Genetics Terms Starting With I
Genetics Glossary: I
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Imprinting
/ im-PRIN-ting / · Old French: empreindre (to impress)
Imprinting is an epigenetic process by which certain genes are expressed in a parent-of-origin-specific manner, with one parental copy silenced through DNA methylation or histone modification.
Imprinted genes carry epigenetic marks established during gametogenesis that persist through fertilization and development. Expression of imprinted genes is monoallelic, meaning only the maternal or paternal copy is active in a given tissue. Disruption of imprinting at chromosome 15q11-q13 causes Prader-Willi syndrome when the paternal copy is lost and Angelman syndrome when the maternal copy is lost, demonstrating that both parental copies are required for normal development even though only one is expressed.
Approximately 100 to 200 imprinted genes have been identified in the human genome, and many cluster in discrete chromosomal regions regulated by shared imprinting control elements.
Imprinting evolved independently in mammals and flowering plants, suggesting it arose as a mechanism to regulate resource allocation between parent and offspring rather than from a single shared ancestral origin.
Imprinting is not the same as X-inactivation. X-inactivation silences one entire X chromosome chosen at random in each somatic cell, while imprinting affects specific individual genes on autosomes or sex chromosomes based solely on which parent transmitted the allele.
In mice (Mus musculus), the Igf2 gene encoding insulin-like growth factor 2 is expressed only from the paternal chromosome, while the neighboring H19 gene is expressed only from the maternal chromosome. Mice that inherit a deletion of the paternal Igf2 copy weigh roughly 40 percent less than normal littermates, illustrating how a single imprinted gene controls fetal growth.
Inbreeding
/ IN-bree-ding / · English: in + breeding
Inbreeding is the mating of individuals more closely related to each other than would be expected by random mating in a population, increasing the proportion of homozygous loci in offspring.
The degree of inbreeding is quantified by the inbreeding coefficient, which measures the probability that both alleles at a locus are identical by descent from a common ancestor. Inbreeding depression occurs when increased homozygosity exposes deleterious recessive alleles, reducing fitness in inbred offspring. Conservation programs actively monitor and manage inbreeding in small or captive populations to prevent genetic deterioration.
The cheetah (Acinonyx jubatus) is a well-documented example, with genome-wide heterozygosity so low that unrelated individuals accept skin grafts from one another, a level of genetic uniformity normally seen only between identical twins.
In humans, first-cousin marriages increase the risk of offspring inheriting two copies of a rare recessive allele by about four to five fold compared to matings between unrelated parents.
Autosomal Recessive Inheritance →Inbreeding does not introduce new harmful mutations into a population. It increases homozygosity, which reveals deleterious recessive alleles that were already present but hidden in heterozygous carriers.
The Florida panther (Puma concolor coryi) population suffered severe inbreeding depression, including poor sperm quality and cardiac defects, before wildlife managers introduced eight Texas pumas in 1995. Within one generation, heterozygosity increased measurably and survival rates of kittens improved, demonstrating the power of genetic rescue in small populations.
Incomplete Dominance
/ in-kum-PLEET DOM-ih-nens / · Latin: in (not) + completus + dominare (to rule)
Incomplete Dominance is a pattern of inheritance in which neither allele is fully dominant, producing a heterozygous phenotype that is intermediate between those of the two homozygotes.
Unlike complete dominance, where heterozygotes resemble the dominant homozygote, incomplete dominance produces a blend of both parental phenotypes in the heterozygote. The classic example is snapdragon (Antirrhinum majus) flower color, where crossing red-flowered and white-flowered plants produces pink-flowered F1 offspring. When two pink-flowered F1 plants are crossed, the F2 generation shows a 1:2:1 ratio of red, pink, and white flowers, recovering both original phenotypes.
At the molecular level, the intermediate phenotype often reflects a reduced total dose of functional protein, since one allele produces a non-functional product and a single working copy generates less pigment than two.
Incomplete dominance in snapdragons was once cited as evidence for blending inheritance, an older theory proposing that parental traits permanently merge in offspring, but the reappearance of red and white flowers in the F2 generation disproved that idea.
White Flowers →Incomplete dominance is not the same as codominance. In incomplete dominance the heterozygote shows a blended intermediate phenotype, while in codominance both alleles are fully and simultaneously expressed as distinct traits in the same individual.
Andalusian chickens show incomplete dominance in feather color: crossing black-feathered and white-feathered birds produces blue-feathered offspring in the F1 generation. Crossing two blue Andalusians yields approximately 25 percent black, 50 percent blue, and 25 percent white offspring, matching the 1:2:1 ratio predicted for incomplete dominance.
Insertion Mutation
/ in-SER-shun myoo-TAY-shun / · Latin: insertio (a grafting in)
Insertion Mutation is a type of mutation in which one or more nucleotides are added to a DNA sequence, potentially disrupting the reading frame and altering the encoded protein.
Single-nucleotide insertions cause frameshift mutations that change every codon downstream of the insertion site, typically producing a nonfunctional protein. Larger insertions may introduce premature stop codons, disrupt splice sites, or add entire functional domains depending on their size and position. Insertions caused by transposable elements moving into gene coding regions are a significant source of spontaneous mutation in many species; transposable elements make up roughly 45 percent of the human genome and continue to generate new insertions at low frequency.
When an insertion is a multiple of three nucleotides and falls within a coding sequence, the reading frame is preserved, and the effect on protein function depends on the chemical properties of the added amino acids.
Fragile X syndrome, the most common inherited cause of intellectual disability, results from an expansion of a CGG trinucleotide repeat in the FMR1 gene, with affected individuals carrying more than 200 repeats compared to the typical range of 5 to 44.
An insertion mutation does not always cause disease. Insertions in non-coding regions, or those that are exact multiples of three nucleotides within a coding sequence, may leave protein function intact or produce only subtle changes in activity.
The insertion of a transposable element called gypsy into the white gene of the fruit fly (Drosophila melanogaster) disrupts pigment biosynthesis and produces a white-eyed phenotype in flies that would otherwise have red eyes. The inserted sequence spans several kilobases and prevents normal transcription of the white gene.
Intron
/ IN-tron / · Latin: intra (within) + -on (unit)
Intron is a non-coding nucleotide sequence within a eukaryotic gene that is transcribed into pre-mRNA but removed by splicing before the mature mRNA is translated.
Introns are removed from pre-mRNA by the spliceosome, a large ribonucleoprotein complex that recognizes conserved splice site sequences at intron boundaries. Most human genes contain multiple introns, and intronic sequences together account for a much larger fraction of the genome than the exons do. Although introns do not encode proteins, many contain regulatory sequences, non-coding RNA genes, and elements that influence gene expression.
Alternative splicing of intron-exon boundaries in a single pre-mRNA can generate dozens of distinct protein isoforms from one gene.
The human dystrophin gene contains 78 introns and spans 2.4 million base pairs of genomic DNA, making it the largest known human gene, yet its mRNA is only about 14,000 nucleotides long.
Building Blocks of Nucleic Acids →Introns are not simply junk DNA. Many introns harbor enhancers, silencers, and non-coding RNA genes that directly regulate gene expression.
Mutations at the splice sites flanking an intron in the CFTR gene can prevent proper intron removal, leading to a truncated or nonfunctional cystic fibrosis transmembrane conductance regulator protein. A splicing error can alter the final mRNA even when every coding exon remains intact.