Download Model Organisms pre-class activity: Huntington disease

Model Organisms pre-class activity: Huntington disease
1.
What is the difference between a “normal” huntingtin gene and a gene that has the Huntington
mutation?
A person with the “normal” gene has 9-35 repeats of a CAG sequence in the huntingtin gene. A person
with the HD mutation has between 36 and 121 repeats.
2. What is the number of repeats in a huntingtin gene related to?
The number of repeats is related to the age of onset of Huntington disease. The greater the number of
repeats the earlier the onset of the disease.
3. How does an excess number of repeats affect the protein created by this gene?
Each CAG is a codon that codes for the amino acid glutamate. When a protein is made from the mutated
huntingtin gene, it will include too many glutamates, causing the huntingtin protein to fold incorrectly.
4. Explain how a child can inherit Huntington disease.
A child inherits the disease by receiving an expanded chromosome number 4 (with an expanded
huntingtin gene) from either their mother or father.
5. How could a child whose biological parents don’t have Huntington disease go on to develop this disorder?
If a father has between 27 and 35 repeats in his gene, this chromosome can gain repeats when the
sperm enters the egg and begins to form an embryo. If this chromosome ends up with more than 40
repeats, the child will develop the disease.
6. (a) Draw a Punnett square showing a cross between an unaffected mother and a father with the
Huntington mutation. What is the chance (in percent) that their child will inherit the disorder?
There is a 50% chance that a child of this couple will develop Huntington disease.
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(b) Explain how it is possible for this set of parents to have one child who will inherit the disease and
two other children who do not inherit the disease.
Each child from this couple will have a 50% chance of inheriting the disorder. The probability is the
same for each child born.
7.
Explain the term autosomal dominant as it relates to Huntington disease.
Autosomal dominant means that the gene is dominant (rather than recessive) and resides on a
chromosome known as an autosome – one of the 22 “numbered” chromosomes, not the X or the Y.
If one parent carries the mutated gene for this disease, there is a 50% chance that their child
will have the disease.
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