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Biological Sciences

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Reproductive Systems Gametogenesis and Early Development

Question
5 out of 10
 

Which of the following statements is true regarding twin formation?

A Identical twins result from the fertilization of one egg by two sperm.
B Fraternal twins result when two eggs are mistakenly released at once, and each is fertilized by one sperm.
C Fraternal twins result when one egg is fertilized by one sperm, and the developing embryo splits apart, each part being able to grow into a new individual.
D Identical twins are not really genetically identical.
Ans. B

Identical twins result when one egg is fertilized by one sperm, as normal, but for some reason the two cells that result after the zygote divides become separated; at this early stage, each of the cells can go on to produce a complete individual. Fraternal twins are created when two eggs are mistakenly released, and are fertilized by separate sperm. Thus identical twins are in fact genetically identical, while fraternal twins are no more genetically related than any siblings. Of course, identical twins are not identical in all of their characteristics, due to the influence of the environment on development. Choice B is the only answer consistent with this information.

Reproductive Systems Gametogenesis and Early Development Flashcard List

10 flashcards
1)
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.
2)
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.
3)
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.
4)
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.
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