Chromosomes are the thread-like DNA molecular structures found in the nucleus of cells. The nucleus is a specialized, membrane-protected cell structure that contains the genes and hereditary information. The nucleus also controls and regulates cell activity such as growth. Genes are located on threadlike structures called chromosomes. Chromosomes have two sections, or “arms,” which give chromosomes their unique shape and help identify the location of specific genes.
Each human cell normally contains twenty-three pairs of chromosomes, for a total of forty-six individual chromosomes. Babies inherit twenty-three chromosomes from the mother and twenty-three chromosomes from the father.
Twenty-two of these pairs are called autosomes, or chromosomes that are not sex chromosomes. These twenty-two pairs look the same in both males and females. The twenty-third pair is the sex chromosome, and it differs between the genders. Females have two copies of the X chromosome, and males have one X and one Y chromosome. Each chromosome is numbered based on its size. Chromosome 1 is the largest chromosome, and chromosome 18 is the smallest chromosome.
The presence of more or less chromosomes than normal is called aneuploidy. [15]