All life is made up of cells. All cells in the human body, except red blood cells, contain chromosomes. Chromosome comes from the Greek word khroma meaning “color” and soma meaning “body”. Chromosomes got their name from the first lab experiments in the 1880s where it was revealed that chromosomes could be stained with dyes, making them easier to study.
A gene is located on a chromosome. Every factor in inheritance is due to a particular gene. Genes specify the structure of particular proteins that make up each cell. Gene comes from the Greek word genea meaning “generation,” “origin,” “beginning” or “kin.” The word “gene” was shortened from “pangene” which means “all-generation.” Genes contain DNA, the chemical basis of heredity.
Think of it this way: DNA is in genes, genes are on chromosomes.
When it was first seriously conceived to ‘map’ all genes on all human chromosomes, it was called the Human Genome Project—a combination of “gene” and “chromosome.”
What happens when a chromosome is abnormal?
Each chromosome has a specific and proper structure and color. Together, all the chromosomes in our body form a distinct pattern called the human karyotype. We have 46 chromosomes (23 from each parent), however there are variations in reproduction due to abnormalities in the karyotype of an individual. A few of these abnormalities are beneficial, most are harmless, and a few are catastrophic. A major chromosome abnormality (MCA) accounts for half of all spontaneous human abortions. An MCA occurs in about 1 in every 100–200 births. Recent medical diagnostic techniques can now detect many MCAs long before the child is born by analyzing the patient’s blood under a microscope. MCAs take one of five forms:
- In duplication, instead of the normal two, the individual has three copies of the chromosome in every cell of their body (trisomic). Most trisomics result in a spontaneous abortion—but not always. Trisomy-21 (Down’s syndrome) occurs 1.5 times in every 1000 births—the individual has three copies of chromosome 21. Trisomy-8 (Edward’s syndrome) occurs 3 times in every 10,000 births—the individual has three copies of chromosome 18. Trisomy-13 (Patau’s syndrome) occurs 2 times in every 10,000 births—the individual has three copies of chromosome.
- In deletion, a piece of chromosome may go missing. The missing piece is usually from the end of the chromosome, but can be missing from the middle as well. The types of symptoms and their severity will vary depending on the size and location of the deletion. Deletion syndromes occur at a frequency of about 1 in every 16,000 births. In Wolf-Hirschhorn syndrome (1 in every 50,000 births) the abnormality occurs on chromosome. In Cri du Chat syndrome (1 in every 50,000 births), the abnormality occurs on chromosome.
- In translocation, a whole or a piece of a chromosome becomes attached to another.
- In inversion, a portion of the chromosome breaks off, turns upside down, and reattaches.
- In rings, a portion of the chromosome breaks off and forms a circle, with or without loss of genetic information.
In the last three forms, there is an alteration in the pattern of genetic information.