What is a locus
The unique chromosomal location defining the position of an individual gene or DNA sequence
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What is allelic heterogeneity
Different mutations in the same locus cause the same disease
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What is a genotype
A list of the alleles present at one or a number of loci
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What is a gene
A gene is a determinant, or a co-determinant, of a character that is inherited in accordance with Mendel’s rules
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What does homozygous mean
If both alleles at that locus are the same, then the person is said to be homozygous
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What does heterozygous mean
If both alleles at the locus are different, then the person is said to be heterozygous
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What is hemizygous
A person is hemizygous if they only have a single allele at a locus
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What is a dominant disease
When a phenotype manifests in a heterozygous person
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What is a recessive disease
When a phenotype does not manifest in a heterozygous person
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Non-Mendelian/multifactorial character
Where a character depends on 2, 3 or many genetic loci
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Mendelian character
Where a particular genotype at a single locus is both necessary and sufficient for a phenotype to be expressed
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When genetic determination involves a small number of loci
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When a genetic determination involves many loci, each of small effect
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Susceptibility genes
Susceptibility genes increase a person’s likelihood of contracting a heritable illness
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Dichotomous character
Characters that you either have or do not have (e.g. extra fingers)
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Quantitative/continuous character
An inherited character that is expressed phenotypically in all degrees of variation, between one (often infinite) extreme to another
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Quantitative trait loci
A loci that contributes to determining the phenotype of a continuous character
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A person serving as the starting point for the genetic study of a family
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Occurs when the contributions of both alleles are visible in the phenotype
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X inactivation
A process by which one of the copies of the X chromosome present in female mammals is inactivated
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Barr body
The inactive X chromosome is visualised as a barr body in interphase cells
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Manifesting heterozygotes
Heterozygous women may be severely affected for an X linked recessive disease due to bad luck, where the healthy X chromosome has been inactivated in critical tissues
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'Porcupine men' is an example of what
Y linked disorder
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PAR1 and PAR2 are _______ regions of the X and Y chromosomes
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Leri Weill dyschondrosteosis is associated with
Pseudoautosomal inheritance
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Matrilineal inheritance
Inheritance of a trait or gene directly from the mother
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A cell can have some mitochondria that have a mutation in the mtDNA and some that do not
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In some cases of mitochondrial disease, every mitochondrial genome carries the causative mutation
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Mitochondrial bottleneck
A stage in which cells contain very few mitochondria, which can result in variation of the number of abnormal mitochondria between mother and offspring
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The theory that each gene directly produces a single enzyme; this is generally, but not exactly correct
One gene one enzyme hypothesis
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Locus heterogeneity
Similar phenotypes result from mutations in several different loci
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Clinical heterogeneity
Mutations on the same gene produce two different diseases
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Allelic heterogeneity
Different mutations in the same locus cause the same disease
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The effect of one gene is dependent on the presence of another gene (modifier gene)
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The common situation where variation in one gene affects several different aspects of the phenotype (e.g MARFAN)
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Complementation test
Can be used in model organisms to determine whether mutations in different strains are on the same gene
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Interallelic complementation
Alleles at the same locus can complement each other
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The tendency of some conditions to become more severe or have an earlier onset in successive generations
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The probability that a person who has the genotype will manifest the character
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Variable expression
Different members of the family show different features of the syndrome, due to other genes, environmental factors and chance
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Certain human characters affect both sexes, but manifest only when inherited from a parent of a particular sex; this is because certain genes are imprinted with their parental origin
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An individual who has two or more genetically different cell lines derived from a single zygote
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An individual who is the result of fusion of two zygotes to form a single embryo
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What is heritability
The proportion of phenotypic variance for a quantitative trait that is attributable to genetic factors
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Mathematical polygenic theory
Assumes that there are very many loci, each with a small effect
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Many factors contribute to a disorder, and the effects of each are small, once the additive effects of the factors pass a critical value, one becomes affected with a disorder – there is a multifactorial causality but a dichotomous phenotype
Polygenic threshold theory
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What is the genome
The complete haploid genetic complement of a typical cell and the content of the organelles in the cell
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Active chromatin often containing DNA sequences which are being transcribed into RNA, there is relatively weak binding by histone H1 molecules and extensive acetylation of the 4 nucleosomal histones
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Inactive chromatin, highly condensed and can be divided into constitutive and facultative
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Carries genetic information from DNA to the protein synthesis machinery, often described as messenger RNA
Coding RNA
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Often involved in assisting the expression of other genes
Non-coding RNA
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Genetic elements that can amplify themselves in a genome and are ubiquitous components of the DNA of many eukaryotic organisms; these sequences are transcribed into RNA, then reverse transcribed back into DNA and inserted into the genome
