Biology 100/101 Genetics Exercise
The Blue People Of Troublesome Creek, KY


Hereditary methemoglobinemia (met-H) is a human genetic disease that is inherited as an autosomal recessive. The gene is located on chromosome 22. People aflicted with this genetic disorder have skin which is literally purple-blue in color and dark purple lips. The hemoglobin in their blood has a reduced ability to carry oxygen which produces the blue color of their skin. The dominant, normal allele is responsible for the production of an enzyme (protein), called diaphorase that reduces the hemoglobin so it can be reused and pick up more oxygen. The recessive mutant allele produces a dysfunctional enzyme that does not reduce the hemoglobin. Blue people seem to be able to continually breakdown and produce new hemoglobin by biosynthesis and live a relatively normal life.


A man and a woman are considering having a child and come to you for information about the inheritance and expression of the diaphorase gene.

The man's mother is a blue person and the woman is herself a blue person.

a. To help these people understand the inheritance of this trait construct paper chromosome models to represent of one of the man's cells at the beginning stage of meiosis. Include only the #22 chromosomes and the two sex chromosomes in your models. LABEL the following terms on your chromosomes and/or be able to explain them as you model the formation of sperm cells by meiosis.

homologous chromosome pair #22 the two sex chromosomes (X and/or Y)
the diaphorase gene locus the dominant alleles (D)
the recessive alleles (d) sister chromatids

Remember how we did this with chromosome models last week.

c. What percentage of the woman's egg cells that she produces during her lifetime will contain the recessive allele (d) for diaphorase and an X chromosome?

d. If these two people have a child, what is the probability (chance) that they will have a boy who is a carrier for the disorder? Show and explain how you arrived at your answer.

e. Make up another "genetics problem" based on this genetic disorder.