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You are watching: Alternative forms of the same gene are called

Morrison PJ, Spence RAJ. Genes for Surgeons. London: Remedica; 2005.

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Adenine (A)

One of the bases making up DNA and RNA (pairs withthymine in DNA and uracil in RNA).

Agarose gelatin electrophoresis

See electrophoresis.


One of two or much more alternative creates of a gene in ~ a offered location(locus). A single allele for each locus is inherited separatelyfrom every parent. In normal human beings there room two alleles because that each locus(diploidy). If the 2 alleles space identical, the individual issaid to be homozygous for that allele; if different, the individualis heterozygous.

For example, the normal DNA succession at codon 6 in thebeta-globin gene is GAG (coding because that glutamic acid), whereas in sickle celldisease the succession is GTG (coding for valine). An separation, personal, instance is said to beheterozygous for the glutamic acid → valine mutation ifhe/she own one common (GAG) and one mutated (GTG) allele. Together individualsare carriers the the sickle cabinet gene and also do not manifest classicalsickle cell an illness (which is autosomal recessive).

Allelic heterogeneity

Similar/identical phenotypes caused by differentmutations in ~ a gene. Because that example, manydifferent mutations in the same gene room now recognized to be linked withMarfan"s syndrome (FBN1 gene in ~ 15q21.1).


Withdrawal the amniotic fluid, usually brought out throughout the 2nd trimester,for the purpose of prenatal diagnosis.


The manufacturing of increased numbers that a DNA sequence.

In vitro.

In the early days of recombinant DNA techniques, the only means to amplify asequence of interest (so that large amounts were available for detailedstudy) was to clone the fragment in a vector(plasmid or phage) and transform bacteria through therecombinant vector. The transformation method generally results in the"acceptance" that a solitary vector molecule by every bacterial cell. The vectoris able come exist autonomously in ~ the bacterial cell, periodically at veryhigh copy number (eg, 500 vector copies per cell). Development of the bacteriacontaining the vector, coupled through a an approach to recuperate the vector sequencefrom the bacterial culture, enables for nearly unlimited manufacturing of asequence that interest. Cloning and bacterial propagation room still provided forapplications requiring either big quantities of product or elseexceptionally pure material.

However, the advent of the polymerase chain reaction (PCR) hasmeant the amplification of preferred DNA sequences deserve to now be performed morerapidly 보다 was the instance with cloning (a couple of hours cf. Days), and also it is nowroutine come amplify DNA sequences 10 million-fold.

In vivo.

Amplification may additionally refer to boost in the variety of DNA sequenceswithin the genome. Because that example, the genomes of many tumors space now recognized tocontain regions that have actually been intensified many fold compared to theirnontumor counterparts (ie, a succession or an ar of DNA that normally occursonce at a specific chromosomal place may be existing in hundreds ofcopies in some tumors). It is thought that countless such areas harboroncogenes, which, when existing in high copy number,predispose to advance of the malignant phenotype.


Possessing an not correct number (abnormal complement) the chromosomes.The common human enhance is 46 chromosomes, any cell the deviates from thisnumber is stated to it is in aneuploid.


The chromosomal problem of a cell or organism with an untrue number ofchromosomes. Individuals with Down"s syndrome are explained ashaving aneuploidy, since they own an extra copy that chromosome 21(trisomy 21), do a complete of 47 chromosomes.


A general phenomenon that describes the monitoring of an increase in severity,and/or decrease in age of onset, the a problem in succeeding generations the afamily (see number 1). Anticipation isnow known, in countless cases, to result directly from the existence of adynamic mutation in a family. In the lack of a dynamicmutation, anticipation might be defined by "ascertainment bias".Thus, prior to the first dynamic mutations were described (in fragile X andmyotonic dystrophy), that was thought that ascertainment prejudice was the completeexplanation because that anticipation. There space two main reasons for ascertainmentbias:

Identical mutations in different individuals oftenresult in change expressions the the associatedphenotype. Thus, people within a family, allof who harbor an identical mutation, may have actually variation in theseverity of their condition.
Individuals through a major phenotype are more likely to existing to themedical profession. Moreover, such individuals are much more likely tofail come reproduce (ie, lock are genetic lethals), often for social,rather than direct physical reasons.

