Chromosome Abnormalities Flashcards
What some features of Trisomy 21?
Most common genetic cause of mental retardation
Can arise from nondisjunction in M1 or M2
Diagnosed at birth from dysmorphic features
What is Aneuploidy?
organism whose chromosome number is not an exact multiple of the typical haploid set for that species
Monosomic - zygote or diploid cell that is missing a chromosome (2N-1
Trisomic - “” has an extra chromosome (2N+1)
What is polyploidy?
Presence of an extra set or sets of total haploid genome
What is Nullisomic and Disomic?
Nullisomic -haploid cell that is missing a chromosome
Disomic - haploid cell that has an extra chromosme
How are karyotypes named?
Number of chromsomes –> sex chromosome designation –> Addition or deletion –> translocation
What can cause aneuploidy?
Chromosome nondisjuntion or chromosome loss during cell division in either mitosis or meiosis
Most are sporadic, but can be hereditary/familial
How can mosaicism occur?
Mitotic nondisjunction occurs after the first division but early in zygote development.
Some tissues will be normal, others will be trisomic
What is the difference between first degree and second degree nondisjunction?
First - nondisjunction occurs in cells with normal chromosome numbers
Second - nondisjunction occurs in aneuploid cells
Second has a higher frequency
Where does the nondisjunction occur for most autosomal trisomy cases?
During maternal meiosis
Why is Y chromosome deficiency or surplus well tolerated?
Y is largely heterochromatic
What autosomal aneuploidies are nonlethal?
21, 18, and 13
Only the trisomy 21 children reaches adulthood
All others are not compatible with survival
Meiotic nondisjunction accounts for what percentage of all trisomy 21 cases? What is the reason for the rest?
95%
The remainder is either caused by an unbalanced Robertsonian translocation or an isochromosome 21
What is the maternal age effect?
The risk for chromosomal abnormalities increases with increasing maternal age
What is a contiguous gene syndrome?
Syndrome that results from deletion of a segment of DNA containing portions from multiple adjacent genes, i.e. loss of function of genes located next to each other
Down syndrome is likely a result of this
What is Trisomy 13?
Most rare trisomy
Cleft lip and palate, ocular abnormalities, polydactyly, clenched fists and rocker-bottom feet
