The question
Name: ______________________ Date: ____________ Nondisjunction in meiosis I and meiosis II (4 points)
Experimental setup
Where did the extra chromosome come from?
Meiosis normally gives each human egg or sperm one copy of each of the 23 kinds of chromosome. Sometimes a pair of chromosomes, or two sister chromatids, fail to separate. A child with Down syndrome has three copies of chromosome 21. Researchers compared a stretch of DNA near the centromere of chromosome 21, where crossing over almost never happens, in the child and both parents. The mother's two copies of chromosome 21 carry different versions of this stretch (types 3 and 4); the father's carry types 1 and 2. The child carries types 1, 3 and 3.
(a) Describe what separates in anaphase I and what separates in anaphase II of normal meiosis. [1 point]
(b) Explain why nondisjunction of one chromosome pair in meiosis I makes all four gametes abnormal, while nondisjunction in meiosis II makes only two of the four abnormal. [1 point]
(c) Using the DNA types, identify the parent and the division (meiosis I or meiosis II) in which nondisjunction produced this child, and justify your answer. [1 point]
(d) A gamete missing chromosome 21 (n − 1) is fertilized by a normal gamete. Predict the number of chromosomes in the zygote, and explain why an embryo with one copy of an autosome rarely survives. [1 point]