Microscopic studies of the chromosomes themselves have revealed that abnormal numbers of chromosomes can be produced by a fault at the moment of fertilisation. These might be caused by changes in numbers or structure of chromosomes. Numerical chromosomal changes include the absence of a chromosome (monosomy), as in Turner's syndrome, or an additional chromosome (trisomy), as in Down's syndrome. Cytogenetic studies have shown that patients with Down's syndrome have an additional chromosome, which is indistinguishable from chromosome 21. Down's syndrome is more common in children born to older women and the eye changes include narrow palpebral fissures with a characteristic slant, cataract, high myopia and rather intriguing grey spots on the iris known as Brushfield's spots. Brushfield's spots are sometimes seen in otherwise normal individuals. Turner's syndrome (one missing X chromosome) and Klinefelter's syndrome (an extra X chromosome) are further examples of disease in which there are known to be abnormalities of the chromosome, which are visible under the microscope. People with these last two diseases are of interest to the ophthalmologist on account of the abnormal but predictable manner in which they inherit colour blindness.
Structural abnormalities occur when recombination or reconstitution in an altered form follows chromosomal breaks. Such changes can be in the form of deletions, duplication inversions, translocations or isochromosomes.
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