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A group of genetic elements found in large numbers in human genome, it encodes a reverse transcriptase and other proteins
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Short DNA sequences
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LTR retrotransposon
Retrotransposons which have direct long terminal repeats ranging from 100bp – 5kb
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What is different in RNA and DNA
In DNA, the sugar is deoxyribose
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In RNA uracil replaces ____
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A sugar with an attached base (in DNA) is called
A nucleoside
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A sugar, with a base and a phosphate group is called
A nucleotide
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A form DNA
Rare form of DNA in which DNA adopts a right handed helix form (spiralling in a clockwise direction) with 11 base pairs per turn
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B form DNA
In most eukaryotic cells, DNA adopts this right handed helix form (spiralling in a clockwise direction) with 10 base pairs per turn
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Z form DNA
Rare form of DNA in which DNA adopts a left handed helix form (spiralling in an anti-clockwise direction) with 12 base pairs per turn
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Unwinds the DNA strands of a helix
DNA helicase
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What does semi-conservative replication mean
Each daughter DNA duplex contains one strand from the parent molecule and one newly synthesised strand
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Origins of replication
Initiation points of DNA replication
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Semi-discontinuous synthesis
Refers to the leading and lagging strands of DNA, in which only the leading strand is synthesised continuously
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Housekeeping genes
Expressed in essentially all cells, often required for the maintenance of basic cellular functions such as gene expression
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The non-template strand during RNA synthesis
Sense strand
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The template strand during RNA synthesis
Antisense strand
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What is a promotor
A collection of closely spaced short DNA sequence elements, often in the immediate vicinity of a gene, which transcription factors recognise and bind to in order to initiate transcription - they are said to be cis-acting
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TATA box, GC box and GAAT box are elements often found within
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What is the basal transcription apparatus
The complex that is required to initiate transcription by an RNA polymerase, it consists of the polymerase plus associated general transcription factors
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A cluster of cis-acting short sequence elements that can enhance the transcriptional activity of a specific gene, these sequences can have very variable distances from the gene they work on. They bind to activator proteins and other transcription app
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Segments of a transcript that are cut out during splicing; they appear to be largely non-functional, yet may contain elements that modify transcription and splicing, or may be the source of small nucleolar RNAs, micro RNAs and other RNAs
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A segment of a gene that is retained during splicing; they may contain coding or non-coding DNA
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A series of reactions whereby the intronic RNA segments are removed and remaining exonic RNA segments are joined end to end to give a shorter RNA product
RNA splicing
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A larger RNA protein complex which mediates RNA splicing in eukaryotic nuclei
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The highly conserved dinucleotides at the end of introns, which are crucial for splicing
GT-AG rule
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Splice junction consensus sequences
Sequences that are immediately adjacent to GT and AG exon limits, which are highly conserved and contribute to marking the limits of an intron
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Branch site
A conserved intronic sequence which is typically located no more than 40 nucleotides upstream of an introns terminal AG
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5' capping
Shortly after the initiation of synthesis of a primary RNA transcript, a methylated nucleotide is linked by phosphodiester bond to the first 5’ nucleotide; this nucleotide protects the transcript from exonuclease attack and facilitates transport
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3' polyadenylation
The enzyme polyA polymerase adds adenylate residues to the 3’ end of a cleaved transcript, this is known as a polyA tail and is thought to aid in transport, stabilisation and enhance recognition
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The ribosomal subunits in eukaryotes are
60S and 40S
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Base wobble
Refers to the relaxation of base pairing at position 3 of a codon in RNA, which is why only 22 types of tRNA are made
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Addition of carbodydrates, lipid groups and post translational cleavage
Post translational processing
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What does haploid mean
Describing a cell which has only a single copy of each chromosome
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What does diploid mean
Having two copies of each type of chromosome; the normal constitution of most human somatic cells
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What is the zygote
The fertilised egg cell
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The point at which paired chromosomes remain in contact and cross over to exchange genetic material
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During interphase, the chromatin exists in an extended state which is diffusely staining and dispersed throughout the nucleus known as
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Constitutive heterochromatin
Condensed and generally inactive, consisting largely of repetitive DNA and found in centromeres and telomeres
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Faculative heterochromatin
Sometimes inactive, sometimes active, for example like that in X chromosome inactivation
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What order are the 5 stages of prophase?
Leptotene, zygotene, pachytene, diplotene, diakinesis
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Chromosomes are digested with trypsin, then stained with Giemsa - positive staining bands are black, and negative staining bands are white
G banding
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Chromosomes are stained with a fluorescent dye that binds to AT rich regions, such as the DAPI stain or quinacrine, and then viewed under fluorescent microscopes
Q banding
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Chromosomes are denatured with saturated barium hydroxide, before being stained with Giemsa - this demonstrates constitutive heterochromatin
C banding
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A summary of the chromosome constitution of a cell or person, such as 46, **, often used more loosely to mean an image showing the chromosomes of a cell sorted in order and arranged in pairs, strictly, a karyogram
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What is allelic heterogeneity


Different mutations in the same locus cause the same disease

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What is a genotype


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What is a gene


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What does homozygous mean


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