Cdm Flashcards
0
Q
locus
A
locus is the specific place on a chromosome where a gene is located.
1
Q
Gene is…
A
Genes are the physical and functional unit of heredity – a segment of DNA.
2
Q
testcross
A
testcross involves crossing a heterozygous individual with a homozygous recessive
3
Q
When doing a dihybrid cross how do you know if the gene is recombinant?
A
- Two equally frequent nonrecombinant classes totaling in excess of 50%.
- Two equally frequent recombinant classes totaling less than 50% .
4
Q
The work of Morgan showed that linked genes in a dihybrid may be present in one of two configurations:
A
- The two dominant alleles are present on the same homolog – cis configuration (adjacent).
- The two dominant alleles are on different homolgs – trans configuration (opposite).
5
Q
Morgan suggested that recombination is brought about by?
A
brought about by chiasma formation.
6
Q
Chiasma formation occurs during
A
zygotene / pachytene of meiotic prophase 1.
7
Q
How do you detect recombination?
A
comparing the inputs into meiosis with the outputs.
8
Q
Why are two gene test crossing not efficient?
A
Genetic maps can be constructed from a series of testcrosses for pairs of genes. not particularly efficient because numerous two-point crosses must be carried out and because double crossovers are missed.
9
Q
Three-point testcross is more efficient because?
A
With three genes the order of the genes can be determined using a single set of offspring and double crossovers can usually be detected.
10
Q
Three linked genes how many types of crossing over can take place?
A
Three types
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Q
Inter chromosomal recombination
A
Independent assortment of chromosomes produces 50% recombination. Testcross progeny show a 1:1:1:1 ratio
12
Q
What is the key to chromosome mapping?
A
frequency of recombination is the key to chromosome mapping.
13
Q
Recombinant frequencies for different genes range
A
Recombinant frequencies for different genes range from 0 – 50%.
14
Q
Properties of Neurospora?
A
- Found in many part of the world growing on dead vegetation.
Haploid- 7
Neurospora can reproduce both asexually and sexual.
When different mating types come into contact their cell walls and nuclei fuse resulting in diploid nuclei.
The diploid nuclei under goes meiosis and mitosis to produce eight ascospores.
15
Q
What is the life cycle of neurospora?
A
From both mating types x and y a hypee goes over and touches the other mating type causing cross fertilisation. Then synchronous division and fusion takes place to form diploid myocytes. The myocytes are in a bag called perithekum. This is where myosin and mitosis takes place forming asci (punches) 8. Which have spores in them.
16
Q
Centromeres in eukaryotes can not be mapped because?
A
they show no heterozygosity.
17
Q
in fungi the centromere can be mapped because?
A
they produce linear tetrads
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Q
How does centromere mapping work?
A
Centromere mapping involves estimating the distance from a locus to the centromere.
By observing the pattern of spore types in the ascus we can directly derived the result of single meiosis.
19
Q
What are the Different patterns of alleles that can appear in the tetrad or octad in the asci as a result of meiosis with crossing over.
A
Two basic patterns are observed a 4:4 or a 2:2:2:2.
20
Q
When does the 4:4 pattern arise in the octad?
A
The 4:4 pattern arises when there is no crossing over between the gene and the centromere.
21
Q
WhEn does the 2:2:2:2 pattern arise?
A
The 2:2:2:2 pattern arises when there is a crossover between the gene and the centromere.
22
Q
How are a large number of human traits been successfully mapped
A
with the use of pedigree data and linkage analysis.Because the number of progeny from any one mating is small, data from several families and pedigrees are usually combined to test for independent assortment.
23
Q
How does in situ hybridisation work?
A
In situ hybridisation is a method for determining the chromosomal location of the gene through molecular analysis.
This method requires a probe for the gene which is single-stranded and complimentary to the gene.
The target, chromosome spread is denatured (single stranded).
The probe and target hybridize on microscope slide (DNA–DNA).
The probe is fluorescently labelled and is visible under UV light.
Sites of hybridization will be easily identified.
The colour of the Fluorescence can be made specific to a particular gene.
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How does Somatic-cell hybridisation work?
Somatic-cell hybridisation involves the fusion of the different types of cells.
Most mature somatic cells can undergo only a limited number of cell division.
However if altered by viruses or derived from tumours these cell lines can culture indefinitely in the laboratory.
Cells from two different species can be fused and the cells possess two nuclei – heterokaryon.
Eventually the two nuclei fuse, for reasons that are not understood chromosomes from one species are lost preferentially.
