You may mean post zygotic isolation/barrier to interbreeding between species. Sterile hybrids, such as mules are a good example of this. The zygote can form from the union of two gametes, but the mature organisms can no longer produce progeny usually due to chromosome mismatch.

After condensing into long strands in the nucleus during Leptotene/Leptonema Chromosomes line up approximately with each other into homologous pairs. Telomeres cluster at one end of the nucleus and synapsis (pairing together of the homologous chromosomes) takes place in Zygotene/Zygonema.

According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern Z-G-T---. That is, eight letter words with 1st letter Z and 3rd letter G and 5th letter T. In alphabetical order, they are:

zygotene

According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 4 words with the pattern Z-GO----. That is, eight letter words with 1st letter Z and 3rd letter G and 4th letter O. In alphabetical order, they are:

zygodont

zygomata

zygosity

zygotene

According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern Z---TEN-. That is, eight letter words with 1st letter Z and 5th letter T and 6th letter E and 7th letter N. In alphabetical order, they are:

zygotene