Section 3: Human Genetic Disorders

A genetic disorder is an abnormal condition that a person inherits through genes or chromosomes caused by either a mutation in the DNA of genes or changes in the overall structure or number of chromosomes.   A pedigree is a diagram or family tree of genetic inheritance used by geneticists to map genetic traits.  This helps to identify the potential for passing down certain traits to offspring.

Many times, if a family has a genetic disorder, they may turn to a genetic counselor for advice.  One tool that genetic counselors use to help is a karyotype, which is a picture of all the chromosomes in a person’s cell.  To make a karyotype, chromosomes are viewed under a microscope and then counted and photographed.  A human karyotype shows twenty-three pairs of chromosomes; the first twenty-two pairs are autosomes, while the twenty-third pair determines the sex of an individual.  The karyotype would help the genetic counselor detect chromosomal disorders or whether a person has the correct number of chromosomes in his or her cells.

An abnormal number of chromosomes causes several genetic disorders.  For example, Down Syndrome, or Trisomy 21, is when there is an extra copy of chromosome twenty-one.  Trisomy is the medical term used for an extra copy of a chromosome.  Edwards Syndrome, or Trisomy 18, is when the individual has three copies of chromosome number 18.  Turner’s Syndrome occurs only in females when there is a missing or incomplete sex chromosome and only a single X present.


  1. What is a genetic disorder?
  2. How is a karyotype useful?
  3. Identify and explain one type of genetic disorder.

Click here to go back to the Table of Contents