Chromosome Abnormalities Flashcards
Numerical Abnormalities
Missing or having extra chromosomes (wrong # chromosomes)
Structural Abnormalities
Chromosome is altered by deletion, duplication, breakage (problems WITHIN a chromosome)
Aneuploidy
The gain or loss of a whole chromosome
(Most common chromosome abnormality in humans)
Aneuploidy is the leading genetic cause of…
spontaneous miscarriage and congenital birth defects
(also seen in most tumors)
Most common cause of aneuploidy…
Nondisjunction
Non-Disjunction
The failure of homologs or sister chromatids to separate during cell division
(= “not coming apart”)
Non-Disjunction can occur at: (3)
Meiosis I –> Anaphase I (homologs don’t separate)
Meiosis II –> Anaphase II (sister chromatids don’t separate)
Mitosis –> Anaphase (sister chromatids don’t separate)
Non-Disjunction in Meiosis I (results)
2 gametes = (n + 1) = Extra chromosome
2 gametes = (n - 1) = Lost chromosome
Non-Disjunction in Meiosis II (results)
1 gamete = (n + 1) = Extra chromosome
1 gamete = (n -1) = Lost chromosome
2 gametes = (n) = Normal
Monosomic Zygote
(2n-1) = Lost chromosome
Normal gamete + (n - 1) gamete
Trisomic Zygote
(2n + 1) = Gained Chromosome
Normal gamete + (n + 1) gamete
What does aneuploidy affect?
1) Gene dosage
2) Recessive “unmasking”
3) Sterility
Gene Dosage
The # of copies of a gene that are actively being expressed
–> Proportionally reflects the RNA and protein products in a cell
Affects of aneuploidy on gene dosage
A missing or extra chromosome throws off the overall gene balance in the chromosome set (dosage)
–> Can lead to more or less RNA/Protein products
Recessive “unmasking” (due to aneuploidy)
A missing chromosome = 1 allele determines the phenotype for those now missing genes
–> Allows any harmful recessive alleles on the single chromosome to be phenotypically expressed (hemizygous)