In human–mouse somatic cell hybrids, the human chromosomes tend to be lost.
A panel of six cell lines, each line containing a different subset of human chromosomes, is examined for the presence of the genes product (such as an enzyme). So if chromosome four is missing and the enzyme in search is missing then you know that chromosome four has the genes to make the enzyme.
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How is the precision of Somatic-cell hybridisation increased?
You use irradiated hybrid cells that can delete portions or whole arms of chromosomes so that you can see which gene was present on that arm of the chromosome.
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Why is it difficult to map human genes?
Efforts in mapping human genes are hampered by the inability to perform crosses.
In addition each human family usually has a small number of progeny.
Nevertheless a large number of human traits have been successfully mapped with the use of pedigree data and linkage analysis.
Because the number of progeny from any one mating is small, data from several families and pedigrees are usually combined to test for independent assortment.
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What was the first human chromosome to be mapped?
X chromosome mapping
The first human chromosome mapped was the X chromosome.
This is because it is more amenable to mapping by recombination analysis.
This is because males are hemizygous for X-linked genes.
If we look at only male progeny from a dihybrid female we are effectively sampling her gametic output.
In other words we have a close approximation to a testcross.
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What were the first genetic markers?
Phenotypic genes because they are observable.
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When were the dna markers discovered? Why were the dna markers better?
1980. There are many of them.
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What are the different Types of DNA markers?
Minisatellite markers are based on variation in the number of tandem repeats 15–100 bp long.
Microsatellite markers are based on variation in the number of a even simpler sequence (2–6 bp), the most common is CA and its compliment GT.
Both minisatellite and microsatellite loci have the same repeat unit but different number of repeats.
Single nucleotide polymorphism (SNPs) are positions in the genome where people differ in a single nucleotide base.
Restriction fragment length polymorphism (RFLPs) is a SNP that alters a restriction enzyme recognition site.
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Genomewide association studies
An alternative approach to mapping genes, looks for non-random associations between a trait (disease) and alleles scattered across the genome.Rather than analysing crosses it looks for associations between traits and a suite of alleles in a population.
When a mutation arises in a population it will appear on a particular chromosome and be associated with a particular set of alleles – haplotype.Because of the physical linkage between the mutation and the haplotype they will tend to be inherited together.
Crossing over breaks up the linkage disequilibrium however the closer the mutation is to the alleles the longer it will persist.
Linkage disequilibrium provides information about the distance between genes.
A strong association between a trait (mutation) and a set of linked markers indicates that the mutation is likely to be near the genetic markers.
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linkage disequilibrium
The non-random association between alleles in a haplotype is called linkage disequilibrium.
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Chromosomal mutations are important because they:
1. allow us to understand how genes work together in the genome.
2. provide insights into meiosis and chromosome architecture.
3. have proven useful tools for genomic manipulation.
4. are the cause of some genetic diseases in humans.
5. have revealed insights into evolutionary processes.
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two types of chromosome mutations
changes in structure and changes in number.Both changes can occur spontaneously and can also be induced by both chemical and physical mutagens.
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Changes in chromosome number are often referred to as
polyploid changes.
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Structural mutations involve novel sequence rearrangements within
one or more DNA molecules.
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Changes in chromosome structure are called
rearrangements
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segment can be moved to a different chromosome resulting in a
translocation
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Breakage & rejoining
The first event is the generation of two or more double stranded breaks in the chromosome.
Double-stranded breaks are lethal unless repaired.
Repair mechanisms in the cell correct double-stranded breaks by joining broken ends together.
If the ends of two different breaks are joined a chromosomal rearrangement can result.
Only DNA molecules with a centromere and two telomeres will survive meiosis.
If a rearrangement duplicates or deletes a segment of chromosome gene function may be affected.
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nonallelic homologous recombination.
An important cause of rearrangements is crossing over between duplicated DNA sequences.
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Deletions, duplications, inversions and translocations can all be produced by such crossing over. This occurs by?
In organisms with repeat DNA sequences within one chromosome or on different chromosomes crossing over can occur between non-homolgous.
If repeats pair that are not in the same position on homologs, crossing over can produce rearrangements.
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Deletion
A deletion is simply the loss of part of a chromosome.
The process of deletion requires two chromosomal breaks to cut out the intervening segment.
The segment will be lost because it does not have a centromere.
The deletion may be terminal – at the end of a chromosome or interstitial – within a chromosome arm.
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Intragenic deletion and Multigenic deletions
Intragenic deletion is a small deletion within a gene that inactivates the gene.
Multigenic deletions involve several or many genes and their consequences are more severe.
