Zygote

A diploid cell resulting from the fusion of two haploid gametes (sperm and egg), marking the beginning of a new organism.

Karyotype

The number and visual appearance of the chromosomes in the nucleus of a cell. It is often used to detect chromosomal abnormalities.

Chromosome

A long, thread-like structure composed of DNA and proteins, carrying genetic information. Humans have 23 pairs of chromosomes in diploid cells.

Sister Chromatids

Two identical copies of a chromosome connected by a centromere. They are formed during DNA replication and separate during mitosis or meiosis II.

Centromere

The region of a chromosome where the two sister chromatids are held together and where the kinetochore forms, playing a key role in chromosome segregation.

Centrosome

The organelle that serves as the main microtubule organizing center of the cell, helping to form the spindle apparatus during cell division.

Kinetochore

A protein complex assembled on the centromere that attaches chromosomes to the spindle fibers during cell division, ensuring proper chromosome separation.

Cohesin

A protein complex that holds sister chromatids together from the time they are formed until they separate during mitosis or meiosis.

Telomere

The repetitive nucleotide sequences at the ends of chromosomes that protect them from degradation and prevent fusion with neighboring chromosomes.

Subtelomere

The region of a chromosome located just inside the telomere. It is less repetitive than the telomere and contains genes and regulatory elements.

Somatic

Refers to body cells (as opposed to gametes) that are diploid and make up most tissues and organs in an organism.

Microtubule

A component of the cytoskeleton made of tubulin proteins that forms the spindle fibers during cell division and helps in intracellular transport.

Spindle

The structure composed of microtubules that segregates chromosomes into daughter cells during mitosis and meiosis.

Checkpoint

Regulatory points in the cell cycle where the cell checks for errors (e.g., DNA damage, chromosome alignment) before proceeding to the next phase.

Metacentric

A chromosome with the centromere located near the middle, making the two arms of the chromosome roughly equal in length.

Submetacentric

A chromosome with a centromere slightly off-center, resulting in one arm being longer than the other.

Acrocentric

A chromosome with a centromere very close to one end, resulting in one very short arm and one long arm.

Telocentric

A chromosome with the centromere at the very end, so that it essentially has only one arm. This type of chromosome is not found in humans.

Karyokinesis

The division of a cell's nucleus during mitosis or meiosis, which precedes cytokinesis.

Cytokinesis

The division of the cytoplasm into two daughter cells, occurring after karyokinesis in both mitosis and meiosis.

Interphase

The phase of the cell cycle during which the cell grows, replicates its DNA, and prepares for mitosis. It consists of G1, S, and G2 phases.

Prophase

The first stage of mitosis, during which chromosomes condense, the nuclear envelope breaks down, and spindle fibers begin to form.

Prometaphase

The phase of mitosis following prophase in which the nuclear envelope fully disintegrates and spindle fibers attach to the kinetochores of chromosomes.

Metaphase

The stage of mitosis where chromosomes align along the metaphase plate (the cell's equator), preparing for segregation.

Anaphase

The phase of mitosis in which sister chromatids are pulled apart by the spindle fibers and move toward opposite poles of the cell.

Telophase

The final stage of mitosis, during which the separated chromosomes arrive at the poles, and the nuclear envelope re-forms around each set of chromosomes, leading to cytokinesis.

Disjunction

The normal separation of homologous chromosomes or sister chromatids during anaphase of meiosis I or meiosis II (or mitosis), respectively, leading to daughter cells receiving the correct number of chromosomes.

Nondisjunction

The failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis, resulting in gametes or cells with an abnormal number of chromosomes (e.g., trisomy or monosomy).

Crossing Over

The process during prophase I of meiosis in which homologous chromosomes exchange segments of DNA, leading to genetic recombination and increased genetic diversity in offspring.

Chiasma/Chiasmata

The visible site(s) where crossing over has occurred between homologous chromosomes during prophase I of meiosis. These regions are points of exchange of genetic material between chromatids.

Pseudoautosomal Region

Regions of homologous sequences of nucleotides on the X and Y chromosomes that allow them to pair and recombine during meiosis. These regions behave like autosomes despite being part of sex chromosomes.

