Immunoglobulin gene superfamily --- A family of evolutionarily related genes composed of human leukocyte antigen (HLA) class I and class II genes, immunoglobulin genes, T-cell receptor genes, and other genes encoding cell surface molecules.
Imprinting --- The phenomenon of different expression of alleles depending on the parent of origin. See Prader-Willi syndrome and Angelman syndrome in the text for examples.
In situ hybridization --- Mapping a gene or segment of DNA by molecular hybridization to a chromosome spread or cell nucleus on a slide by use of a labeled DNA sequence as a probe corresponding to the gene or DNA segment to be mapped. Usually involves fluorescently labeled probes, in which case it is referred to as fluorescence in situ hybridization (FISH).
Inborn error of metabolism？ific protein defect produces a metabolic block that may have pathological consequences.
Inbreeding --- The mating of closely related individuals. The progeny of close relatives are said to be inbred. (Note that some consider the term inbreeding to be pejorative when it is applied to human populations.)
Incompletely dominant --- A trait that is inherited in a dominant manner but is more severe in a homozygote than in a heterozygote (synonym: semidominant).
Indel --- A polymorphism defined by the presence or absence of a segment of DNA, ranging from one base to a few hundred base pairs. Includes simple indels, microsatellites, and minisatellite polymorphisms.
Index case --- The family member affected with a genetic disorder who is the first to draw attention to a pedigree. See proband.
Induction --- The determination of the fate of one region of an embryo by extracellular signals from a second, usually neighboring, region.
In-frame deletion --- A deletion that does not destroy the normal reading frame of the gene.
Inner cell mass --- A small group of cells within the preimplantation mammalian embryo that will become the primitive ectoderm (or epiblast) after implantation and, ultimately, give rise to the embryo proper and not the placenta.
Insert --- In molecular biology, a fragment of foreign DNA cloned into a vector.
Insertion --- A chromosomal abnormality in which a DNA segment from one chromosome is inserted into another chromosome.
Intergenic complementation --- The ability of cells from patients with similar phenotypes due to mutations in different genes to correct one another.
Intergenic DNA --- The untranscribed DNA of unknown function that makes up a large proportion of the total DNA in the genome.
Interphase --- The stage of the cell cycle between two successive mitoses.
Intervening sequence --- See intron.
Intron --- A segment of a gene that is initially transcribed but then removed from within the primary RNA transcript by splicing together the sequences (exons) on either side of it.
Inversion --- A chromosomal rearrangement in which a segment of a chromosome is reversed end to end. If the centromere is included in the inversion, the inversion is pericentric; if not, it is paracentric.
In vitro fertilization --- A reproductive technology in which sperm are allowed to fertilize an egg in tissue culture and the fertilized eggs are then introduced back into the uterus to allow implantation.
Isochromosome --- An abnormal chromosome in which one arm is duplicated (forming two arms of equal length, with the same loci in reverse sequence) and the other arm is missing.
Isodisomy --- See uniparental disomy.
Isolate --- A subpopulation in which matings take place exclusively or usually with other members of the same subpopulation.
Isolated case --- An individual who is the only member of his or her kindred affected by a genetic disorder, either by chance or by new mutation. See also sporadic.
Karyotype --- The chromosome constitution of an individual. The term is also used for a photomicrograph of the chromosomes of an individual systematically arranged and for the process of preparing such a photomicrograph.
kb (kilobase) --- A unit of 1000 bases in a DNA or RNA sequence.
Kindred --- An extended family.
Kinetochore --- A structure at the centromere to which the spindle fibers are attached.
Library --- In molecular biology, a collection of recombinant clones that contain a sample of the DNA or RNA (as cDNA) of a tissue.
Ligation --- In molecular biology, the process of joining two double-stranded DNA molecules to form a recombinant DNA molecule, by means of phosphodiester bonds, with use of the enzyme DNA ligase.
LINE sequences --- A class of repetitive DNA made up of long interspersed nuclear elements, up to 6 kb in length, occurring in several hundred thousand copies in the genome (also called L1 family).
Lineage --- The progeny of a cell, generally determined by experimentally labeling the cell so that all of its descendants can be identified. See clone.
Linkage --- Genes on the same chromosome are linked if they are transmitted together in meiosis more frequently than chance would allow. Compare with synteny.
Linkage analysis --- A statistical method in which the genotypes and phenotypes of parents and offspring in families are studied to determine whether two or more loci are assorting independently or exhibiting linkage during meiosis.
