Variations In Chromosone Number Flashcards
How are chromosomes treated with chemicals to view patterns
First the fixed chromosomes on a slide are treated with trypsin so that the DNA loosens up bit
-trypsin is a protease which, when used for a short time will partially digest proteins associated with the chromosome
- Next, the fixed chromosomes are stained with a purple dye called Geimsa
- some regions of the chromosomes pick up more dye resulting in a banding pattern
-The pattern of staining is viewed under the microscope and photographed
What can cause variations in two haploid sets?
- Change in total number of chromosomes
- Deletion or duplication of genes/segments of chromosome
- Rearrangements of genetic material within or among chromosomes
What is aneuploidy?
- Variations in chromosome number
- Organism gains or loses one or more chromosomes(not entire set)
What are the types of aneuploidy?
Monosomy
Trisomy
Polyploidy
Euploidy
Triploid
What is Monosomy?
Loss of single chromosome in diploid genome
What is trisomy?
Gain of single chromosome
What is euploidy?
Complete haploid sets of chromosomes are present
What is polyploidy?
More than two sets of chromosomes are present
What is triploid?
Three sets of chromosomes
How can triploidy(3n) happen?
- Most common cause is dispermy (one egg and two spermatozoa)
- Could be fertillization of a primary oocyte
- Could be a failure to separate homologous chromosomes in Meiosis 1
- Could be a failure to separate homologous chromosomes in Meiosis II
- Could be retention of a polar body
How often does triploidy (3n) occur?
Occurs in 2-3% of conceptuses and ~ 20% of chromosomal abnormal first-trimester miscarriages
How can tetraploidy(4n) happen?
- Rare occurrence, may be chromosome separation failed during meiosis 1 or II in the egg cell and then dispermy
- Failed sister chromatid separation in an early embryo
What is nondisjunction ?
Failure to separate
- Gives rise to chromosomal variation
- Paired homologs fail to disjoin during segregation
- Nondisjunction during meiosis 1 or II
What can result from Monosomy?
- Loss of one chromosome
- Produces 2n-1 complement
- Haploinsufficiency: when one copy is not sufficient for organism to survive
Loss of sex chromosomes is compatible with life
46, XX loss of X= 45,X. 47,XY loss of Y= 45, X
Turner Syndrome
Contrast the effect of a loss of sex chromosome and loss of autosomes
- Loss of any one of the autosomes is not compatible with life
- loss of a sex chromosome IS compatible with life
What are the effects of trisomy of autosomes?
2n+1 chromosomes
Most trisomy involving the autosomal chromosomes are lethal
Three exceptions which result in live-born children
- trisomy 21(Down syndrome)
- trisomy 18(Edward syndrome)
- trisomy 13 (Patau Syndrome)
What disease results from trisomy 21?
Down syndrome
What disease results from trisomy 18?
Edward syndrome
What disease results from trisomy 13?
Patau syndrome
Differentiate trisomy and triploidy
Triploidy-extra chromosome added to each pair resulting in 69, XXX or 69, XXY or 69, XYY
Trisomy adds an extra chromosome to a single pair (e.g. chromosome 21 pair gets gets another to cause Down syndrome) 47, XX or 47, XY
What is the frequency of Down syndrome?
1: 800 live births
What is the life expectancy of a person with Down syndrome?
Live expectancy on average is about 60 years
What are the phenotypic characteristics of Down syndrome?
- short stature
- depressed nasal bridge
- upslating palpebral fissures
- Brushfield spots on iris of the eye
- Epicanthal folds (skin fold from the nose to upper eyelid)
- short neck
- low-set small, folded ears
Aside from phenotypic characteristics, how may down syndrome affect an individual?
- Intellectual disability and heart malformations
- affected individuals bear a striking resemblance to one another
