What are Genes & Chromosomes?

Humans grow and develop based on the instructions contained in our genetic material. A normal human cell should contain exactly 46 chromosomes. They are matched into 23 pairs. Pairs one through 22 are numbered by size and appearance, and are the same in males and females. The 23rd pair is referred to as the sex chromosomes. Typically, females have 2 X chromosomes, while males have an X and a Y. The shorthand way to refer to a normal set of chromosomes is 46,XX for women and 46,XY for men.

46,XY		46,XX

At conception, an egg containing 23 chromosomes from the mother combines with a sperm containing 23 chromosomes from the father. The two sets of genetic information combine so that the growing embryo has 23 pairs, or 46 total, chromosomes and is a mixture of genes from both biological parents.

Sometimes an error occurs when an egg or sperm develops and it may have extra or missing chromosomes. It is not known why this happens in a particular egg or sperm. When the abnormal egg or sperm is involved in a conception, there will not be exactly 46 chromosomes, and the resulting pregnancy will have a chromosome abnormality.

As a woman gets older, the chance that she may have an abnormal pregnancy involving an extra or missing chromosome increases. There appears to be no association with most family histories, ethnicity, diet, or lifestyle.

A common example is Down Syndrome. In Down syndrome there is an extra copy of chromosome #21, creating a total of 47 chromosomes. For this reason, it is also known as Trisomy 21. The chromosomes of a person who has Down syndrome are shown below.

47,XX,+21

Approximate chances of having a liveborn child with a chromosome abnormality:

Maternal age         Chance for abnormality
Age 20                                   1 in 525
Age 25                                   1 in 475
Age 30                                   1 in 380
Age 35                                   1 in 180
Age 38                                   1 in 105
Age 40                                   1 in 65
Age 42                                   1 in 40
Age 45                                   1 in 20

What are Genes?

Chromosomes contain smaller units of genetic material called DNA. DNA is a sequence of letters that spell out the genetic code. The DNA is organized into words and sentences called genes. Humans have approximately 20,000 genes and each one influences a part of development. These genes usually come in pairs. One copy is inherited from our mother and one from our father. A change in the spelling of a DNA sequence or the gene is called a mutation. Every person’s DNA contains mutations which are usually harmless. However, some mutations are responsible for causing specific disease.