I. What are chromosomes? Chromosomes are the basic substances that make up the nucleus of a cell and are the carriers of genes. In other words, chromosomes are equivalent to a train that carries all the genes that control our looks, height, weight, as well as growth and development, birth and death. The number of human chromosomes is 46 (23 pairs), of which 44 (22 pairs) are autosomes and 2 (1 pair) are sex chromosomes. Jiang Miao, Genetic and Prenatal Diagnostic Center, Zhengzhou University First Affiliated Hospital II. Diseases caused by abnormalities in the number or structure of chromosomes are chromosomal disorders. Because there are many genes on each chromosome, an abnormal number of chromosomes or a small structural abnormality will cause the addition or deletion of many genes, resulting in a variety of abnormalities or abnormal syndromes, i.e. chromosomal disorders. Chromosomal disorders are subdivided into two major categories, one is autosomal disorders, i.e., disorders caused by congenital abnormal number or structural aberrations of chromosomes 1-22. They are characterized by congenital non-progressive mental retardation and growth retardation, often accompanied by malformations of the five senses, limbs, and internal organs ((except for chromosome translocation carriers). The other category is sex chromosomal disorders, which are diseases caused by congenital numerical abnormalities or structural aberrations of the X and Y chromosomes. Clinical features are gonadal hypoplasia or hermaphroditism, while some patients only show reduced fertility, secondary amenorrhea, slightly poorer intelligence, and abnormal behavior. Nearly 10,000 human chromosomal number abnormalities and structural aberrations have been identified, and more than 100 chromosomal disease syndromes. Despite the discovery of so many chromosomal disorders, we all may not be familiar with chromosomal disorders, thinking that they are very rare and have little relevance to our lives. In fact, chromosomal disorders have an important impact on our lives. Numerous studies have shown that 50% of miscarried embryos are caused by chromosomal abnormalities; 0.8% of stillborn babies and 0.6% of neonatal deaths are also caused by chromosomal abnormalities. Chromosomal imbalance carriers often have serious illnesses and die prematurely because of gene loss or duplication, while chromosomal balance carriers do not have gene loss or duplication and do not have abnormalities in appearance or body organs, but they can be inherited and have chromosomal abnormal children with a 50%-100% chance.