Task 2 : The photo 1 shows multiple structural abnormalities after irradiation of an individual with high dosage of X-rays. Describe structural chromosomal aberrations on the photo. Photo 1 gap breakage triradial quadriradial chromatid breakage chromosomal (double-chromatid) breakage
The mother is carrier of balanced robertsonic translocation of chromosomes 14 and 21. She is healthy but her offspring has increased risk of the Down syndrome.
Risk for further offspring Normal karyotype Carrier M. Down Trisomy 14 Monosomy 21 Monosomy 14 Lethal during prenatal development Theoretical risk 1/3 … 33% Empiric risk 8 – 10% Chromosomal constitution of mother carrier:
A boy (see photo) with mental retardation , long narrow face, large ears, a high arched palate, flat feet and overly flexible joints (especially the fingers) had been cytogenetically examined. The karyotype contained abnormality on chromosome X – see partial karyotype.
Describe X- chromosomal abnormality and determine the cytogenetic finding.
Gene responsible for fragile X syndrome is called FMR1 (fragile X mental retardation 1).
The gene appears in three forms that are defined by the number of repeats of a pattern of DNA called CGG repeats:
Individuals with less than 60 CGG repeats have a normal gene.
Individuals with 60 – 200 CGG repeats have a premutation ( they carry an unstable mutation which can expand in future generations ) .
Individuals with over 200 repeats have a full mutation which causes fragile X syndrome. The full mutation causes the gene to shut down or methylate a region of the FMR-1 gene.
Normally, the FMR-1 gene produces an important protein called FMRP. When the gene is turned off, the individual does not make fragile X mental retardation protein (FMRP). The lack of this specific protein causes fragile X syndrome .