WHAT CAUSES ABNORMAL NUMBERS OF CHROMOSOMES? Chromosomes are tiny structures in the cells of the human body that contain roughly 30,000 to 35,000 gene pairs. The genes determine traits like eye and hair color, and also how our bodies grow and develop in the womb. Each person normally has 23 pairs of chromosomes (46 in all), inheriting one chromosome per pair from each parent. Unlike other cells in the body, sperm and egg cells only have 23 unpaired chromosomes; when a sperm and egg unite, they form a fertilized egg with all 46 chromosomes. But sometimes, in the process of cell division, an error occurs, so that a sperm or egg cell has too many or too few chromosomes. Scientists don't know what exactly causes this, but the resulting embryo has a chromosomal abnormality. About 70 percent of such pregnancies result in a miscarriage, but if carried to term, the baby could have any number of disorders because of the abnormality.
ABOUT DOWN SYNDROME AND CHROMOSOMAL ABNORMALITIES: Down syndrome is the most common of these disorders, affecting about 1 in every 800 to 1,000 live-born babies. Babies with Down syndrome have three copies of a particular chromosome instead of two. Children with Down syndrome can suffer from intellectual and developmental disabilities as well as heart defects. Babies born with extra copies of other chromosomes can be severely retarded with many physical birth defects; most die before their first birthday. Other chromosomal abnormalities include small missing sections of the chromosome; a missing single gene; a section of one chromosome attaching itself to another; and a chromosome that is somehow snipped out and reinserted upside down in the sequence.
HOW WE WALK: Walking is different from a running gait because only one foot at a time lifts off the ground. During forward motion, the leg that leaves the ground swings forward from the hip, like a pendulum. Then the leg strikes the ground with the heel and rolls through the toe in a motion similar to an inverted pendulum. The motion of the two legs is coordinated so that one foot or the other is always in contact with the ground -- a so-called 'double pendulum' strategy. The process of walking recovers about 60 percent of the energy expended thanks to the pendulum dynamics and the ground reaction force. The legs act as long levers that transfer ground reaction force to the spine.