Final answer:
An individual with fewer or additional chromosomes than normal is an aneuploid. Monosomy usually leads to lethal outcomes, while some cases of trisomy, such as Down Syndrome, can be viable but present challenges due to gene overexpression. Sex chromosome aneuploidies often have milder effects due to compensation mechanisms.
Step-by-step explanation:
Individuals who lack an appropriate number of chromosomes for their species, specifically in humans, are referred to as aneuploid. Human zygotes missing any autosome do not generally survive to birth because they lack essential genes, highlighting the critical nature of gene dosage. However, there are exceptions with certain sex chromosome aneuploidies, such as Turner Syndrome (XO), where individuals can survive with only one sex chromosome.
Monosomy refers to the loss of an autosome and invariably leads to non-viable embryos due to the absence of essential genes. Trisomy, on the other hand, such as the all-too-familiar Trisomy 21 causing Down Syndrome, can sometimes result in viable offspring. These individuals usually present with developmental challenges due to the overexpression of genes, which is less tolerated in autosomes than sex chromosomes.
Moreover, sex chromosome aneuploidies like the XXY composition in Klinefelter syndrome and triple X syndrome typically have milder effects due to mechanisms like X chromosome inactivation which mitigate the impact of the extra genetic material.