chromosomal mutationschromosomal mutations

I.I. Chromosomal mutationsChromosomal mutationsA.A. Changes in chromosome numberChanges in chromosome number

B.B. Changes in chromosome structureChanges in chromosome structure

II.II. Chromosome testingChromosome testingA.A. KaryotypingKaryotyping

B.B. High resolution analysisHigh resolution analysis

C.C. Postnatal genetic testingPostnatal genetic testing

I. Chromosomal MutationsI. Chromosomal Mutations

possibly affecting more than one gene (multi-possibly affecting more than one gene (multi-gene level)gene level)

A.A. Changes in NUMBERChanges in NUMBER Monoploid number (2n – 1)Monoploid number (2n – 1) Euploidy (multiples of n)Euploidy (multiples of n) Polyploid (3n, 4n, 5n…)Polyploid (3n, 4n, 5n…) Triploid, tetraploid, pentaploid, hexaploidTriploid, tetraploid, pentaploid, hexaploid

1. Polyploidy1. Polyploidy

Usually Usually lethallethal in mammalsin mammals

Does occur in some animals - Reproduction via Does occur in some animals - Reproduction via parthenogenesis, Flatworms, leeches, brine parthenogenesis, Flatworms, leeches, brine shrimp, lizards, salamanders, salmonidsshrimp, lizards, salamanders, salmonids

Polyploidy in plants: much more common because it can be tolerated by plants, can reproduce asexually…

Important role in the evolution of plants –wheat: 2n = 14, 28, 42chrysanthemum: 2n = 18, 36, 54, 72, 90

sympatric speciation:e.g. polyploidy in plants..

•Autopolyploidy: due to meiosis error. Offspring can self fertilize.

•Allopolyploidy:2 different species mating, produce a hybrid that is polyploid:•The hybrid is fertile because the polyploid condition provides the homologous chromosomes for pairing during meiosis…

Endopolyploidy Endopolyploidy only certain cells in only certain cells in the organism are the organism are polyploid polyploid

Liver cells, plant Liver cells, plant tissue (stem), tissue (stem),

larval gut tissue larval gut tissue (mosquitos)(mosquitos)

2. Aneuploidy – the total # is not an 2. Aneuploidy – the total # is not an exact multiple of a setexact multiple of a set

(2n +/- x)(2n +/- x)

Caused by NondisjunctionCaused by Nondisjunction = failure of = failure of normal chromatid division during normal chromatid division during meiosis, two chromosomes go to one meiosis, two chromosomes go to one pole, none in the other. pole, none in the other.

Results in the wrong number of Results in the wrong number of chromosomes.chromosomes.

Results in a Results in a gene imbalancegene imbalance

Fertilization of one of these affected gametes produces a zygote w/ either 3 members (trisomy) or only one member (monosomy) of the chromosome.

gene imbalance - THE problemgene imbalance - THE problemAneuploids are more abnormal than polyploids, why? Aneuploids are more abnormal than polyploids, why?

(polyploid plants are completely viable and usually (polyploid plants are completely viable and usually bigger, whereas in Drosophila the only aneuploids that bigger, whereas in Drosophila the only aneuploids that survive are trisomics and monosomics for chromosome survive are trisomics and monosomics for chromosome 4, the smallest chromosome)4, the smallest chromosome)

Normal physiology of a cell depends on the Normal physiology of a cell depends on the proper ratio of gene products in the euploid cell.proper ratio of gene products in the euploid cell.The amount of expression is correlated with the The amount of expression is correlated with the

number of genes in a cellnumber of genes in a cell If 3 copies present: 150% of the normal amount of If 3 copies present: 150% of the normal amount of

protein will be madeprotein will be made If 1 copy present: 50% of the normal amount of If 1 copy present: 50% of the normal amount of

protein will be madeprotein will be made

Nondisjunction responsible for Turner’s syndrome and Kleinfelter’s syndrome…

Turner’s syndrome produces sterile females with a normal # of autosomes and 1 X chromosome (XO). These are the only human monosomics that survive…

Klienfelter’s syndrome individuals are trisomic: XXY, they are sterile males that are typically tall, and thin and some degree of mental retardation.

