Reference no: EM13707465

 Autosomal Recessive Trait

Introduction:

Watch segment 3, "One Wrong Letter", of the PBS "Nova" PROGRAM, "Cracking The Code of Life".

The first two segments of "Cracking The Code of Life" provide a good background for understanding this particular disease. Watch them if you are interested.

Disease Description:

Tay-Sachs disease is an autosomal recessive disorder that results in a degeneration of the nervous system, producing symptoms at about 6 months of age. Affected children loose hearing, sight, and the ability to move. Children with Tay-Sachs disease normally die by age 3.

The gene involved in this disease is HEXA, located on chromosome #15 (8th from the bottom of the right-hand list of gene locations). The normal, dominant allele for the HEXA gene codes for the alpha subunit of the enzyme β-hexosaminidase. The HEXA enzyme is packaged in the lysosomes of nerve cells and breaks down a fatty material called GM2 ganglioside. This fatty material builds up and degrades the nerve cells of a person who has two recessive alleles for the HEXA gene locus.

Question 1

Autosomal Recessive Trait (6 points)

In the video linked in the introduction above, Allison and Tim have discovered that their baby is suffering from Tay-Sachs disease. After some gene testing, they find that they are both heterozygous for the HEXA gene that causes the disease. Both of them have inherited one dominant allele and one recessive allele. For an autosomal recessive disease like Tay-sachs, a heterozygous person is called a "carrier" because they carry a hidden recessive allele that can be passed on to their children.

Question 1, Part A - Draw a simple, but accurate diagram of the chromosomes in one of Tim's cells in his testes at the BEGINNING stage of meiosis. LABEL the following terms and HIGHLIGHT EACH TERM ONCE in YOUR diagram.

homologous chromosome pair #15 the two sex chromosomes
the HEXA gene locus the dominant (HEXA) alleles
the recessive (hexa) alleles sister chromatids
Question 1, Part B - Make a similar drawing of one of Allison's cells in her ovary at the beginning stage of meiosis, include her #15 chromosomes and sex chromosomes. Include labels for the terms in the TABLE above.

Question 2. (6 points)

Draw a series of three SETS of diagrams that follow the two #15 chromosomes, the HEXA and hexa alleles, and the sex chromosomes through meiosis to produce sperm cells.

Diagram Set #1; Illustrate the chromosomes lined up in the center of the cell during the first division of meiosis. SHOW THE TWO POSSIBLE CHROMOSOME ALIGNMENTS of the #15 and sex chromosomes (remember independent assortment?).

Diagram Set #2; Illustrate all possible combinations of the #15 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 #15 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 HEXA gene locus and the sex chromosomes (X or Y).

Question 3. (4 points)

What percentage of Allison's egg cells that she produces during her lifetime will carry the recessive (hexa) allele and the X chromosome?

Question 4. (4 points)

If Allison and Tim have another child:

a. What is the probability (chance) that they will have a boy who is heterozygous for the Tay-Sachs gene locus?

b. Will this child have Tay-Sachs disease?

c. Show and explain how you arrived at your ANSWER. (Punnett square with an explanation - Be sure to indicate the appropriate genetic combination(s) in your Punnett square.)

Question 5.

Autosomal Dominant Trait (5 points)

Fatal Familial INSOMNIA (FFI) is caused by an autosomal dominant allele that produces an abnormal version of a prion protein normally present in the brain. The abnormal proteins fold unnaturally, clump together, destroy nerve cells, and cause spongy holes in the brain. People who carry just one dominant allele for FFI develop the disease in middle age. Symptoms progress fairly rapidly from mild insomnia and panic attacks, to hallucinations, to complete lack of ability to sleep, and finally, death. Average age of onset is 50 with death following 7 to 36 months later. See also, FFI in Wikipedia.

The gene associated with FFI is located on chromosome #20 (see the third gene locus from the top on left list).

A young woman learns that her mother is developing symptoms of FFI disease. The young woman's mother is heterozygous for the FFI disease. Her father is normal (homozygous recessive).

a. Is there a chance that the young woman's child will develop FFI disease? (The father of the child is homozygous recessive for the FFI gene.)

b. Show or explain how you arrived at YOUR ANSWER. (Punnett square with an explanation - Be sure to indicate the appropriate genetic combination(s) in your Punnett square.)

Question 6.

Aneuploidy and Nondisjunction (5 points)

Triple-X Syndrome (47 XXX) results when a person inherits two sex chromosomes from one of her parents, and a third from the other parent, resulting in a condition in which the individual has three X chromosomes.

Triple X women are normal women in all regards. The extra X chromosome seems to have no real detrimental effect. There is some statistical data that indicate triple-X women are a little taller than average. Triple-X girls may be a little quieter as babies than normal girls.

Triple X Syndrome

See the "Abnormal chromosome numbers" section of lesson 10 FOR MORE INFORMATION on aneuploidy and nondisjunction. These two animations are also useful:

Nondisjunction in the First Division of Meiosis

Nondisjunction in the Second Division of Meiosis

a. Draw a diagram showing how a mistake during meiosis in the development of gametes would RESULT in a gamete with two X chromosomes.

b. Diagram or explain how this would result in a child with the XXX chromosome combination.

c. Could the mother or the father (or both) be "responsible" for this aneuploid condition in a child? Explain your choice.

Question #7.

Sex-linked Recessive (X chromosome) (5 points)

Severe combined immunodeficiency (SCID) is an X-linked recessive disorder. Affected individuals have little or no immune response. You may have heard about the condition as "bubble boy disease". With no immune system, a person is susceptible to recurrent infections from many common diseases that most of us consider minor. A mutated, recessive gene fails to produce an interleukin receptor protein that is necessary for the immune system to function and protect a person from common pathogens.

Bubble Boy disease from MSNBC

A man and a woman are having a child.

The man does NOT have SCID.

The woman is a carrier for SCID.

a. What is the probability that their child will be a boy who does not have SCID?

b. Show how you arrived at your ANSWER. (Punnett square solution with an explanation - Be sure to indicate the appropriate genetic combination(s) in your Punnett square.)

Question #8.

Gene Expression by Protein Synthesis (10 points)

Xeroderma Pigmentosum (XPA) is an autosomal recessive disorder. Affected individuals are exceptionally sensitive to the ultra violet light in sunlight. All of us experience DNA damage (mutation) in our skin cells when we are exposed to sunlight. CONTINUED mutation of DNA of skin cells can result in skin cancer, however most of us have genes that produce enzymes (proteins) that REPAIR these mutations in the DNA. People who have xeroderma pigmentosum have inherited two recessive XPA alleles and do not produce an active form of one of these DNA repair enzymes. As a consequence, homozygous recessive individuals must protect themselves from UV light at all times.

The XPA Gene - Genetics Home Reference

Xeroderma Pigmentosum Society (Read about how affected people and families deal with this condition.)

a. Explain the CONNECTIONS between the normal XPA allele and the production of the XPA DNA repair protein. In other words, how does an allele "produce" a protein (in this case the functional XPA protein)?

Be sure to use ALL of these terms appropriately in your explanation. (5 points)

Dominant XPA allele mRNA PROCESSING TRANSLATION
tRNA Transcription mRNA
RNA Polymerase Amino Acids Codons
Anticodons Protein Folding Ribosomes
introns exons


b. Why does the recessive XPA allele result in the production of a non-functional DNA repair protein? (3 points)

c. How is the functional XPA protein responsible for a healthy condition? (1 Point)

d. What happens in terms of the production of functional XPA protein when a person is heterozygous for XPA? (1 Point)