Oogonium/Oogonia

The diploid germ cells in the ovaries that give rise to primary oocytes through mitotic divisions. Precursors of mature eggs (oocytes).

Oocyte

The female gametocyte or germ cell that undergoes meiosis to form an ovum (egg). Can be primary (before meiosis I is completed) or secondary (after meiosis I and before meiosis II).

Spermatogonium/Spermatogonia

The diploid male germ cells in the testes that divide by mitosis to produce primary spermatocytes, which then undergo meiosis to form sperm.

Spermatocyte

A male germ cell that undergoes meiosis to produce sperm. Can be primary (undergoing meiosis I) or secondary (undergoing meiosis II).

Spermatid

A haploid male germ cell resulting from the second meiotic division of spermatocytes. Differentiate into mature sperm cells during spermiogenesis.

Synapsis

The pairing of homologous chromosomes during prophase I of meiosis. This close association allows for crossing over between homologous chromosomes.

Synaptonemal Complex

A protein structure that forms between homologous chromosomes during meiosis and mediates synapsis and crossing over by holding homologs together.

Polar Body

A small cell that is a byproduct of oocyte meiosis. Do not develop into eggs and usually degenerate. They result from the unequal division of cytoplasm during oogenesis.

Recombination

The exchange of genetic material between homologous chromosomes during meiosis (via crossing over), which increases genetic variation in gametes.

Bivalent/Tetrad

The pair of homologous chromosomes that align and synapse during prophase I of meiosis. Consists of four chromatids (two from each homologous chromosome).

True-Breeding

Organisms that, when mated, produce offspring with the same phenotype for a particular trait over many generations. These organisms are homozygous for the traits being observed.

Homozygote

An individual that has two identical alleles for a particular gene (e.g., AA or aa).

Heterozygote

An individual that has two different alleles for a particular gene (e.g., Aa).

Compound Heterozygote

An individual who has two different mutant alleles at a particular gene locus, one on each chromosome of a pair.

Monohybrid Cross

A genetic cross between two individuals that are heterozygous for one particular gene of interest.

Monogenic

Refers to a trait or disorder that is determined by a single gene.

Dominant

Refers to an allele that is expressed in the phenotype even when only one copy is present (heterozygous condition).

Recessive

Refers to an allele that is only expressed in the phenotype when two copies are present (homozygous condition), and not in the heterozygous state.

Genotype

The genetic constitution of an individual, or the specific alleles an individual has for a given gene.

Phenotype

The observable physical or biochemical characteristics of an individual, determined by both the genotype and environmental influences.

Dihybrid Cross

A genetic cross between two individuals that are both heterozygous for two different genes.

Test Cross

A genetic cross between an individual with an unknown genotype and a homozygous recessive individual, used to determine the unknown genotype.

Gene

A sequence of DNA that encodes information to synthesize proteins, which determine specific traits.

Allele

One of two or more alternative forms of a gene that can exist at a specific locus on a chromosome.

Wild-Type

The most common or standard allele of a gene found in nature, typically considered the normal variant for a given species.

Autosomal

Refers to genes located on autosomes (non-sex chromosomes), meaning that both males and females inherit them equally.

Vertical Pattern of Inheritance

A pattern seen in pedigrees where a trait or disorder is passed from one generation to the next, characteristic of dominant inheritance.

Horizontal Pattern of Inheritance

A pattern in which a trait or disorder appears to affect siblings within the same generation, characteristic of recessive inheritance.

Trait

A specific characteristic or feature of an organism, which can be physical or biochemical and is determined by genes.

Punnett Square

A diagram used to predict the outcome of a genetic cross by showing the possible combinations of alleles in offspring.

Mutant

Refers to an individual, gene, or allele that has undergone a mutation and exhibits characteristics different from the wild-type.

Consanguineous/Consanguinity

Refers to individuals who are related by blood, often leading to an increased risk of genetic disorders due to the inheritance of identical alleles.

De novo

Refers to a genetic mutation that arises for the first time in an individual, occurring spontaneously in the germ cell or early embryo, rather than being inherited.

Exome

The portion of the genome that consists of all the exons, which are the protein-coding regions of genes. Sequencing focuses on these regions to identify mutations related to diseases.

