Chromosome Abnormality ​
Often occurs during crossing over in prophase I
Structural abnormalities ​
Deletion ​
Removal of genetic sequence
Often harmful <= disrupts gene balance
Lethal if embryo homozygous for large deletion or if occurring on X in male
Duplication ​
Repetition of genetic sequence
Often harmful <= disrupts gene balance
Inversion ​
Reversed orientation of genetic sequence
Usually benign <= preserves gene balance
Translocation ​
Relocation of genetic sequence from one chromosome to another
May join a nonhomologous chromosome
Often harmful <= may preserve or disrupt gene balance
Reciprocal translocation ​
Exchange of segments between nonhomologous chromosomes
Usually benign <= preserves gene balance
Robertsonian translocation ​
Fusion of 2 nonhomologous chromosomes
Short arms often lost
Usually harmless <= preserves gene balance because the short arms don't contain significant genetic material
10-15% of child with Familial Down Syndrome (hereditary)
Ajitani Hifumi