Question

3. (4 pt.) Think about the nondisjunction you modeled with the orange- and green-tipped chromosomes. What can you say about the origin of two X chromosomes for ND at meiosis I vs. ND at meiosis II. Are the two X chromosomes in each case former sister chromatids or former are they from former homologous chromosomes? Explain why. No points will be earned without an accompanying explanation a. In meiosis I ND? b. In meiosis l ND?

Answer 1

In meiosis I ND :

the X chromosomes are homologous chromosomes. they do not get separated during meiosis 1, as a result, out of four gamets, two gamets have two X chromosomes and other two gamets lack X chromosomes.

yes the X chromosomes in case of meiosis I ND are sister chromatids. the X chromosomes are present in 2 gamets and other 2 gamets lack X chromosomes.

In meiosis II ND:

out of four gamets 3 gamets have X chromosomes and 1 gamet lack X chromosomes. out of 3 gamets having X chromosomes one gamet has 2 X chromosomes and the other 2 gamets has 1 X chromosome each.

ര' (ര) ) ( 1 ) 4

first figure shows ND at meiosis II and the second figure shows ND at meiosis I.