What is a chromosome?
A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information.
Chromosomes are the things that make organisms what they are. They carry all of the information used to help a cell grow, thrive, and reproduce. Chromosomes are made up of DNA. Segments of DNA in specific patterns are called genes. Your genes make you who you are.
DNA and histone proteins are packaged into structures called chromosomes.
Everybody has 23 pairs of chromosomes, or 46 chromosomes in total. Each pair is inherited from your parents - one from your mother and one from your father. Two of your chromosomes are responsible for determining what sex you are - for males the sex chromosome is XY and for females it is XX.
Chromosome abnormalities are problems that exist in the genetic structure of a baby's chromosomes. Also referred to as chromosome disorders, these abnormalities can appear in different ways.
Most chromosome abnormalities involve an extra copy of a particular chromosome. Sometimes chromosomes might be broken or arranged in the wrong order.
Abnormal chromosomes are caused by a problem in the development of the sperm or egg cell. No one really knows what causes these problems, but chromosomal abnormalities due seem to appear more frequently as a mother ages.
COMMON CHROMOSOMAL ABNORMALITIES:
A. Down syndrome (trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are 47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely:
Karyotype of a boy with Down Syndrome:
B. Patau syndrome (trisomy 13): serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months.
C. Edward's syndrome (trisomy 18): almost every organ system affected 1:10,000 live births. Children with full Trisomy 18 generally do not live more than a few months.
D. Deletion: a portion of one chromosome is lost during cell division. That chromosome is now missing certain genes. When this chromosome is passed on to offspring the result is usually lethal due to missing genes.
Example - Cri du chat (cry of the cat): A specific deletion of a small portion of chromosome 5; these children have severe mental retardation, a small head with unusual facial features, and a cry that sounds like a distressed cat.
E. Duplication: if the fragment joins the homologous chromosome, then that region is repeated
Example - Fragile X: the most common form of mental retardation. The X chromosome of some people is unusually fragile at one tip - seen "hanging by a thread" under a microscope. Most people have 29 "repeats" at this end of their X-chromosome, those with Fragile X have over 700 repeats due to duplications. Affects 1:1500 males, 1:2500 females.
