Chromosomes are structures within a cell nucleus that are made up of many genes. Genes contain deoxyribonucleic acid (DNA), which contain the genetic information used to synthesize proteins.
Chromosomes are long strands within a cell that can contain hundreds or thousands of genes. Humans have anywhere from 20,000 to 30,000 genes. Each human cell has a pair of 23 chromosomes, which yields a total of 46 chromosomes. Genes help form traits, and more than one gene can create a certain trait. Genes sometimes contain genetic abnormalities or acquired mutations, which in turn influence how traits develop. Within genes, deoxyribonucleic acid (DNA) contains four building blocks, which are adenine, cytosine, guanine and thymine. The order of these bases shapes the genome's instructions.
Despite chromosomes having a large number of genes, those genes are arranged in a very specific sequence. Each gene has a special place within a chromosome, which is called its locus. Most cells in the human body have 23 pairs of chromosomes, with the exception of a few cells like red blood cells, egg cells and sperm. Each pair of chromosomes contains genetic information from a mother cell and a father cell. Of the 23 pairs of chromosomes within a cell, 22 are nonsex chromosomes, or autosomal chromosomes. The last pair of chromosomes contains the sex chromosomes, which are X and Y. The pairing of the sex cells determines the offspring's gender. Males have one X chromosome and one Y chromosome. The X originates from the mother, and the Y chromosome comes from the father. Females, in contrast, have two X chromosomes. One chromosome comes from the mother and the other comes from the father. Ordinarily, genes in the nonsex chromosomes can be fully expressed. Occasionally, however, problems arise wherein this is not the case. Depending on the abnormality, people can be born with or develop mild to severe developmental problems.
Abnormalities that arise in chromosomes can take several different forms. Chromosomal abnormalities can arise as a result of an abnormal number of chromosomes or when an area of the chromosome develops abnormally, such as sections that are accidentally deleted or placed in a different chromosome, which is called translocation. Having an abnormal number of chromosomes can create serious complications. Having an extra nonsex chromosome or lacking a nonsex chromosome, for example, can be fatal to a fetus. It can also cause developmental problems, such as Down syndrome, which appears in genetics as a person having three copies of chromosome 21. Large areas of chromosome that are abnormal can be due to a genetic disorder or an acquired mutation. One example of this abnormality is chronic myelogenous leukemia, which arises when part of chromosome 9 is translocated to chromosome 22.
Sometimes, people have genetic abnormalities without suffering from harmful effects. Humans have about 300 to 400 abnormal genes, but they are less likely to develop problems if one part of the affected chromosome is still normal. Disorders arise when an individual has two copies of the same abnormal genes. The likelihood of an individual developing a disorder is higher in people whose parents have genetic abnormalities than in people whose parents do not have any genetic problems.