Which of the following steps would not lead to variation of genetic material?

Question

crossing over of non sister chromatids

crossing over of sister chromatids

the alignment of chromosomes during metaphase 1

the combination of sperm and egg genes

Answer 1

Sister chromatids are not identical - they can be from different chromosomes.

From that list, the only one that doesn't happen is the first.  So that's your answer.

Answer 2

Sister chromatids are identical, so crossing over of sister chromatids does not increase genetic variation.

Answer 3

OK ... the first two answers are all mixed up and, as a result, have some correct parts and some incorrect.

First, sister chromatids ARE on homologous chromosomes.  Homologous chromosomes carry alleles on the chromatids that are for identical TRAITS.  Crossing over is the switching of chromatids (or portions of chromatids) on homologous chromosomes during prophase I of meiosis and leads to variation.

If crossing over occurred in non-sister chromatids, the gamete would either have too many of an allele for a trait or be totally lacking an allele for a trait.  

Therefore, crossing over does NOT occur in non-sister chromatids as non-sister chromatids are on chromosomes that are NOT homologous and would not lead to variation.

The other two choices are, to some degree, dependent on crossing over and, therefore, also lead to variation.