# Huntington's chorea genotype - HH?

1. Apr 12, 2006

### Math Is Hard

Staff Emeritus
We were doing Punnett squares in my bio class the other night and one of the problems I was working out was about chances of a person inheriting Huntington's chorea.

I thought I had done the problem correctly. Since Huntington's is a dominant disease, I counted any resulting Hh genotypes (one dominant and one recessive allele) as having Huntington's. But then the teacher worked the problem out and said that the genotype would have to be HH (the person MUST have BOTH dominant alleles) in order for the person to have Huntington's chorea. I am completely confused now.

I did some Googling and what I am finding seems to support my original assumptions. For instance this:

http://evolution.berkeley.edu/evolibrary/article/0_0_0/medicine_05

And I have looked at some other sites which state that an individual inheriting the HH genotype has a very low chance of survival, and will not make it to adulthood.

Can anyone help me understand this? Thanks!

2. Apr 12, 2006

### Staff: Mentor

According to the site you posted, Hh would have the disease, but HH would probably have a more severe form. So it appears that while Hh can survive to adulthood (IIRC - Woody and Arlo Guthrie), the HH would not. So Hh can survive to pass on the gene to another.

If the partner is hh, then Hh risk is 0.50. If parter is Hh, then risk of HH is 0.25 and risk of Hh is 0.5.

See also - http://en.wikipedia.org/wiki/Huntington's_Disease - but this is not under strict QC.

3. Apr 12, 2006

### Math Is Hard

Staff Emeritus
But the question remains: why does my professor state that only those with the HH genotype have Huntington's disease? She excludes Hh, and that does not make sense to me.

4. Apr 12, 2006

### Staff: Mentor

Well, from the literature on the internet, it would appear that your professor is incorrect.

University of Iowa - http://www.uihealthcare.com/topics/neurologicalhealth/neur3530.html

National Institutes of Health - http://www.nlm.nih.gov/medlineplus/ency/article/000770.htm

http://biology.wsc.ma.edu/biology/students/posters/caseit/10499m/Huntington/ [Broken] - see bottom of page for more links to other sites.

Last edited by a moderator: May 2, 2017
5. Apr 12, 2006

### Math Is Hard

Staff Emeritus
Funny you should mention the Guthries, 'Nuc. I am going to post the exact question as given. Possibly there is some nuance in the wording that I have missed:

The allele for Huntington's Chorea is dominant. Woody Guthrie was heterozygous for this trait, and Mrs. Guthrie did not have Huntington's Chorea. What are the chances that their son Arlo will develop the symptoms?

I say: 50%

Prof says: 0%

6. Apr 13, 2006

### Staff: Mentor

Woody Guthrie is perhaps one of the most well-known (famous) people who died from Huntingdon's disease.

The Wikipedia bio says that his mother did have the disease, so presumably she is heterozygous -
http://en.wikipedia.org/wiki/Woody_Guthrie

I am trying to remember is Arlo has it. He was very reluctanct to get the test for it - he just didn't want to know, and if he had it, then he likely would (or has) passed it along to his children.

A site devoted to Woody simply mentions that his mother was institutionalized - http://www.woodyguthrie.org/biography.htm [Broken]

And this site that mentions he inherited HD from his mother -
http://www.songaweek.com/woody/songs/huntington.html

So somebody has got it wrong - Woody's mother either had it or she didn't.

Last edited by a moderator: May 2, 2017
7. Apr 13, 2006

### Hootenanny

Staff Emeritus
I would agree with you, if Woody was a heterozygote (Hh) and his spouse was homozygous recessive (hh) a simple monohybrid cross reveals that the genotype of any offspring have a 50% chance of being either heterozygous (Hh) or homozygous recessive (hh) and thus a 50% chance of the phenotypes being normal : HC patient. Perhaps, your proffessor meant to ask for the probability of the offspring having the more sevre case?

Reagrds,
-Hoot

8. Apr 13, 2006

### Math Is Hard

Staff Emeritus
Thanks. I am more and more convinced that my response is accurate, from reading things like this:
http://www.emunix.emich.edu/~rwinning/genetics/beyond3.htm [Broken]

and
http://www.biology.iupui.edu/biocourses/N100/2k4ch10mendelexcept.html

so I am going to have to go back to the prof and see what she meant.

Does anyone know how long someone who is homozygous dominant for Huntington's can live? I think I remember reading that embryonic death is common, and best case scenario, he/she will survive into early childhood.

Last edited by a moderator: May 2, 2017
9. Apr 13, 2006

### Staff: Mentor

It's apparently somewhat complicated because it depends on how many times the errent CAG sequence is repeated.

How Does It Persist?: Huntington's Disease as a Case Study in Population Genetics - http://www.hdac.org/features/article.php?p_articleNumber=22

The CAG repetition was mentioned in one of the other links I posted.

So the professor may be thinking that Hh person will not get the disease, which may be the case if the person does not have the necessary number of CAG repeats.

See also - http://www.hdsa.org/site/PageServer [Broken]

Last edited by a moderator: May 2, 2017
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