Non-disjunction

Each somatic cell in an individual with Down syndrome contains _____ chromosomes.

47

In theory, when a nondisjunction for chromosome 18 occurs during meiosis I, four gametes can be produced. If these gametes are fertilized with unaffected gametes from the second parent, what observations would you make concerning the resulting embryos? a. Two of the embryos will be trisomic for chromosome 18, and two will contain a single copy of chromosome 18. b. Two of the embryos will be trisomic for chromosome 18, and two will be normal. c. All four possible embryos will be trisomic for chromosome 18. d. Two of the embryos will be normal, and two will contain a single copy of chromosome 18.

a

karyotyping A) shows chromosomes as they appear in metaphase of meiosis II. B) can reveal alterations in chromosome number. C) examines points of crossing over. D) reveals the results of independent orientation of chromosomes during meiosis I.

a

Mr. and Mrs. Smith have three sons in elementary school. Two of their children are progressing normally, but their youngest son, Charles, has been much slower than his siblings in developing speech and language skills. His parents are concerned that he has a learning disability and decide to investigate further. Since some learning disabilities can be genetically based, their pediatrician recommends a chromosomal analysis. The results show that Charles has a trisomy of the sex chromosomes, diagnosed as XYY, which is caused by nondisjunction in the formation of the father's sperm. The nondisjunction resulted in an extra copy of the Y chromosome. The extra copy was passed on to Charles during fertilization. Most often, this chromosomal change causes no unusual physical features or medical problems, but those with trisomy of the sex chromosomes do have a higher than normal risk of delays in learning development. During which stage of meiosis could this nondisjunction have occurred?

anaphase II

Which of the following indicates Turner syndrome? a. XYY b. XXY c. XO d. XXX

c

A karyotype is most like a. a movie showing the stages of the reproductive cycle of a beetle. b. the answer key to a multiple-choice exam. c. a map showing the hidden location of buried treasure. d. photographs of every couple at a high school prom.

d

nondisjunction

occasional mishap in the process that distributes chromosomes to daughter cells without error