Alterations in Chromosome Structure and Number Flashcards
What happens when there’s an atypical number of chromosomes
- In most cases, embryos don’t survive
- When they survive, serious developmental disorders occur including: down syndrome, patau syndrome, edwards syndrome, klinefelter syndrome
Nondisjunction
Errors caused by changes in chromosome number
* Spindle pulls incorrect amount of chromosomes to a side
Meiotic spindle makes an error in distribution of chromosomes
* Homologous chromosomes don’t separate during anaphase I or II
* Results in gametes w either too many or too little chromosomes
Abnormal gamete + normal gamete = zygote that contains atypical number of chromosomes
* Happens in meiosis anaphase
Trisomy 13: Patau Syndrome
Statistics
* Affects 1 in 16000
* Occurs on chromosome 13
* 5-10% babies survive past first year
Symptoms:
* Severe intellectual disability
* heart defects
* Brain or spinal cord abnormalities
* Physical abnormalities: small eyes, extra fingers/toes, cleft lip
Trisomy 18: Edwards Syndrome
Statistics:
* Affects 1 in 5000
* Occurs on chromosome 18 (3 chromosomes instead of 2)
* 5-10% of babies survive past first year
Symptoms:
* Severe intellectual disability
* Low birth weight
* Small head, jaw, and mouth
* Clenched fists
* Heart defects and other organ abnormalities
Trisomy 21: Down Syndrome
Statistics:
* Affects 1 in 800 babies
* Occurs on chromosome 21 (3 chromosomes instead of 2)
* Older the mother, the higher the risk for down syndrome
Symptoms:
* Characteristic facial appearance
* Poor muscle tone (hypotonia)
XXY: Klinefelter Syndrome
Statistics
* Affects 1 in 500 to 1 in 1000 males
* Occurs on sex chromosome (site 23) - 3 chromosomes instead of 2
Affects male sexual development:
* Results in small testes that don’t produce enough testosterone
* Shortage of testosterone can lead to: breast enlargement, reduced facial and body hair, infertility
Testosterone injections can help treat
Mutation
Any change to DNA of a cell
* Some mutations only change one or a few base pairs in a DNA sequence, others occur at chromosome level
* Chromosomal mutations normally occur during an error in crossing over
Types of Damage to Chromosome Structure
Chromosomal mutations
Deletions
Duplications
Inversions
Translocation
Type of chromosomal mutation
* Translocation = occurs when a fragment of one chromosome attaches to a nonhomologous chromosome
Translocation Down Syndrome
translocation occurs between chromosome 21 and 14
* A person inherits 2 normal copies of chromosomes 14 and one that has a piece of chromosomes 21 attached to it
Deletions
when a fragment of a chromosome is lost
* Small deletions are not normally harmful
Prader-Willi Syndrome
deletion of chromosome 14, causing:
* Developmental delays
* Learning Disabilities
* Behavioural problems
* Obesity
* Decreased sex glands that produce little to no hormones
Duplication
Part of chromosome is repeated
* Sometimes includes an entire gene
* If a region is only repeated once, duplication may have no effect
* If a duplication happens more than once, it can cause: seizures, developmental delays
Inversion
reverses a fragment of the original chromosome
* Can cause problems w the synapsis of chromosomes during meiosis
* Result: person w a chromosomal inversion may have difficulty conceiving a baby
Genetic testing (types)
- Karyotype analysis
- Carrier testing = tests if one is a carrier
- Presymptomatic screening = tests for disorders that appear after birth or later in life before symptoms appear
- Diagnostic Genetic Testing = to confirm when symptoms are present
- Prenatal testing = used to detect small scale mutations or chromosomal alterations in the fetus
- Newborn screening → occurs at birth
