- A chromosome abnormality or a chromosomal disorder is a missing or irregular or extra portion of chromosomal DNA.
- usually occur when there is an error in cell division following meiosis or mitosis.
- are generally not inherited but occur as random events during the formation of reproductive cell.
- an error in cell division called non-disjunction results in the reproductive cells with an abnormal number of chromosome.
- Non-disjunction – the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during anaphase (meiosis), usually resulting in  abnormal distribution of chromosomes in the daughter nuclei.
- examples of chromosomal disorders = Down syndrome, Edwards syndrome, Patau syndrome, Cri du Chat syndrome, Turner syndrome etc
Fig: Non-disjunction (wikipedia)
Down Syndrome:
- also called trisomy 21 or 47, + 21 or DS or DNS
- chromosomal disorder caused by the presence of whole or a part of third copy of chromosome 21.
- first characterized by English physician John Langdon Down in 1862.
- occurs in about one per 1000 babies born each year.
- sign & symptoms = mental illness, stunted growth, shortened hands, slanted eyes, abnormal teeth, short neck, flat head, large and protruding tongue, poor muscle tone (hypotonia), extra space between big toe and second toe etc
- prone to respiratory disease, heart malformation, keratoconus and leukemia.
- most individuals with Down syndrome have mild IQ (50-70) or moderate IQ (35-50).
- characteristic cause = non-disjunction of chromosome 21 during meiosis – paired homologous chromosome failed to disjoin either during anaphase – I or II that may lead to gametes with n +1 chromosome composition.
- main source (about 95%) of trisomy 21 is the ovum.
- other cause = chances dramatically increase as the age of mother increases.
At 30 –> 1 in 1000
At 40 –> 1 in 100
At 45 –> 1 in 5o
- parental diagnosis = Amniocentesis (AFT) or Chorionic Villus Sampling (CVS)
- therapeutic abortion is one option currently available to parents with number of religious and ethical issues.
Fig: Karyotype – Down Syndrome