X-chromosome linked disease

[Function]

Hello

 

In preparation for the acceptance exam for Med school, inheritance is very important. I just discovered how I can see whether a disease is dominant or recessive.

There's only one problem: I don't know how to tell if the disease is so-called 'gender-related' (or X-chromosome linked).

Take the following example:

 

Healthy brother has an affected sister. They have an affected father and a healthy mother. The affected father has a healthy sister. They are the result of healthy parents. The mother has a healthy brother. They are the result of a healthy father and an affected mother.

 

I can tell this disease is recessive. I do this by reasoning as following: (please tell me if there's a shorter way)

 

Let's assume the disease is recessive and displayed by genotype aa. Healthy would thus mean a genotype Aa or AA.

 

Daughter has sickness: aa

Her brother (son): Aa or AA

Father: aa

Mother: Aa or AA

 

Should mother be AA, then daughter should also be Aa, which is not the case --> mother: Aa

Result: brother: Aa

 

Mother's father: aa; since mother: Aa, mother's mother must be Aa.

Result: mother's brother: Aa

 

Since father: aa, both his healthy parents should be Aa. Result: father's brother: Aa.

 

No problems are found, so the disease is recessive.

 

Affirmation: suppose the disease is dominant.

 

Daughter: Aa/AA

Brother: aa

Mother: aa

Father: Aa (since daughter must have an A)

 

Father's healthy parents: aa; since both his parents should be aa, he could not be Aa.

Result: the system is in error, so the disease cannot be dominant.

 

So, it's confirmed the sickness is recessive.

2 questions:

* Can this be done in a shorter way?

* How can I see this disease is not X-chromosome linked? I don't really get this part, because I'd say that it is X-chromosome linked, for both the daughter and her father have the disease. In some way, thus, I guess mother's mother must have something to do with it?

 

Thanks.

 

Ta-ta

 

Function

Edited April 8, 2014 by Function

[chadn737]

Its not X-linked. The key is here:

 

"Healthy brother has an affected sister. They have an affected father and a healthy mother. The affected father has a healthy sister. They are the result of healthy parents. The mother has a healthy brother. They are the result of a healthy father and an affected mother."

 

Males, remember have only a single X-chromosome in normal circumstances and they get it from the mother. You are right, this is a recessive trait, which means that the great-grandmother has two copies of the disease causing allele. If it were X-linked, the grandmother's brother would be affected as it is almost certain he would get a disease causing allele.

[Function]

 

You are right, this is a recessive trait, which means that the great-grandmother has two copies of the disease causing allele. If it were X-linked, the grandmother's brother would be affected as it is almost certain he would get a disease causing allele.

 

I'm afraid I still don't really get it. Why do you suddenly bring in a great-grandmother? I thought it was just the grandmother who has two copies of this recessive disease-allele? And nothing is given of grandmother's brother. He's not in this family tree. Perhaps you mean mother's brother?

 

If this is the case, I think I get it: uncle gets one of the two affected X-chromosomes of grandmother, so should have the disease (I just accepted this; is the Y-chromosome not affecting the result, even when it doesn't carry the recessive-disease-causing allele? I mean, will a X-linked disease always affect a male when his mother has two copies of the causing allele, even when he has a completely healthy father, resulting in chromosomes XdYD? (Xd is X-chromosome with causing allele, YD is Y-chromosome with no causing allele)), but he hasn't, so the disease is not X-linked but autosomal?

[chadn737]

 

I'm afraid I still don't really get it. Why do you suddenly bring in a great-grandmother? I thought it was just the grandmother who has two copies of this recessive disease-allele? And nothing is given of grandmother's brother. He's not in this family tree. Perhaps you mean mother's brother?

 

If this is the case, I think I get it: uncle gets one of the two affected X-chromosomes of grandmother, so should have the disease (I just accepted this; is the Y-chromosome not affecting the result, even when it doesn't carry the recessive-disease-causing allele? I mean, will a X-linked disease always affect a male when his mother has two copies of the causing allele, even when he has a completely healthy father, resulting in chromosomes XdYD? (Xd is X-chromosome with causing allele, YD is Y-chromosome with no causing allele)), but he hasn't, so the disease is not X-linked but autosomal?

 

I meant the affected grandmother and the unaffected Uncle. I counted that wrong yesterday.

 

Yes. In an X-linked recessive disease, any sons of an affected Mother will have the disease. The Y-chromosome does not compensate for this. If this disease were X-linked then the Uncle should be affected. That he is not means it is autosomal.

[Function]

 

I meant the affected grandmother and the unaffected Uncle. I counted that wrong yesterday.

 

Yes. In an X-linked recessive disease, any sons of an affected Mother will have the disease. The Y-chromosome does not compensate for this. If this disease were X-linked then the Uncle should be affected. That he is not means it is autosomal.

 

Ah, wonderful. I think I get it +2

Thanks!

Edited April 9, 2014 by Function

[Function]

One more thing, just to be sure:

 

Imagine a recessive, X-linked disease.

Father is carrier of the allele, and must thus be sick. Mother is not carrier of the allele. Conclusion: all the daughters are carriers of the allele, and none of the sons are, and none of the children is affected by the recessive disease.

 

Reasoning: X_DX_D + X_dY_D = X_DX_d, X_DX_d, X_DY_D, X_DY_D

With (X/Y)_D a sex-chromosome without the disease causing allele, and X_d an X-chromosome with the disease causing allele.

Healthy dominates sick, so none has the disease, but 2/4 are carriers.

 

Correct?

Edited April 9, 2014 by Function

[chadn737]

One more thing, just to be sure:

 

Imagine a recessive, X-linked disease.

Father is carrier of the allele, and must thus be sick. Mother is not carrier of the allele. Conclusion: all the daughters are carriers of the allele, and none of the sons are, and none of the children is affected by the recessive disease.

 

Reasoning: X_DX_D + X_dY_D = X_DX_d, X_DX_d, X_DY_D, X_DY_D

With (X/Y)_D a sex-chromosome without the disease causing allele, and X_d an X-chromosome with the disease causing allele.

Healthy dominates sick, so none has the disease, but 2/4 are carriers.

 

Correct?

 

If one half are daughters correct.