For both reasons, the is much more likely the a mildly affected parent will certainly beascertained with a severely influenced child, 보다 the reverse. Therefore, theseverity the a condition shows up to boost through generations.


The 3-base succession on a transfer RNA (tRNA) molecule the iscomplementary to the 3-base codon that a messenger RNA(mRNA) molecule.

Ascertainment bias

See anticipation.

Autosomal disorder

A disorder linked with a mutation in an autosomalgene.

Autosomal leading (AD) inheritance

An autosomal disorder in which the phenotype isexpressed in the heterozygous state. This disorders room notsex-specific. Fifty percent the offspring (when only one parent is affected) willusually manifest the disorder (see Figure2). Marfan syndrome is a an excellent example the an advertisement disorder; affectedindividuals own one wild-type (normal) and one mutated alleleat the FBN1gene.

Autosomal recessive (AR) inheritance

An autosomal disorder in i m sorry the phenotype ismanifest in the homozygous state. This pattern of inheritance isnot sex-specific and also is difficult to trace v generations due to the fact that bothparents must contribute the abnormal gene, however may not necessarilydisplay the disorder. The youngsters of 2 heterozygous AR parentshave a 25% opportunity of manifesting the disorder (see number 3). Cystic fibrosis (CF) is a an excellent example the anAR disorder; influenced individuals possess 2 mutations, one ateach allele.


Any chromosome, various other than the sex chromosomes (X orY), the occurs in pairs in diploid cells.


Barr body

An inactive X chromosome, clearly shows in the somatic cellsof people with more than one X chromosome (ie, all regular females and allmales with Klinefelter"s syndrome). For individuals with nX chromosomes,n–1 Barr bodies room seen. The visibility of a Barr body in cellsobtained through amniocentesis or chorionic villus samplingwas previously used as an indication of the sex the a baby before birth.

Base pair (bp)

Two nucleotides held together through hydrogen bonds. InDNA, guanine always pairs with cytosine,and thymine through adenine. A basic pair is likewise thebasic unit because that measuring DNA length.



An individual who is heterozygous because that a mutant allele(ie, dead one wild-type and also one mutated copy of thegene under consideration).

CentiMorgan (cM)

Unit of genetic distance. If the opportunity of recombination between twoloci is 1%, the loci are stated to it is in 1 cm apart. On average, 1 cM suggests aphysical distance of 1 Mb (1,000,000 base pairs) but significantdeviations native this preeminence of thumb occur because recombination frequencies varythroughout the genome. Therefore if recombination in a certain an ar isless most likely than average, 1 cM may be tantamount to 5 Mb (5,000,000 basic pairs)in that region.


Central constriction the the chromosome wherein daughterchromatids are joined together, separating the brief (p) fromthe long (q) eight (see figure 4).

Chorionic villus sampling (CVS)

Prenatal diagnostic procedure for obtaining fetal organization at an earlier stage ofgestation 보다 amniocentesis. Typically performed ~ 10 weeks,ultrasound is offered to overview aspiration of tissue from the villus area of thechorion.


One of the two parallel similar strands that a chromosome, connectedat the centromere during mitosis andmeiosis (see number 4).Before replication, each chromosome consists of only one chromatid. Afterreplication, two identical sister chromatids are present. In ~ the finish of mitosisor meiosis, the 2 sisters separate and also move come opposite poles prior to the cellsplits.


A easily stained substance in the nucleus of a cabinet consisting ofDNA and also proteins. Throughout cell department it coils and also folds toform the metaphase chromosomes.


One of the threadlike "packages" that genes and also other DNAin the cell core of a cabinet (see number 4).Humans have actually 23 bag of chromosomes, 46 in total: 44 autosomes andtwo sex chromosomes. Each parental contributes one chromosome to eachpair.

Chromosomal disorder

A disorder that outcomes from gross alters in chromosome dose. Mayresult from enhancement or ns of whole chromosomes or just portions ofchromosomes.


A team of genetically similar cells v a typical ancestor.


A 3-base coding unit that DNA that mentions the duty of acorresponding unit (anticodon) of transfer RNA(tRNA).

