While analyzing a karyotype, geneticists identify a missing chromosome. which term describes this chromosomal abnormality?
aneuploidy
polyploidy
triploidy
tetraploidy

The answer is aneuploidy.

The normal karyotype of the most of the species is diploid (2n). That means that in every somatic cell of the body, all chromosomes are present in duplicates. Aneuploidy is the situation when one of the chromosomes is present in a different number of copies than two (2n + 1, 2n - 1) in diploid organisms. 
Triploidy is the one extra copy of all chromosomes, so in diploid cells, there are no 2n, but 3n. Similarly, in tetraploidy (4n for diploid cells instead of 2n), there are 4 copies of all chromosomes.

It forms into a full nebula/star