Huntington’s disease, which killed folksinger Woody Guthrie, is a dominant genetic illness. A single mutant allele produces an abnormal version of the Huntington protein; this abnormal protein accumulates in particular regions of the brain and gradually kills the brain cells. By middle age, this progressive brain damage produces severely disturbed physical movements, loss of intellectual functions, and personality changes. The pedigree shown below diagrams three generations of a hypothetical family with Huntington’s disease.

    This pedigree demonstrates the  features of autosomal dominant inheritance. Notice that all affected individuals have at least one parent with the disease. Unlike recessive inheritance, there is no such thing as a carrier: the disease will affect all heterozygous individuals. Among the offspring of an affected heterozygote and an unaffected person, 50 percent will be affected and 50 percent will be unaffected. None of the children born to two unaffected individuals will have the disease. (Although not shown in this pedigree, homozygous dominant mutations often produce very severe cases of the disease, because the amount of the abnormal protein is doubled and the normal protein is entirely absent.)

 

 

    Recessive and dominant result from the mutation of a single gene. Some genetic disorders result from the gain or loss of an entire chromosome. Normally, paired homologous chromosomes separate from each other during the first division of meiosis. If one pair fails to separate, an event called nondisjunction, then one daughter cell will receive both chromosomes and the other daughter cell will receive none. When one of these gametes joins with a normal gamete from the other parent, the resulting offspring will have either one or three copies of the affected chromosome, rather than the usual two.

 

    Trisomy

 

    A single chromosome contains hundreds to thousands of genes. A zygote with three copies of a chromosome (trisomy), instead of the usual two, generally cannot survive embryonic development. Chromosome 21 is a major exception to this rule; individuals with three copies of this small chromosome (trisomy 21) develop the genetic disorder called Down syndrome. People with Down syndrome show at least mild mental disabilities and have unusual physical features including a flat face, large tongue, and distinctive creases on their palms. They are also at a much greater risk for various health problems such as heart defects and early Alzheimer’s disease.

 

    Monosomy

 

    The absence of one copy of a chromosome (monosomy) causes even more problems than the presence of an extra copy. Only monosomy of the X chromosome (discussed below) is compatible with life.

 

    Polyploidy

 

    Polyploidy occurs when a failure occurs during the formation of the gametes during meiosis. The gametes produced in this instance are diploid rather than haploid. If fertilization occurs with these gametes, the offspring receive an entire extra set of chromosomes. In humans, polyploidy is always fatal, though in many plants and fish it is not.