Define aneuploidy.

Answer

### Step-by-Step Solution:

1. **Definition of Aneuploidy**:
- Aneuploidy is defined as a condition in which an organism has an abnormal number of chromosomes.

2. **Understanding Normal Chromosome Number**:
- In humans, the normal chromosome number is 46, which consists of 23 pairs. This includes 22 pairs of autosomes and 1 pair of sex chromosomes.

3. **Types of Aneuploidy**:
- Aneuploidy can occur in two main ways:
- **Addition of Chromosomes**: This occurs when there is an extra chromosome, leading to a total of 47 chromosomes. This condition can result in genetic disorders, such as Down syndrome (Trisomy 21).
- **Loss of Chromosomes**: This occurs when one chromosome is missing, resulting in a total of 45 chromosomes. This can lead to conditions such as Turner syndrome (monosomy X).

4. **Impact of Aneuploidy**:
- The presence of an abnormal number of chromosomes can lead to various diseases and developmental issues, depending on which chromosome is affected.