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Academic Integrity in Biology 100/101

I have read and understand Part #4 of the University of Illinois Student Code, which deals with Academic Integrity.

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Don't overlook the links to web resources and references to text pages below that might be useful in answering these questions. Activities in lecture, discussion, and laboratory classes will also address these questions

Completion of these questions will help you achieve the Objectives for lectures #10 and #11.

Introduction:

Phenylketonuria (PKU) is an autosomal recessive disease that results in mental retardation, organ damage, and unusual posture. The gene responsible for this genetic disorder is located on chromosome #12. (See the gene locus listed fifth from the bottom of the left-hand list of genetic disease loci on chromosome 12.)

The gene is designated PAH for the enzyme it produces (phenylalanine hydroxylase). Phenylalanine hydroxylase normally converts the amino acid phenylalanine to another amino acid, tyrosine. When PAH enzyme is not present in a person's cells, the resulting build-up of toxic levels of phenylpyruvic acid produced by the partial breakdown of phenylalanine coming from food proteins. The disease can be controlled by strictly watching the diet to restrict the intake of foods containing phenylalanine. Newborn babies are tested for PKU and several other genetic diseases and placed on the low protein diet if the PKU tests are positive. See Astronomer, Dr. Tracy Beck's web pages detailing her experience living with PKU.

PKU is inherited as an autosomal recessive disorder. That means that the gene for the disease is located on one of the numbered chromosomes (autosomes) and is not sex-linked (located on a sex chromosome - X or Y).  The dominant PAH gene exists as the normal, functional genetic sequence that produces a functional PAH enzyme. This is the dominant form of the gene or dominant allele. 

There are more than 320 known mutations of the PAH gene. Each of these mutations represents a different form of the gene, or recessive allele.  Some of the mutations result in little or no change in the structure and functionality of the PAH enzyme.  Some mutations change the shape and functionality of the PAH enzyme and moderately reduce the function of the enzyme.  Other mutations radically change the structure of the enzyme and destroy the ability of the enzyme to break down phenylalanine.  The large number of different mutations in the PAH gene means that there are many different degrees of "recessiveness" for this disorder.  Depending on the two alleles inherited, some people have very mild symptoms, moderate symptoms, or severe symptoms.

As long as a person inherits one dominant (normal) allele from one parent he/she will be able to make enough of the PAH enzyme to remain healthy, even if the allele inherited from the other parent is unable to produce functional PAH enzyme. 

A person who inherits two recessive PAH alleles and can produce no PAH enzyme will have PKU and have to spend a lifetime on a low protein diet to avoid the symptoms of the disorder.

A person who inherits an allele that produces a moderate amount of the enzyme and one that produces none will be able to produce some of the enzyme and will have moderate symptoms.

See PKU News for more information, including personal stories.

Question 1.

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

The couple has learned that they are both carriers (heterozygous) for this recessive disease. Both parents have one normal, dominant allele (PAH) and one severely mutated, non-functional, recessive allele (pah).

Draw a simple, but accurate diagram of the chromosomes in one of the man's cells in his testes at the beginning stage of meiosis. LABEL the following terms and HIGHLIGHT EACH TERM ONCE in your diagram.

Question 2.

Draw a series of three sets of diagrams that follow the two #12 chromosomes, the PAH and pah alleles, and the sex chromosomes through meiosis to produce sperm cells. Remember how we did this with chromosome models in discussion and lab and in the lecture activity in lecture 12.

• Diagram Set #1; Illustrate the chromosomes lined up in the center of the cell during the first division of meiosis. SHOW ALL POSSIBLE CHROMOSOME ALIGNMENTS of the #12 and sex chromosomes (remember independent assortment?).

• Diagram Set #2; Illustrate all possible combinations of the #12 and sex chromosomes in the cells at the end of meiosis I based on the alignment of chromosomes in your drawings in Diagram Set #1.

• Diagram Set #3; Illustrate all possible combinations of #12 and sex chromosomes that could be found in the sperm cells at the end of meiosis II based on the alignment of chromosomes in your drawings in Diagram Set #2.

In each of your three sets of diagrams LABEL and HIGHLIGHT the locations of the dominant and recessive alleles for the PAH gene locus and the sex chromosomes (X and/or Y).

Question 3. What percentage of the woman's egg cells that she produces during her lifetime will carry the dominant (PAH) allele and the X chromosome?

Question 4. If these two people have a child, what is the probability (chance) that they will have a girl who is homozygous dominant for the PAH gene locus? Show and explain how you arrived at your answer.