Chapter 6 Flashcards
karyotype
- display of a complete set of chromosomes
Chromosomal mutation Types
- chromosomal rearrangements
- aneuploids
- euploids (polyploidy)
Aneuploids
- the number of chromosomes is altered (addition/deletion)
euploids/polyploids
- one or more complete sets of chromosomes are added
Tandem duplication
- duplicated region is immediately adjacent(next to)
displaced duplication
- located a distance away (even on another chromosome)
What is a characteristic of duplication?
- loop forming during meiosis
Why does duplication alter the phenotype?
- related to gene dosage, shifts to one side
Gene dosage
- balance of gene products must be maintained for some pathways
- interaction of certain gene products can lead to abnormal developments (position effect)
Deletion of centromere can lead to
- loss of the entire chromosome because no segregation in meiosis/mitosis
What happens if you lose essential genes?
- lethal if homozygous for loss
What occurs in heterozygous deletions?
- gene products will be imbalanced
- allows recessive alleles on undeleted chromosome to be expressed (pseudo-dominance)
- some genes have to be present in two copies to produce enough gene product (haploin-insufficient)
Example of a haploin-insufficient case
- Notch Locus
- heterozygous notch deletion
Deletions lead to problems during what?
- chromosome pairing
Inversions are another example of _______
- troubling chromosomal rearrangements
Paracentric Inversions
- inversions that do not include the centromere in the inverted region
Pericentric inversions
- inversions that include the centromere in the inverted region
Inversion
- gene order is changed, can break a gene into 2
What is the position effect?
- regulation of genes is sometimes context dependent
- order related/which genes are interacting
The inversion loop is ______, present in _____
- characteristic
- meiosis
In pericentric inversion a dicentric bridge can occur what is this?
- bridge between two chromosomes attached to a centromere
Translocation
- movement between nonhomologous chromosomes
- not crossing over
- between homologous recombination
Nonreciprocal translocation
- movement one way
- from one chromosome to another without equal exchange
Reciprocal translocation
- movement 2 ways
- from one chromosome to another with equal exchange
What can result from translocation?
- altered gene expression due to position effect (ex: Burkitt lymphoma)
- Break may occur in a gene
- lose gene completely
Robertsonian translocation
- translocation and deletion
- forms a metacentric chromosome
- causes some forms of down syndrome
What can result from robertsonian translocation?
- some forms of down syndrome
2. possible origin of a reciprocal translocation between two nonhomologous chromosomes
What can happen if segragation in translocation occurs during meiosis
- 1/3 possible outcomes
1. alternate segregation
2. adjacent 1 segregation
3. adjacent 2 segregation
What happens in alternate and adjacent 1 segregation?
- homologous centromeres segregate to opposite poles
After anaphase 2 in alternate segregation what happens to the gametes?
- they are viable
After anaphase 2 in adjacent 1 and 2 segregation what happens to the gametes?
- they are nonviable
Aneuploidy
- changes the number of individual chromosomes
How can aneuploidy occur?
- loss of chromosome which has lost its centromere
- robertsonian translocation
- nondisjunction
Types of aneuploidy
- Nullisomy
- Monosomy
- Trisomy
- Tetrasomy
Nullisomy
- loss of both members of homologous pairs
- 2n-2
n is equal to..
- the haploid number of chromosomes
- humans 2n=46
Monosomy
- loss of a single chromosome
ex: Turners syndrome
Trisomy
- gain of a single chromosome, resulting in three homologous chromosomes
ex: down syndrome (21)
Tetrasomy
- gain of two homologous chromosomes, resulting in four homologous chromosomes
- 2n+2
What is an organism with an extra copy of two different chromosomes
- double trisomic
Jimson Weed
- all different chromosomes, all different trisomics
What is the best tolerated aneuploidy?
- XYY has very little impact, due to the lack of genetic info on the Y
Examples of human aneuploidys
- Turners Syndrome (XO)- lethal in males, females is viable
2. Klinefelter Syndrome (XXY)
Are autosomal or sex chromosome aneuploidys more tolerated?
- autosomal because smaller chromosomes
- down syndrome (21) has about 300 genes
Most aneuploidys result from nondisjunction in father or mother?
- 75% is from nondisjunction in mother
Incidence of primary down syndrome increases with what?
- maternal age
Familial down syndrome
- translocation of a part of chromosome 21
- runs in families, parents have undergone a robertsonian translocation
Down syndrome results from?
- a trisomy of chromosome 21
- nondisjunction in meiosis 1 or 2 in female gamete
Trisomy 18
- type of autosomal aneuploids
- Edward Syndrome, severe problems, most die by age of 1
Trisomy 13
- type of autosomal aneuploids
- Patau syndrome
- 50% die within 1 month
- 95% die by age 3
Trisomy 8
- mosaic individuals can have normal life expectancy
Trisomic Rescue
- Trisomy that is resolved by loss of a chromosome during embryogenesis
Uniparental Disomy
- occurs when both chromosomes are inherited from one parent
- can result from trisomic rescue
Chromosome Mosaicism
- an organism can have normal tissues and tissues that have chromosome abnormalities that result from nondisjunction
Example of chromosome mosaicism
- Gynandromorph
- is both sexes XX and XO
- has a red eye and wild type wing
- also has a white eye and mini wing
How does polyploidy arise?
- errors in meiosis
- events at fertilization
- errors in mitosis following fertizilization
Dispermy
- one at fertilization mechanism
- simultaneous fertilization of one egg with two sperm
- 75% have two sets of paternal chromosomes
What is the most common Polyploidy in humans?
- triploidy
- found in 15-18% of spontaneous abortions
Autopolyploidy
- extra chromosome sets are derived from same species
Allopolyploidy
- extra chromosome sets are derived from different species
What are the problems with meiosis when polyploidy?
- usually results in sterility
How does alloploidy occur?
- as a result of hybridization followed by chromosomes doubling
What is an example of an alloploidy
- modern wheat
What are fragile sites
- a heritable gap of a chromosome that can be induced to generate chromosome breaks
Fragile X syndrome
- most common form of inherited mental impairment
- results from increase in number of trinucleotide repeats
What does trinucleotide expansion and premutation lead to
- methylation and blockage of transcription
- fragility is related to length of repeat region
Genetic anticipation
- the number of repeats increases in future generations causing symptoms to worsen
Polyploidy
- addition of entire chromosome sets
Types of duplications (can be good)
- gene redundancy and amplification
- gene families
- pseudogenes- looks like a gene but is nonfunctional
Robertsonian translocation and evolution have the only major structural differences of
- a number of pericentric and paracentric inversions
- the recent fusion of two chromosomes to form human chromosome 2
- a reciprocal translocation between the gorilla chromosomes which correspond to human chromosomes 5 and 17
Types of aneuploidy that are NOT a result of nondisjunction
- inversion- seg bks and reverses orientation and reattaches
- deletion- seg bks off and never returns
- duplication- seg bks off and attatches to homologous chromosome, deletes from og
- translocation- seg bks off and attatches to a nonhomologous chromosome