XYY – trisomic males have mild mental retardation

ConditionCondition FrequencyFrequency syndromesyndrome

Trisomy-21Trisomy-21 1/8001/800 DownDown

Trisomy-18Trisomy-18 1/6,0001/6,000 EdwardEdward

Trisomy-13Trisomy-13 1/15,0001/15,000 PatauPatau

XXYXXY 1/1,0001/1,000 KlinefelterKlinefelter

XYYXYY 1/1,0001/1,000 JacobsJacobs

XXXXXX 1/1,5001/1,500 SuperfemaleSuperfemale

XX 1/5,0001/5,000 TurnerTurner

Aneuploid Conditions in Humans

Inherited disorders associated with aneuploidy. Trisomies and variations in the sex chromosomes result in mental retardation, organ defects, sexual immaturity, etc.

Trisomy 18, diaphragmatic hernia

Turner’s syndrome, developmental abnormality

polydactyly

Trisomy 21, abnormal creases

Why is monosomy so bad?Why is monosomy so bad?

Monosomics for all human Monosomics for all human autosomes die in uteroautosomes die in utero

Any deleterious recessive Any deleterious recessive alleles present on monosomic alleles present on monosomic autosome will be automatically autosome will be automatically expressedexpressed

B. Changes in chromosome B. Changes in chromosome structurestructure

1)1) A) DeletionsA) Deletions

2)2) B) DuplicationsB) Duplications

3)3) C) InversionC) Inversion

4)4) D) TranslocationD) Translocation

Deletion loop

1. Deletions1. Deletions

a)a) Spontaneous Spontaneous breakage and breakage and rejoiningrejoining

Interstitial deletionInterstitial deletion Terminal deletionTerminal deletion

b)b) Crossing over Crossing over between repetitive between repetitive DNADNA

Region w/centromere usually maintained during division, the other part will be lost

Multigenic deletionsMultigenic deletions

If both homologs have the same deletion If both homologs have the same deletion then it will be then it will be lethallethal

If only on one homolog, the deletion can If only on one homolog, the deletion can “uncover” lethal recessives in the “uncover” lethal recessives in the heterozygous conditionheterozygous condition

PsuedodominancePsuedodominance = when recessive = when recessive alleles are expressed due to a deletion alleles are expressed due to a deletion eventevent

“partial monosomy”

Caused by a heterozygous deletion of the tip of the p arm of chromosome #5 – phenotype: distinctive cat-like cry made by infants, microencephaly & moon-like face

2. Duplications2. Duplications

Extra copy of some particular region… Extra copy of some particular region… Rare, and difficult to detectRare, and difficult to detectUsually due to Usually due to unequal crossing overunequal crossing over during meiosis, or through replication error during meiosis, or through replication error prior to meiosisprior to meiosisNot as problematic as deletions, but some Not as problematic as deletions, but some problems are associated:problems are associated: Bar eye in Bar eye in DrosophilaDrosophila (gene imbalance) (gene imbalance)

3. Inversions3. InversionsRegion breaks, Region breaks, rotates 180 degreesrotates 180 degrees and rejoins and rejoins

Generally viable, and show no abnormalities at the Generally viable, and show no abnormalities at the phenotypic levelphenotypic level

Paired homologs form an

During synapsis, one chromosome must twist into a loop to pair up w/the genes on the other…

1) Paracentric – centromere outside of the inversionCross over products: dicentric and acentric

chromosome2) Paricentric – inversion spans centromere

Cross over products: duplication, and deletion

Types of inversions

During meiosis, homologs still pair up, even w/inversions

-Inversion loop makes this possible

Crossing over produces affected chromatids:

Duplication &Deletion events

Semisterility = an organism that is heterozygous for a reciprocal translocation usually produces about half as many offspring as normal

due to difficulty in chromosome segregation in meiosis.

Translocation cross = because of the translocations, the pairing of homologous regions leads to the unusual structure that contains four pairs of sister chromatids.

