Extensions of Mendelian Inheritance and Chromosome Structure

A complete set of vocabulary flashcards covering non-Mendelian inheritance patterns, sex-influenced and sex-limited traits, epistasis, organellar/extranuclear inheritance, imprinting, gene linkage and recombination mapping, and structural and numerical chromosomal variations.

Wild-type allele

The allele most commonly found in a natural population, which usually encodes a functional protein produced in normal amounts.

Genetic polymorphism

A condition in a population where a gene has two or more common wild-type alleles.

Recessive allele

An allele often caused by mutations that reduce or eliminate the function of the encoded protein, expressed usually only when an individual carries two copies.

Lethal allele

An allele that causes the death of an organism when expressed.

Conditional lethal allele

An allele that is lethal only under particular environmental conditions, such as a specific temperature range.

Incomplete dominance

A pattern of inheritance where the phenotype of a heterozygote is intermediate between the phenotypes of the two homozygotes.

Codominance

A pattern of inheritance occurring when both alleles in a heterozygote are fully expressed, resulting in both characteristics being visible rather than an intermediate phenotype.

Penetrance

The proportion of individuals with a particular genotype who express the expected phenotype, measured at the population level.

Variable expressivity

Differences in the degree or form of a phenotype among individuals who possess the same genotype.

Sex-influenced trait

A trait expressed differently in males and females, where the same allele may be dominant in one sex but recessive in the other due to hormonal or physiological differences.

Cattle scurs trait

A sex-influenced trait in cattle where the same genotype produces different phenotypic outcomes (scurs vs. no scurs) depending on whether the individual is male or female.

Sex-limited trait

A trait that is expressed in only one sex, even though the genes responsible may be present in both sexes.

Sexual dimorphism

The condition in which males and females of the same species differ in physical form.

Epistasis

A gene interaction that occurs when alleles of one gene mask or alter the effects of alleles at another gene locus.

Pleiotropy

A phenomenon where a single gene affects multiple, often apparently unrelated, phenotypic traits.

Complementation

A phenomenon occurring when two individuals with recessive mutations in different genes produce offspring with a wild-type phenotype.

Extranuclear inheritance

Also called cytoplasmic inheritance, this involves genetic material located outside the cell nucleus, such as in mitochondrial DNA or chloroplast DNA.

Paternal leakage

An unusual event in which mitochondrial DNA is inherited from the father rather than exclusively from the mother.

Heteroplasmy

The presence of more than one type of mitochondrial genome within a single cell.

Leber hereditary optic neuropathy (LHON)

A mitochondrial disease caused by mutations in mtDNA that produces sudden, irreversible loss of central vision due to damaged cells in the optic nerve.

Maternal effect

An inheritance pattern in which the mother's genotype determines the phenotype of her offspring, regardless of the offspring's own genotype.

Genomic imprinting

A form of non-Mendelian inheritance in which gene expression depends on whether an allele was inherited from the mother or the father.

Monoallelic expression

The expression pattern of an imprinted gene where only one parental allele is active while the other copy is silenced.

Imprinting control regions (ICRs)

DNA regions associated with imprinted genes that govern the establishment and maintenance of parental imprinting marks.

Gene linkage

The phenomenon where genes located close together on the same chromosome tend to be inherited together as a unit.

Crossing over

The process during pachytene of prophase I in meiosis where corresponding chromosome segments are exchanged between homologous chromosomes.

Twin spotting

A visual outcome of mitotic recombination where two genetically distinct cell populations arise from a single original cell, creating visibly different tissue patches.

Centimorgan (cM)

A unit of measure for genetic map distance, where one centimorgan (or map unit) is approximately equal to a $1\%$ recombination frequency.

Testcross

A cross performed between an individual heterozygous for the genes of interest and an individual homozygous recessive for those same genes.

Three-point cross

A genetic cross used to analyse three linked genes simultaneously to determine their relative order and map distances on a chromosome.

p arm

The short arm of a chromosome.

q arm

The long arm of a chromosome.

Metacentric chromosome

A chromosome with its centromere located near the middle.

Submetacentric chromosome

A chromosome with its centromere located slightly off-centre.

Acrocentric chromosome

A chromosome with its centromere located close to one end.

Telocentric chromosome

A chromosome with its centromere located at or very near the end.

Terminal deletion

A chromosomal deletion that removes genetic material from the end of a chromosome, often involving the telomere.

Interstitial deletion

A chromosomal deletion that removes a segment from the middle of a chromosome while leaving both chromosome ends intact.

Cri-du-chat syndrome

A genetic disorder caused by a deletion of part of human chromosome $5$.

Gene family

A group of related genes within a genome that originated from a common ancestral gene through duplication.

Paralogs

Homologous genes located within the same species that arose via gene duplication.

Position effect

A change in a gene's expression resulting from its relocation to a different chromosomal environment influenced by different regulatory elements.

Pericentric inversion

A chromosomal inversion that includes the centromere, with breakpoints occurring on opposite chromosome arms.

Paracentric inversion

A chromosomal inversion that does not include the centromere, with both breakpoints occurring on the same chromosome arm.

Reciprocal translocation

A chromosomal rearrangement involving an exchange of chromosome segments between two non-homologous chromosomes.

Balanced translocation

A translocation in which genetic material is rearranged without any net gain or loss of genetic material.

Unbalanced translocation

A translocation that involves extra or missing genetic material and may cause abnormal development or disease in offspring.

Robertsonian translocation

A specific type of translocation occurring between acrocentric chromosomes.

Euploidy

The state of possessing one or more complete sets of chromosomes.

Polyploidy

The state of having more than two complete chromosome sets.

Endopolyploidy

A condition in which somatic cells undergo DNA replication without cell division, producing multiple chromosome sets within a single cell.

Polytene chromosomes

Chromosomes that contain many replicated copies of the same DNA molecule lying side-by-side.

Allodiploid

An organism containing two chromosome sets derived from two different species through hybridisation.

Aneuploidy

A condition involving an abnormal number of individual chromosomes rather than a complete extra chromosome set.

Nondisjunction

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

Mosaicism

A condition in which an individual possesses genetically different cell lines that arose after fertilisation.