Complementary DNA (cDNA)

DNA synthesized indigenous messenger RNA (mRNA) usingreverse transcriptase. Differs from genomic DNAbecause it lacks introns.


The wild-type allele that a gene compensates because that a mutantallele the the same gene so the the heterozygote"s phenotype iswild-type.

Complementation analysis

A genetic test (usually performed in vitro) that determineswhether or not 2 mutations that produce the samephenotype space allelic. It allows the geneticist to determinehow many distinct genes are associated when challenged with a numberof mutations that have similar phenotypes.

Occasionally it can be observed clinically. Two parents who both endure fromrecessive hearing deactivated (ie, both are homozygous for amutation causing deafness) may have actually offspring that have normal hearing. IfA and also B describe the wild-type (normal) develops of the genes, and a and also b themutated forms, one parent can be aa,BB and the other AA,bb. Ifalleles A and also B are distinct, each son will have actually thegenotype aA,bB and also will have actually normal hearing. If A and also B areallelic, the child will it is in homozygous in ~ this locus and will alsosuffer native deafness.

Compound heterozygote

An individual with two different mutant alleles at the samelocus.

Concordant Consanguinity

A pair that twins that manifest the same phenotype as each other.Sharing a typical ancestor, and also thus genetically related. Recessivedisorders are seen with enhanced frequency in consanguineous families.


An separation, personal, instance seeking genetic advice.

Contiguous gene syndrome

A syndrome result from the simultaneous practical imbalance the a team ofgenes (see number 5).The nomenclature for this team of obstacle is rather confused, greatly as aresult of the background of their elucidation. The state submicroscopicrearrangement/deletion/duplication and microrearrangement/deletion/duplicationare frequently used interchangeably. Micro or submicroscopic refer to the fact thatsuch lesions room not detectable with standard cytogenetic viewpoints (where thelimit that resolution is normally 10 Mb, and also 5 Mb in only the most fortuitous ofcircumstances). A newer, and also perhaps much more comprehensive, term the is currentlyapplied to this group of disorders is segmental aneusomy syndromes (SASs). Thisterm embraces the opportunity not just of lose or get of a chromosomal regionthat harbors many genes (leading come imbalance of all those genes), but also offunctional imbalance in a team of genes, together a result of one abnormality of themachinery involved in your silencing/transcription (ie,methylation-based instrument that depend on a master manage gene).

In practice, most contiguous gene syndromes result from theheterozygous deletion that a segment that DNA the islarge in molecular terms but not detectable cytogenetically. The size of suchdeletions is usually 1.5–3.0 Mb. It is usual for one to 2 dozengenes to be involved in together deletions, and the resultantphenotypes are often complex, involving multiple body organ systemsand, practically invariably, discovering difficulties. A great example that a contiguousgene syndrome is Williams" syndrome, a sporadic disorder the is because of aheterozygous deletion at chromosome 7q11.23. Affected individualshave characteristic phenotypes, consisting of recognizable facial appearance andtypical behavior traits (including moderate finding out difficulties).Velocardiofacial syndrome is at this time the many commonmicrodeletion known, and is resulted in by deletions that 3 Mb atchromosome 22q11.

Crossing over

Reciprocal exchange of genetic material between homologouschromosomes at meiosis (see figure 6).


The examine of the framework of chromosomes.

Cytosine (C)

One of the bases comprising DNA and RNA (pairs withguanine).


Cells acquired from fetal chorionic villi through chorionic villus sampling(CVS). Supplied for DNA and chromosomeanalysis.

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A certain kind the mutation that involves the ns of a segment ofDNA native a chromosome with succeeding re-joiningof the 2 extant ends. It can refer come the remove of one or more bases withina gene or come a much bigger aberration entailing millions the bases.The ax deletion is not completely specific, and also differentiation should be madebetween heterozygous and also homozygous deletions. Largeheterozygous deletions are a common cause of complicated phenotypes(see contiguous gene syndrome); large germ-line homozygousdeletions are very rare, however have been described. Homozygous deletions arefrequently explained in somatic cells, in association v themanifestation the the malignant phenotype. The 2 deletions in a homozygousdeletion need not it is in identical, but must an outcome in the complete absence of DNAsequences the occupy the "overlap" region.