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Terminal deletion and Interstitial deletion
Terminal deletion = end of a chromosome being lost
| Interstitial deletion = middle of the chromosome being lost
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multigenic deletion
If a multigenic deletion is made homozygous by inbreeding the consequences are always lethal.
If the deletion includes the centromere the remaining chromosome will not segregate in meiosis or mitosis and will be lost.
Multigenic deletions never revert back to the wild type.
No crossing over can occur in the region spanning the deletion.
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pseudodominance
Deletions allow the expression of phenotypes carried as recessive alleles
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deletion mapping.
This can then be used to map genes to specific chromosome segments – deletion mapping.
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Duplication
A duplication is a mutation with an extra copy of a chromosome region.
Duplications can be tandem (adjacent) or located elsewhere in the genome – insertional duplications.
A diploid cell containing a duplication will have three copies of the chromosome region.
Duplication is an effective mechanism for increasing the number of copies of genes and also genome size.
Detection involves looking for duplication of chromosome banding patterns and presence of loops at meiosis in heterozygous individuals.
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Drosophila and position effect
Occasionally a fly carries three copies of the duplication on its X chromosome (double bar) the eye is extremely reduced – position effect.
The Bar mutation results from a small duplication on the X chromosome.
Drosophila individuals with the Bar mutation have a reduced number of facets, making the eye small and bar shaped.
The trait is inherited as a incomplete or partial dominant X linked trait.
Heterozygous females have somewhat smaller eyes while homozygous females and hemizygous males have much reduced eyes.
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segmental duplications
These duplications range in size from 10 – 50 kbp in length and encompass whole genes and the regions between them.
Most of these duplications are dispersed but there are some tandem duplications also.
About 4% of the human genome consists of segmental duplications.
The origin of the segmental duplications is not known.
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Inversions
Inversions involve two chromosome breaks in the same chromosome, the region is flipped and reinserted.
The fragments produced may or may not contain the centromere.
If the centromere is outside the inversion it is called paracentric inversions.
In contrast if the centromere is inside the inversion it is called pericentric inversions.
These paracentric and pericentric have different consequences for meiosis.
Inversions do not change the overall amount of genetic material and therefore individuals with inversions are usually normal.
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inversion heterozygote
Most analysis of inversions are carried out on diploid cells containing one normal and one chromosome carrying the inversion – inversion heterozygote.
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inversion loop.
In meiosis one chromosome twists at the ends of the inversions so that it can pair with the untwisted homolog
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paracentric inversion crossing over within the inversion loop causes
a dicentric bridge and an acentric fragment to form.
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pericentric inversions crossing over can result in
duplications and deletions
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Genetic consequences of inversion
Inversions appear to act as crossover suppressors.
Inversions can lead to the creation of ‘super genes’.
Change the linkage relationships of genes.
Reduce fertility when crossing over occurs in the inverted region.
Relatively common in human populations occurring in about 2% of individuals.
Often detected when patients are having fertility reproduction problems.
Implicated in speciation and evolution.
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Translocations
A translocation involves the movement of genetic material between nonhomologous chromosomes or within the same chromosome.
Compare this to crossing over which is the exchange betweem homologous chromosomes.
There are two types of translocation, reciprocal and nonreciprocal translocations.
In a nonreciprocal translocation genetic material moves from one chromosome to another without any reciprocal exchange.
Translocations change chromosome structure.
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Effects of translocations
Translocations can physically link genes that were previously located on different chromosomes.
These new linkage relationships may affect gene expression – may come under the control of different regulatory sequences – position effect.
The chromosomal breaks that bring about the translocation may occur in a gene and disrupt its function.
Translocations can result in abnormal meiotic configurations.
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Neurofibromatosis
a genetic disease characterised by numerous fibrous tumours of the skin and nervous system was mapped using translocation break points.
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Reciprocal translocations & meiosis. heterozygote with a reciprocal translocation has?When homologous chromosomes pair in prophase I? With a reciprocal translocation homologous centromeres separate and the chromosomes can segregate in?
A heterozygote with a reciprocal translocation has one normal and one translocated chromosome.
When homologous chromosomes pair in prophase I, a crosslike configuration will form, consisting of all four chromosomes.
With a reciprocal translocation homologous centromeres separate and the chromosomes can segregate in three different ways.
The gametes of two of the segregation patterns are not viable, therefore only ~ 50% of the gametes will be functional.
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Pseudolinkage in a translocation heterozygote
Genes on translocated chromosomes act as though they are linked.
