Final answer:
In diploid organisms, a trisomic individual has an extra chromosome and is designated 2n+1, with this condition leading to genetic imbalances such as those observed in Down syndrome. Option A is the correct answer.
Step-by-step explanation:
In organisms that are normally diploid, a trisomic individual has one extra chromosome; therefore, they are designated 2n+1. This is because normal diploid organisms have two sets of chromosomes (2n), and in trisomy, there is an addition of a single chromosome beyond the pair. Examples of trisomy include conditions like Down syndrome, where individuals have three copies of chromosome 21 instead of the usual two.
The difference in the gene dosage, which refers to the amount of gene product produced from the chromosomes, is significant in trisomics, as they have three copies (2n+1) instead of the usual two (2n), potentially disrupting normal biological functions. Offspring with 2n+1 chromosomes are more likely to survive compared to those with 2n-1 chromosomes, as monosomic (2n-1) often fail to develop to birth due to the lack of essential genes.
Thus, the correct option for a trisomic individual in normally diploid organisms is a) 2n+1.