The number of nuclei present in a zygote is, for the most part, about 99.9% the same. The exceptions to the rule, however, are known as aneuploidy and the presence of an extra chromosome. Aneuploidy is when the number of chromosomes is off by one or two. It’s important to note that the percentage of aneuploidy is a function of the number of chromosomes present.
The presence of an extra chromosome is not always an indication of aneuploidy. It can be an indicator of a very rare condition called trisomy 21, or just be a normal pregnancy. The prevalence of trisomy 21 will vary from 0% to 50%, depending on the number of chromosomes present. So, for instance, if a zygote had two chromosomes, it would still be considered aneuploidy.
The zygote is a fertilized egg that’s about to develop into a zygote. The zygote is about to develop into an embryo for the first time. The presence of two or more chromosomes within a zygote can indicate aneuploidy.
There are two primary forms of aneuploidy, trisomy 21 and trisomy X. The former is caused by a trisomy of the whole chromosome 21, while the latter is caused by an extra copy of chromosome X.
So, a zygote with two chromosomes has trisomy 21, but a zygote with two chromosome 21s has aneuploidy. As for the number of nuclei present in a zygote, the only way to tell is by counting the chromosomes. If a zygote has two chromosomes, then it has 21 chromosomes, which means it has two nuclei.
Another form of aneuploidy is trisomy X, which is caused by an extra copy of a chromosome X. A zygote with two chromosomes X (21) has trisomy X, but two chromosome Xs (21) has aneuploidy.
Since you can only count chromosomes, all you need to know is the number of chromosomes present in the zygote. And if you have two X 21s, then you have two nuclei, which is, of course, the same as aneuploidy.
So, with the caveat that counting chromosomes is a very different process from counting nuclei, we can learn if something is aneuploidy by looking at both nuclei and chromosomes.
As it turns out, there are a lot of nuclei in a zygote, and there are lots of chromosomes. What’s more, the number of nuclei in a zygote is the same thing as the number of chromosomes present in that zygote. So if you have two chromosome Xs 21s and a zygote X 21, you have two nuclei, which means you have two chromosomes, which is the same as aneuploidy.
The thing that makes a zygote so deadly is that when you look at the genome of a zygote, it looks as though it’s a zygote, and the DNA of that zygote looks like a zygote of the chromosomes. So if the zygote is a zygote, it’s a zygote of the chromosomes, which means it’s not a zygote of the chromosomes.