4. Translocation-movement of chromosomal 4. Translocation-movement of chromosomal fragments to a new location.fragments to a new location.

Nonreciprocal translocation – (unbalanced)Centromeric regions of two nonhomologous

acrocentric chromosomes become fused to form single centromere.

-Down Syndromechromosome 21 & 14

rearrangement leads to familial Down Syndrome. The heterozygote is normal, the 3 chromosomes must separate during meiosis (only 2/6 are normal, the rest either monosomic or trisomic)-Cancer (CML) type of leukemia, translocation between chromosome 9 & 22, leads to the movement of a gene where it will be overexpressed

Fragile sites – susceptible to Fragile sites – susceptible to breakagebreakage

Fragile X syndromeFragile X syndrome Most common form of inherited mental retardation (1/4000 Most common form of inherited mental retardation (1/4000

males, 1/8000 females)males, 1/8000 females) FMR1 gene, has several trinucleotide repeats CGG in the FMR1 gene, has several trinucleotide repeats CGG in the

5’UTR region5’UTR regionNormal individuals = 6 to 54 repeatsNormal individuals = 6 to 54 repeats

Affected individuals = >230 repeats, region becomes modified (bases Affected individuals = >230 repeats, region becomes modified (bases are highly methylated & gene NOT expressed)are highly methylated & gene NOT expressed)

Link between fragile sites & cancerLink between fragile sites & cancer Chromosome #3 FRA3B region, FHIT gene often altered or Chromosome #3 FRA3B region, FHIT gene often altered or

missing in tumor cells taken from individuals w/ cancermissing in tumor cells taken from individuals w/ cancer

II. Chromosome Testing II. Chromosome Testing

Chromosomes:Chromosomes:A.A. KaryotypingKaryotyping

B.B. High resolution chromosome analysisHigh resolution chromosome analysis

A. karyotypingA. karyotypingadding a dye to metaphasic chromosomes; different dyes that adding a dye to metaphasic chromosomes; different dyes that affect different areas of the chromosomes are used for a range affect different areas of the chromosomes are used for a range of identification purposes. of identification purposes. Giemsa dye is effective because it markedly stains the bands Giemsa dye is effective because it markedly stains the bands on a chromosome; Each chromosome can then be identified on a chromosome; Each chromosome can then be identified by its banding patternby its banding patternAmniocentesisAmniocentesisChorionic Villi BiopsyChorionic Villi Biopsy

Prenatal genetic testing cont.Prenatal genetic testing cont.

Maternal Serum & Amniotic fluidMaternal Serum & Amniotic fluid

Alpha-fetoprotein (AFP)Alpha-fetoprotein (AFP)

Unconjugated estriol (uE3)Unconjugated estriol (uE3)

Dimeric inhibin A (DIA)Dimeric inhibin A (DIA)

Fetal cell sortingFetal cell sorting

B. High resolution chromosome analysisB. High resolution chromosome analysis

1)1) SKY – uses probes. Each of the SKY – uses probes. Each of the individual probes complementary individual probes complementary to a unique region of one to a unique region of one chromosome - together, all of the chromosome - together, all of the probes make up a collection of probes make up a collection of DNA that is complementary to all DNA that is complementary to all of the chromosomes within the of the chromosomes within the human genome.human genome.Each probe is labeled with a Each probe is labeled with a fluorescent color that is fluorescent color that is designated for a specific designated for a specific chromosome..chromosome..the probes hybridize, the the probes hybridize, the fluorescent probes essentially fluorescent probes essentially paint the full set of chromosomes, paint the full set of chromosomes, can be analyzed to determine can be analyzed to determine whether any of them exhibits whether any of them exhibits translocations or other structural translocations or other structural abnormalities.abnormalities.

2) In situ hybridization – used to map specific deletions & insertions

No binding, 13.1-13.3 deleted

(FISH) analysis of a normal individual (D) and patient with a chromosome 22 deletion using a probe for the UFD1 gene. The patient has only one copy of UFD1 seen in blue (white arrows). Chromosome 22 was labeled with a red fluorescent marker (yellow arrows).

http://www.ggc.org/clinical.htm