Human Genetics Key Terms

Comprehensive vocabulary flashcards covering key concepts, syndromes, chromosomal abnormalities, and inheritance patterns from human genetics lecture notes.

Karyotype

An organized visual profile of an individual's complete set of chromosomes, aligned and numbered according to size and structure.

Autosome

Any chromosome that is not a sex chromosome; numbered chromosomes that are identical in both males and females.

Sex Chromosome

A type of chromosome involved in sex determination, typically designated as $X$ or $Y$ in humans.

Metaphase Arrest

The experimental halting of the cell cycle during metaphase, when chromosomes are condensed and optimal for cytogenetic evaluation.

Colchicine

A chemical agent that inhibits spindle microtubule formation, commonly used to achieve metaphase arrest for chromosome analysis.

Giemsa Banding (G-Banding)

A cytogenetic staining method using Giemsa dye to yield reproducible alternating light and dark bands along condensed chromosomes.

Chromosome Arm ($p$ arm / $q$ arm)

The structural sections of a chromosome divided by the centromere, with $p$ denoting the short arm and $q$ denoting the long arm.

Metacentric

A chromosome classification where the centromere is positioned near the middle, resulting in arms of approximately equal length.

Submetacentric

A chromosome classification where the centromere is positioned off-center, producing distinct short ($p$) and long ($q$) arms.

Acrocentric

A chromosome classification where the centromere is situated close to one end, resulting in a very short $p$ arm.

Fluorescence In Situ Hybridization (FISH)

A molecular cytogenetic technique utilizing fluorescent probes that bind complementarily to specific DNA sequences on chromosomes.

Pedigree Analysis

The systematic charting and evaluation of ancestral family relationships to determine the inheritance pattern of specific traits or conditions.

Autosomal Recessive

A mode of inheritance requiring two copies of an altered gene on an autosome for the phenotype or condition to be expressed.

Autosomal Dominant

A mode of inheritance where a single copy of a mutated gene on an autosome is sufficient to produce the phenotype.

X-Linked Recessive

An inheritance pattern caused by a mutation on the $X$ chromosome, manifesting primarily in hemizygous males and homozygous females.

X-Linked Dominant

An inheritance pattern where a single copy of a dominant mutation on the $X$ chromosome results in expression in both males and females.

Y-Linked (Holandric) Inheritance

A transmission pattern of genes located exclusively on the $Y$ chromosome, passed directly from father to son.

Consanguinity

Mating between genetically related individuals who share at least one common ancestor, increasing the probability of sharing identical mutant alleles.

Obligate Carrier

An individual who must carry a specific genetic mutation based on their position in a pedigree, regardless of phenotypic presentation.

Mitochondrial (Maternal) Inheritance

The transmission of non-nuclear mitochondrial DNA exclusively through maternal cytoplasm to all offspring.

Incomplete Penetrance

The failure of some individuals carrying a pathogenic genotype to express the corresponding clinical phenotype.

Variable Expressivity

The variation in clinical presentation or severity of a genetic trait among individuals possessing identical genotypes.

Late-Onset Inheritance

A genetic pattern wherein clinical signs and symptoms do not manifest until later in life, despite the underlying mutation being present from conception.

Genetic Heterogeneity

The occurrence where different gene mutations or loci produce identical or clinically indistinguishable phenotypes.

Dosage Compensation

The regulatory mechanism balancing gene expression from the $X$ chromosome between males with one $X$ and females with two $X$ chromosomes.

Lyon Hypothesis

The principle stating that one $X$ chromosome in female somatic cells is randomly and permanently inactivated during early development.

X-Inactivation

The transcriptional silencing of one $X$ chromosome in female cells to ensure equivalent dosage of $X$-linked gene products.

Barr Body

The dense, heterochromatic, condensed inactive $X$ chromosome visible at the periphery of the nucleus in female somatic cells.

XIST Gene

The $X$-inactive specific transcript gene that encodes non-coding RNA critical for initiating and maintaining $X$-inactivation.

Cellular Mosaicism

The coexistence of two or more genetically distinct cell lines within an individual that originated from a single zygote.

Turner Syndrome ($45, X$)

A condition in females defined by the complete or partial absence of one sex chromosome, resulting in a $45, X$ karyotype.

Klinefelter Syndrome ($47, XXY$)

A chromosomal disorder in males characterized by an extra $X$ chromosome, producing a $47, XXY$ karyotype.

Trisomy X ($47, XXX$)

A female sex chromosome anomaly characterized by the presence of three $X$ chromosomes, resulting in a $47, XXX$ karyotype.

Jacobs Syndrome ($47, XYY$)

A sex chromosome condition in males caused by an additional $Y$ chromosome, yielding a $47, XYY$ karyotype.

Nondisjunction

The failure of paired homologous chromosomes or sister chromatids to separate correctly during cell division.

Meiotic Nondisjunction (Meiosis I vs. Meiosis II)

Chromosomal separation failure occurring in either Meiosis I (involving homologous chromosomes) or Meiosis II (involving sister chromatids), resulting in abnormal gametic chromosome counts.

Aneuploidy

A condition marked by an abnormal chromosome number that differs from the normal diploid count by one or more individual chromosomes.

Monosomy

The presence of only one member of a chromosome pair instead of the standard diploid complement ($2n - 1$).

Trisomy

The presence of three copies of a particular chromosome instead of the standard homologous pair ($2n + 1$).

Down Syndrome (Trisomy 21)

A chromosomal disorder resulting from the presence of a third copy of chromosome 21.

Edwards Syndrome (Trisomy 18)

A severe chromosomal disorder caused by the presence of an extra copy of chromosome 18.

Patau Syndrome (Trisomy 13)

A severe chromosomal condition caused by an additional copy of chromosome 13.

Maternal Age Effect

The statistical increase in the probability of meiotic nondisjunction and resulting fetal aneuploidy associated with advancing maternal age.

Chromosomal Deletion

A chromosomal aberration resulting from the physical loss of a segment of genetic material from a chromosome.

Chromosomal Duplication

A structural alteration that produces an extra copy of a specific segment within a chromosome.

Chromosomal Inversion (Paracentric vs. Pericentric)

A chromosomal rearrangement involving a $180^\circ$ reversal of a chromosome segment; paracentric excludes the centromere, while pericentric spans across the centromere.

Reciprocal Translocation

A structural chromosomal abnormality characterized by mutual exchange of segments between two non-homologous chromosomes without altering total chromosome number.

Robertsonian Translocation

A specific structural translocation occurring between acrocentric chromosomes that fuse at or near their centromeres with the loss of their short arms.

Non-Invasive Prenatal Testing (NIPT / cfDNA)

A prenatal screening approach that evaluates cell-free fetal DNA circulating in maternal blood to detect fetal chromosomal aneuploidies.

Amniocentesis

An invasive prenatal diagnostic procedure involving the extraction and genetic testing of amniotic fluid containing fetal cells.

Chorionic Villus Sampling (CVS)

An invasive prenatal diagnostic technique that samples placental tissue from the chorionic villi to identify chromosomal and genetic conditions.

Inborn Errors of Metabolism (PKU)

A group of genetic metabolic conditions, exemplified by phenylketonuria (PKU), arising from defects in enzymes that catalyze biochemical pathways.

Genomic Imprinting

An epigenetic mechanism causing genes to be expressed in a parent-of-origin-specific manner, silencing one parental allele via methylation or chromatin modification.