Reproductive Systems Gametogenesis and Early Development
Several genetic disorders are caused by trisomies or monosomies, the presence of one too many or one too few of a particular chromosome, respectively. Some such conditions include Down syndrome, which is caused by the presence of 3 copies of chromosome 21 (trisomy 21); Turner’s syndrome, caused by the presence of only one sex chromosome (monosomy X, denoted XO); and Klinefelter’s syndrome, caused by the presence of an extra sex chromosome (XXY).
Individuals with Down syndrome are somewhat mentally retarded, exhibit characteristic facial features, and suffer from a wide variety of chronic heath problems. Turner’s individuals appear female, and may suffer slight retardation, while Klinefelter’s individuals appear male, and likewise may be somewhat retarded. While nondisjunction occurs randomly, no adults with trisomies or monosomies involving any chromosomes other than the sex chromosomes and chromosome 21 are ever observed; if such a condition occurs, it causes miscarriage or early death.
The major cause of trisomies and monosomies is nondisjunction (failure to separate) of homologous chromosomes or chromatids during meiosis, so that gametes are created with 2 copies or no copies of a particular chromosome (when there should be one of each).
The fact that trisomies of only chromosome 21 and the sex chromosomes are observed in adults implies that:
|A||Chromosome 21 and the sex chromosomes are more prone to nondisjunction.|
|B||Trisomies or monosomies of any other chromosomes have such detrimental effects that they do not allow development and cause death.|
|C||There are no genes for fundamental life processes located on chromosome 21.|
|D||Both B and C are correct.|
The passage clearly states that nondisjunction is random, so choice A is ruled out. It also states that if other trisomies or monosomies exist, they are either miscarried or die early; this implies that the effects of such events are extremely detrimental (choice B is true). C also must be true, because if there were genes for fundamental life processes on chromosome 21, Down syndrome individuals could not live to adulthood.