelter syndrome

A human sex-chromosome aneuploidy designated as $47, \text{XXY}$ occurring in $1\text{ in }1000$ males, characterized by variable secondary sexual traits, infertility, and frequent breast swelling.

Microdeletion

A small chromosomal deletion that is too small to be detected by standard microscopic chromosome banding techniques.

Pseudodominance

The phenotypic expression of a recessive allele occurring when the corresponding dominant allele on the homologous chromosome has been deleted or lost.

Deletion mapping

A gene-mapping method that determines the cytological location of a gene by identifying which overlapping chromosomal deletions allow recessive alleles to exhibit pseudodominance.

Paracentric inversion

A structural chromosomal aberration where a broken segment of a chromosome rotates $180^\circ$ and reinserts without spanning the centromere.

Pericentric inversion

A structural chromosomal aberration where a segment of a chromosome reverses orientation and spans across the centromere.

Inversion loop

A circular loop structure formed during prophase I synapsis in an inversion heterozygote to align homologous sequences on inverted and standard chromosomes.

Dicentric bridge

An abnormal chromosome structure containing two centromeres formed during crossing over within a paracentric inversion loop, which stretches between spindle poles and ruptures randomly during anaphase I.

Acentric fragment

A nonviable chromosomal fragment that lacks a centromere, produced following crossing over within a paracentric inversion loop, and is lost during cell division.

Reciprocal balanced translocation

A chromosomal rearrangement resulting from the reciprocal exchange of segments between nonhomologous chromosomes without altering the total genetic content.

Tetravalent complex

A cross-shaped structure formed by the association of four chromosomes during prophase I of meiosis in reciprocal translocation heterozygotes.

Alternate segregation

A meiotic segregation mode in translocation heterozygotes where diagonal, alternate centromeres migrate to the same pole, yielding approximately $50\%$ viable, balanced gametes.

Adjacent-1 segregation

A meiotic segregation mode in translocation heterozygotes where adjacent nonhomologous centromeres migrate together, resulting in nonviable gametes with duplications and deletions.

SRY gene

The sex-determining region located on the mammalian Y chromosome that directs the undifferentiated bipotential embryonic gonads to develop into testes.

ZW sex determination system

A sex-determination system used by birds, butterflies, and select reptiles and fish in which females are the heterogametic sex ($\text{ZW}$) and males are the homogametic sex ($\text{ZZ}$).

Cosmo

A gene-edited bull calf born at UC Davis produced using CRISPR technology by inserting the bovine $SRY$ gene onto autosomal chromosome 17.