A.  Mutations
           1.  Definition - a rare, random and inheritable change in a cell's genetic material (DNA)

           2.  Recessive Alleles - most mutations produce recessive alleles (If they produced dominant alleles that resulted in fatality, the mutations would soon be eliminated from the population.)

           3.  Human Example:
                a.  Hemophilia = a genetic disorder where the blood does not clot properly; a moderate cut can be life-threatening for a hemophiliac
                b.  Normal DNA --- Production of Functional Clotting Factor --- Normal Blood Clotting Ability
                c.  Mutated DNA --- Production of Nonfunctional Clotting Factor --- No Blood Clotting Ability
                d.  Possible Genotypes:
                     1)  Homozygous Dominant - NN - Normal Clotting
                     2)  Heterozygous - Nn - Normal Clotting
                     3)  Homozygous Recessive - No Clotting

                   

     B.  Lethal Alleles
           1.  Definition - a mutated allele that fails to code for the production of a functional protein that is vital for life

           2.  Dominant vs. Recessive:
                a.  Dominant Lethal Allele - Quickly eliminated from the population, because usually causes death before the individual can reproduce.
                b.  Recessive Lethal Allele - Will only cause death in the homozygous recessive condition.

           3.  Heterozygous Condition:
                a.  Carrier - an individual who is heterozygous for a lethal allele or an allele causing any genetic disease
                     1.  These individuals are able to live and reproduce.
                     2.  Carriers do not phenotypically express the genetic condition.
                     3.  These individuals, however, can pass the lethal allele onto offspring.
                 b.  The average human is heterozygous for 3-5 lethal alleles.
                 c.  Partially explains problems associated with inbreeding.

           4.  Human Example:
                a.  Brachydactyly - a genetic condition in which the fingers are abnormally short in heterozygotes; however, this condition is fatal during infancy to homozygous recessive individuals due to major skeletal defects

                b.  Marriage between Two Brachydactyl People:

                          F₁ Genotypic Results:  1 BB : 2 Bb : 1 bb
                          F₁ Phenotypic Results:  1 Normal Child
                                                              2 Brachydactyl Children
                                                              1 Infant Death

     C.  Human Genetic Diseases
           1.  Sickle Cell Anemia - genetic condition that is often lethal in the homozygous recessive condition
                a.  Involved gene codes for the production of hemoglobin, which is an oxygen-carrying protein found in red blood cells
                b.  The alleles involved are governed by Codominance.
                c.  Genotypes and Phenotypes Involved:
                     1)  NN - Homozygous Dominant - Normal Blood Cells

                                 

                     2)  Nn - Heterozygous Carrier - Both Normal and Sickled Red Blood Cells Produced - Nn Genotype Conveys Resistance to Malaria (a potentially fatal mosquito-borne illness prevalent in third world nations)

                          

                     3)  nn - Homozygous Recessive - Sickle Cell Anemia - 50% chance of death by age 20

                           

           2.  Tay-Sachs Disease - genetic condition that is lethal to all homozygous recessive individuals by age four
                a.  Incurable Metabolic Disorder
                b.  Results in Brain Deterioration
                c.  1 in every 30 people of East European Jewish descent are carriers of the Tay-Sach's recessive allele


           3.  Cystic Fibrosis - genetic condition that is lethal to all homozygous recessive individuals by age 30
                a.  Incurable disorder in which thick mucus accumulates in the lungs leading to constant and dangerous respiratory infections
                b.  Most common lethal allele in the United States' Caucasian Population
                       

           4.  Huntington's Disease - genetic condition caused by a dominant lethal allele
                a.  This dominant allele is still present in the population because the disease does not affect individuals until after reproductive stage of life (40-50 years of age).
                b.  Symptoms include twitching and depression
                c.  Results in Brain Degeneration and death within five years of onset

              

     D.  Inborn Metabolic Errors
           1.  Definition - genetic defect caused by a mutation of a gene coding for proteins that function as enzymes in the body's metabolic pathways

           2.  Phenylketonuria - PKU - genetic condition in which individual lacks the enzyme to convert phenylalanine to tyrosine
                a.  Can result in mental retardation and brain damage without treatment.
                b.  All babies born in the United States are tested for PKU before leaving the hospital.
                c.  Treatment is following a special diet avoiding such things are nutrasweet and diet foods.
                d.  Warning labels are on food products containing phenylalanine to warn those with PKU.

           3.  Albinism - genetic condition in which individual lacks the enzyme to convert tyrosine to melanin, which is a skin pigment
                a.  True albinos have very little melanin.
                b.  White hair, very light skin, and red pupils result from this severe lack of pigmentation.
                c.  Albinos must be very careful of sun exposure.

       

To test your knowledge about Mutations and Diseases, click on the Mutation Questions Link at the top of this page.  After you answer the questions, be sure to check your responses by clicking on the Mutation Answers Link.

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