ly viable "superfemales." Exceptional eggs with no Xs, fertilized by Y-bearing spermatozoa, will give zygotes without X chromosomes; such zygotes die in early stages of development.Chromosomal AberrationsTwo general types of chromosomal abnormalities occur: numerical and structural. Numerical aberrations result from nondisjunction; that is, from the failure of a pair of homologous chromosomes or a pair of sister chromatids to separate during cell division. As described above, when nondisjunction occurs during meiosis two types of germ cells will be formed, those with an extra chromosome and those with a missing chromosome. If one of the former combines with a normal germ cell, the new fertilized egg and all the cells of the individual it produces will have an extra chromosome ; if one of the latter combines with a normal germ cell, the fertilized egg will lack a chromosome. If nondisjunction occurs after fertilization, the resulting individual will be a mosaic and will have two or more populations of cells differing in chromosomal number.Structural aberrations result from chromosome breakages. Chromosomes may break spontaneously, or they may be broken by such environmental agents as radiation, viruses, and toxic chemicals. If a chromosomal segment breaks off and is not rejoined, it may be lost entirely in the gametes or somatic cells that derive, respectively, from meiosis or mitosis. Such a loss is called a deletion. In other instances, the broken-off segment may rejoin its chromosome but with its position inverted 180; such inversions can alter the sequence of genetic information along the chromosome. In other cases, the segment may become translocated; that is, it may become attached to a different chromosome. When such a rearrangement occurs between two nonhomologous chromosomes without net loss or gain of chromosomal material, it is called a balanced, or reciprocal, translocation, and the individual is not phenotypically affected. ...
