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 observations that Turner’s individuals appear female and Klinefelter’s individuals appear male may five us information about sex determination in humans. From this information we can tell that:
|A||Sex is determined by the number of X chromosomes (1=male, 2=female).|
|B||Sex is determined by the presence of absence of the Y chromosome.|
|C||Sex is determined by the egg, and not the sperm.|
|D||Any of the above are consistent with the observations.|
Since we know that normal human males are chromosomally XY and females are XX, it would not be unreasonable to put forth either of the following hypotheses: 1.) Sex is determined by the number of X chromosomes present; or 2.) Sex is determined by the presence or the absence of the Y chromosome. Consider that Turner’s individuals appear female, and yet only possess one X, while those with Klinefelter’s syndrome appear male, and yet contain two X’s. However, they do contain a Y, while the Turner’s individuals do not. Thus, we can postulate that sex is in fact normally determined by the presence or absence of the Y chromosome (which is known to be the case). Choice C is incorrect; since the Y chromosome determines sex, it is the sperm, and thus the father, that contributes the information about the sex of the offspring.