When a translocation heterozygote is testcrossed the recombinants created do not survive.
The recombinants carry unbalanced genomes (duplications and deletions which make them nonviable).
The only viable progeny are those bearing the parental genotypes.
The apparent linkage of genes normally located on separate homologous chromosomes is called pseudolinkage.
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Robertsonian translocation
Deletions frequently accompany translocations.
In a Robertsonian translocation, the long arms of two acrocentric chromosomes become joined to a common centromere.
This generates a metacentric chromosome with two long arms and another chromosome with two very short arms.
The smaller chromosome often fails to segregate and is lost.
This loss can lead to an overall reduction in chromosome number.
Robertsonian translocation are the cause of a small number of Down’s syndrome.
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Numerical mutations. Euploidy, Aneuploidy
Euploidy changes in the number of chromosome sets.
| Aneuploidy changes in the number of individual chromosomes.
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Polyploidy
Polyploidy are organisms that have more than two sets of chromosomes.
Polyploidy is very common in plants but rarer in animals.
Polyploidy is one of the major mechanism by which new plant species have evolved.
In polyploidy there is often a correlation between the number of copies of the chromosome set and the size of the organism.
In many organisms cell volume is correlated with nuclear volume and genome size.
Therefore the increase in chromosome number is often associated with a increase in cell size, the higher the ploidy level, the larger the size of the organism.
Genetic ratios in polypliods differ from those in diploids.
Generally uneven polyploids (3x, 5x etc.) will be sterile because of pairing in meiosis.
Even polyploids can also have reduced fertility due to the formation of univalents and trivalents.
Polyploid greatly increases the number of allelic combinations.
Polyploid populations usually show greater genetic diversity than their diploid progenitors.
Mutations can occur without deleterious effects.
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Autopolyploids
Autopolyploids, for example triploid (3n), arise naturally when meiosis fails and diploid gametes are produced.
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Triploids
Triploids can also be produced by crossing a 4n (tetraploid) and a 2n (diploid).Triploids are sterile because pairing can only occur between two of the three chromosomes. This happens for every chromosome threesome therefore it is unlikely that a gamete will receive two or one of every type.
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Polyploidy can also be made by treating plants with
colchicine, a drug that inhibits spindle formation. Colchicine may be applied to generate a tetraploid from a diploid. Colchicine added to mitotic cells during metaphase and anaphase disrupts spindle formation, preventing the migration of chromatids after the centromere has split.
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Allopolyploids
An allopolyploid is a plant that is a hybrid between two or more species.
The resulting polyploid plants contain chromosome sets derived from two or more species.
Allopolyploids arise as a result of hybridization between two closely related species with subsequent chromosome doubling.
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Aneuploidy is a change in the number of individual chromosomes.
Aneuploidy can this arrives in several ways:
1. A chromosome maybe lost in the course of mitosis or meiosis if for example its centromere is deleted.
2. Small chromosomes generated by translocations may be lost in mitosis or meiosis.
3. Aneuploid gametes may arise through nondisjunction, the failure of homologous chromosomes or sister chromatids separating in mitosis or meiosis.
Nullisomy is the loss of both members of a homologous pair of chromosomes 2n - 2. Tetrasomy is gain of two homologous chromosomes 2n + 2.
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Nondisjunction
However, meiotic nondisjunction is more common.
In meiotic nondisjunction the gametes are affected leading to the whole organism being aneuploid.
In meiotic nondisjunction the chromosomes may fail to separate at either the first or second meiotic division.
Nondisjunction at meiosis I is more common.
Crossing over is necessary for normal chromosome pairing and with out it nondisjunction at meiosis I increases.
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Human aneuploidy – Down syndrome
Approximately 92% of those with Down syndrome have three full copies of chromosome 21 – primary Down syndrome.
Approximately 4% of people with Down syndrome result from a translocation most commonly between chromosome 21 and 14 – familial Down syndrome
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Human mtDNA
Human mtDNA is a circular molecule and encodes 2 rRNAs, 22 tRNAs and 13 proteins.
The two DNA strands have a different base pair composition: the heavy (H) strand has more guanine (G) while the light (L) has more cytosine (C).
The origin of replication for the heavy strand is a region known as the d-loop.
Human mitochondrial DNA is highly economic in its organisation, few noncoding nucleotides between genes, all messenger RNA is translated and no introns.
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yeast mtDNA
Although the yeast mtDNA is much bigger it only has six extra genes.
Most of the extra DNA in yeast consists of introns and noncoding sequences.
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heteroplasmy
Cells can be a mixture of mutant and normal organelles
