What are chromosomes?

Chromosomes contain the genes which direct the structure and function of all parts of the body like an architectural plan. Within each cell of our bodies, except for the mature red blood cells, the nucleus contains the 46 human chromosomes. The chromosomes may be seen under the microscope and are divided into 23 pairs based on size and structure. One pair convey the genetic gender of the individual. As a cell prepares to divide, an exact duplicate of each chromosome forms. This way each daughter cell has a complete set of all the chromosomes and all the genes.

Occasionally, when an egg cell (oocyte or ovum) or a sperm cell are made, the chromosomes do not split evenly. An egg or sperm with an extra chromosome or missing a chromosome is then made. If the abnormal egg or sperm goes on to a conception, the fetus will have an abnormal number of chromosomes. Women who are over age 34 at the delivery of a baby are at increased risk for this type of problem, but it may happen at any maternal age.

This situation is like having a mistake in an architectural plan. Children with too few or too many chromosomes have medical problems which may be very severe. A typical syndrome in which a child has too many chromosomes is Trisomy 21, or Down syndrome. In this syndrome, the heart, intestines, stature, facial structure, intellect, and other body parts may be affected. Other chromosome abnormalities may be more severe, which may lead to birth defects, a miscarriage, or the death of a child.

What are genes?

Genes are the smallest unit of organized material we inherit from our parents. Genes are made of DNA and are found in a specific order on the chromosomes within each cell of the body.

The genes are like the different parts of an architectural plan for a building. For example, the differences in each of our faces allows us to recognize individuals as distinct from others. These differences are due in part to variations in the genes which contribute to the formation of the human face. Similarly, hair color, eye color, and stature add to our distinction as specific people and are controlled by genes.

Each of us contains a mixture of variations of the thousands of genes necessary to create a human being. We inherit roughly half of our genes from our mothers in the egg (oocyte) and nearly half from our fathers in the sperm. This inheritance of pairs of genes is why members of a family often resemble each other.

What is DNA?

DNA is a complex chemical compound (deoxyribonucleic acid) made up of four "building block" units in specific patterns and organized to form genes and chromosomes. The building blocks or nucleotides are named adenine, cytosine, guanine, and thymine or A, C, G, and T. DNA forms as long strands which pair and coil around each other. This coil is called a double helix. There are 46 long double helices called chromosomes inside each cell.

The pattern of the nucleotides within genes is very specific, and these are linked together to form an array of individual genes in the chromosomes. The nucleotide pattern can be interpreted by a cell to make proteins from the thousands of genes. The thousands of proteins are the gene products which make up the structure and function of cells, organs, and the whole body.

What is a mutation?

A mutation is a change in the DNA in or close to a gene, which then causes a change in the structure or the amount of the product of the gene. A mutation is different than a normal variation because it is associated with a disorder or disease while a variation in the DNA is not usually associated with a disease. A gene change may be inherited or may have arisen as a new mutation in an individual. Mutations or changes in genes may convey an increased risk of developing heart disease or a predisposition to cancer, for example.

Normally, each person's genes are paired with one copy from the mother and the other copy from the father. For some gene pairs, a change in only one of the copies will cause a disorder to develop in the person that carries the mutated copy. For other gene pairs, both copies must be defective before a disorder develops in the affected person. In this case, the person received a defective copy of the gene from each parent. If each parent only had one defective copy, they would be called silent carriers. In fact, each of us silently carries one mutation in six or eight or more of this type of gene pair known as autosomal recessive. Some of the autosomal recessive conditions are common enough for pregnant patients to be offered genetic screening tests to detect carriers. Ask your healthcare professional if genetic testing is appropriate for you.