Final answer:
An organism with three copies of a specific chromosome is described as trisomic, which is a form of aneuploidy. Trisomy, such as trisomy 21 leading to Down syndrome, causes a genetic imbalance and characteristic developmental and physical features.
Step-by-step explanation:
An organism that has three copies of a particular chromosome instead of the normal two copies is said to be trisomic. This condition is a type of aneuploidy, where there is an atypical number of chromosomes in an organism. Humans typically have 22 pairs of autosomes and one pair of sex chromosomes, making them euploid. However, in cases of trisomy, such as the common trisomy 21, which leads to Down syndrome, the individual has 47 chromosomes with an extra chromosome 21. This genetic imbalance can create a surplus of gene products from that extra chromosome, causing developmental challenges and distinct physical features. Older women have a higher chance of giving birth to a child with trisomy 21, indicating a correlation between maternal age and the occurrence of Down syndrome.