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 older a woman gets, the greater the chances that she will give birth to a Down syndrome child. Which of the following statements is true?
|A||Only the mother (through the egg) can cause Down syndrome in a child; the father (through the sperm) cannot.|
|B||Older eggs have a greater chance of undergoing nondisjunction, since all of a female’s eggs are present at birth, and meiosis is only completed after fertilization.|
|C||The age of a man is completely irrelevant with respect to the chances of his sperm contributing to the formation of a Down syndrome child.|
|D||Both B and C are correct.|
Question one has established that nondisjunction in either parent can in fact lead to Down syndrome, and there is no reason to believe that it can not, so choice A must be incorrect. It is true that a female is born with all the eggs she will ever release already formed, but arrested in the early stages of meiosis; the longer they remain in her body, the greater the chances that mutations will occur which might result in non-disjunction (choice B is correct). In addition, males would have the same probability of creating an aberrant sperm at any time in their lives, since sperm are made and turned over continuously until death (C is correct).