Linkage disequilibrium --- The occurrence of specific combinations of alleles in coupling phase at two or more linked loci more frequently than expected by chance from the frequency of the alleles in the population.
Linkage disequilibrium block？age disequilibrium with each other. Usually occupies a region of the genome from a few kilobases to a few dozen kilobases in length.
Linkage map --- A chromosome map showing the relative positions of genes and other DNA markers on the chromosomes, as determined by linkage analysis.
Locus --- The position occupied by a gene on a chromosome. Different forms of the gene (alleles) may occupy the locus.
Locus control region (LCR) --- A DNA domain, situated outside a cluster of structural genes, responsible for the appropriate expression of the genes within the cluster.
Locus heterogeneity --- The production of identical phenotypes by mutations at two or more different loci.
LOD score --- A statistical method that tests genetic marker data in families to determine whether two loci are linked. The lod score is the logarithm of the odds in favor of linkage. By convention, a LOD score of 3 (odds of 1000 : 1 in favor) is accepted as proof of linkage and a LOD score of -2 (100 : 1 against) as proof that the loci are unlinked.
L1 family --- See LINE sequences.
Loops --- Arrangement of chromatin, packaged as solenoids, attached to the chromosome scaffold. Thought to be a structural or functional unit of chromosomes.
Loss-of-function mutation --- A mutation associated with a reduction or a complete loss of one or more of the normal functions of a protein.
Loss of heterozygosity (LOH) --- Loss of a normal allele from a region of one chromosome of a pair, allowing a defective allele on the homologous chromosome to be clinically manifest. A feature of many cases of retinoblastoma, breast cancer, and other tumors due to mutation in a tumor-suppressor gene.
Lyonization --- Term used for the phenomenon of X inactivation, which was first described by the geneticist Mary Lyon. See X inactivation.
Major histocompatibility complex (MHC) --- The complex locus on chromosome 6p that includes the highly polymorphic human leukocyte antigen (HLA) genes.
Maleficence --- Behavior that harms others. Avoidance of maleficence is one of the cardinal principles of ethics. See beneficence.
Male-to-male transmission --- A pattern of inheritance of a trait from a father to all of his sons and none of his daughters (also referred to as holandric inheritance).
Malformation syndrome --- A recognizable pattern of dysmorphic features having a single cause, either genetic or environmental.
Manifesting heterozygote --- A female heterozygous for an X-linked disorder in whom, because of nonrandom X inactivation, the trait is expressed clinically with approximately the same degree of severity as in hemizygous affected males.
Maternal inheritance --- The transmission of genetic information only through the mother.
Maternal serum screening --- Laboratory test that relies on measurement of the levels of particular substances, such as alpha-fetoprotein, human chorionic gonadotropin, and unconjugated estriol, in a pregnant woman’s blood to screen for fetuses affected with certain trisomies or with neural tube defects.
Mb (megabase) --- A unit of 1,000,000 bases or base pairs in genomic DNA.
Meiosis --- The type of cell division occurring in the germ cells, by which gametes containing the haploid chromosome number are produced from diploid cells. Two meiotic divisions occur: meiosis I and meiosis II. Reduction in chromosome number takes place during meiosis I.
Mendelian --- Patterns of inheritance that follow the classic laws of Mendel: autosomal dominant, autosomal recessive, and X-linked. See single-gene disorder.
Mesoderm --- The middle germ layer in the early embryo; the source of cells that go on to make bones, muscles, connective tissue, heart, hematopoietic system, kidney, and other organs.
Messenger RNA (mRNA) --- An RNA, transcribed from the DNA of a gene, that directs the sequence of amino acids of the encoded polypeptide.
Metacentric --- A type of chromosome with a central centromere and arms of apparently equal length.
Metaphase --- The stage of mitosis or meiosis in which the chromosomes have reached their maximal condensation and are lined up on the equatorial plane of the cell, attached to the spindle fibers. This is the stage at which chromosomes are most easily examined.
Metastasis --- Spread of malignant cells to other sites in the body.
Methemoglobin --- The oxidized form of hemoglobin, containing iron in the ferric rather than the ferrous state, that is incapable of binding oxygen.
MicroRNA --- A particular class of noncoding RNAs that are processed into short interfering RNAs (siRNA), double stranded RNAs approximately 22 nucleotides in length, that affect mRNA stability or translation. siRNAs are involved in gene regulation in development and differentiation.微RNA：一种非编码RNA，与小干扰RNA（siRNA）有关，为双链RNA，长约22个核苷酸，影响mRNA的稳定和翻译。siRNA与发育和分化的基因调控有关。