Incomplete Dominance

A form of inheritance in which the heterozygous phenotype is an intermediate between the two homozygous phenotypes. Neither allele is completely dominant over the other, resulting in a blending of traits (e.g., red and white flowers producing pink offspring).

Codominance

A form of inheritance where both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows both traits without blending (e.g., blood type AB, where both A and B antigens are expressed).

Phenocopy

A phenotype that is produced by environmental factors that mimic a phenotype typically produced by a genetic mutation, but without a genetic basis (e.g., a condition caused by exposure to chemicals that resembles a genetic disorder).

IA, IB, i

The three alleles that determine ABO blood type in humans.

• IA: Codes for A antigens on red blood cells.

• IB: Codes for B antigens on red blood cells.

• i: The recessive allele that does not produce antigens. IA and IB are codominant, while i is recessive.

Compound Heterozygote

An individual who carries two different mutant alleles at a particular gene locus, one on each chromosome of a homologous pair. This can result in a recessive genetic disorder if both alleles are defective.

Dominance Series

An order of dominance among alleles of a gene, where some alleles are more dominant than others. This hierarchy determines how traits are expressed when multiple alleles are present.

Antigen

A substance (typically a protein or carbohydrate) that is recognized by the immune system and can trigger an immune response. In the context of blood types, they are found on the surface of red blood cells and determine compatibility for blood transfusions.

Recessive Lethality

A genetic condition where two copies of a recessive allele result in death of the organism, often during embryonic development. Individuals with one copy of the allele (heterozygous) can survive, but those with two copies (homozygous recessive) do not.

Genetic Heterogeneity

A situation where mutations in different genes or different mutations within the same gene can produce the same phenotype or genetic disorder.

• Allelic Heterogeneity: Different mutations within the same gene.

• Locus Heterogeneity: Mutations in different genes causing the same disorder.

Epistasis

A genetic interaction where one gene interferes with or masks the expression of another gene. The gene whose effect is masked is hypostatic, while the gene doing the masking is epistatic.

Bombay Phenotype

A rare genetic condition in which individuals with blood type O lack the H antigen (precursor to A and B antigens). Even if they have IA or IB alleles, they will express blood type O because they cannot produce A or B antigens.

Centimorgan

A unit of genetic linkage that represents a 1% chance of recombination occurring between two loci during meiosis. It is a measure of genetic distance between genes or markers.

Map Unit

A synonym for a centimorgan (cM), used to measure genetic distance between loci on a chromosome based on recombination frequency.

Heteroplasmy

The presence of more than one type of mitochondrial DNA within a cell or organism. This can result in variation in the severity of mitochondrial diseases depending on the proportion of mutated versus normal mitochondria.

Independent Assortment

Mendel’s second law stating that alleles of different genes segregate independently of each other during the formation of gametes. This results in the random combination of alleles from different genes in offspring.

LOD Score (Logarithm of the Odds)

A statistical measure used to evaluate the likelihood that two genes or genetic markers are linked rather than assorting independently. A score of 3 or higher suggests significant evidence of linkage.

Linkage

The phenomenon where genes located close to each other on the same chromosome tend to be inherited together because they do not assort independently.

Parental

Refers to offspring that inherit the same combination of alleles as one of the parents, without any recombination between linked genes.

Recombinant

Refers to offspring that inherit a combination of alleles that differs from that of either parent, due to crossing over and recombination during meiosis.

Linkage Map

A genetic map that shows the relative positions of genes or markers on a chromosome based on recombination frequencies. The distances between genes are measured in centimorgans.

Haplotype

A group of alleles or genetic markers that are inherited together from a single parent due to their close proximity on the same chromosome.

Linkage Disequilibrium

The non-random association of alleles at different loci in a population. When two alleles are in linkage disequilibrium, they are inherited together more often than would be expected by chance.

Genetic Marker

A specific DNA sequence with a known location on a chromosome, used in genetic mapping and identification of genes associated with particular traits or diseases.

GWAS (Genome-Wide Association Study)

A study that involves scanning entire genomes from many individuals to identify genetic variations (usually single nucleotide polymorphisms, or SNPs) that are associated with specific traits or diseases.