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).
Which of the following cases of nondisjunction could lead to the development of an individual with Turner’s syndrome?
|A||Failure of the X and Y chromosome to separate during meiosis I in the father.|
|B||Failure of the two X chromosomes to separate during meiosis I in the mother.|
|C||Failure of the two Y chromatids to separate during meiosis II in the father.|
|D||All of the above events could lead to Turner’s syndrome.|
All of these events could lead to the production of a Turner’s individual in the following ways. If the X and Y chromosomes fail to separate in the father at meiosis I, some of the resulting gametes will contain both X and Y, and some will contain no sex chromosomes. If the latter type joins with a normal egg, an XO individual will result. Likewise, failure of the X’s to separate during meiosis I in the mother would lead to some eggs containing two X chromosomes, and some containing none; if the latter combined with a normal male sperm carrying an X chromosome, Turner’s would result. Finally, if the two chromatids of the replicated Y chromosome failed to separate during meiosis II, some gametes would end up with two Y chromosomes, and some with no sex chromosomes; if the latter combined with a normal female egg, an XO individual would be produced.