Pedigree Analysis - Determining Type of Inheritance Pattern of a Trait

[sam777]

Hi,

I'm currently doing Biology 1/2. I just have a question about a particular question on pedigree analysis. Though in general, I find this topic rather difficult. Any tips on how to identify the inheritance pattern quickly? But mainly, I think a walkthrough of this question (attached in an image,) would be very helpful.

[tiredandstressed]

Memorise key trends that are specific for each mode of inheritance
Autosomal dominant: all affected individuals have at least one parent that is affected (equal sex ratio)
Autosomal recessive: two unaffected parents have an affected child OR two affected parents have all affected children (automatically has to be autosomal recessive)
X-linked dominant: all affected daughters, have affected fathers
X-lined recessive: all affected sons, have affected mothers

These are the clear rules that can help you EXCLUDE a mode of inheritance
Here are some steps to answer the question
1. Is there an equal or unequal sex ratio (equal = autosomal, unequal = x-linked)
In your case, it is unequal so I would SUSPECT (but not confirm) it is x-linked
2. Notice trends, based on the rules above ^
Look at that all affected fathers have affected daughters.
This is likely x-linked dominant.

[sam777]

Memorise key trends that are specific for each mode of inheritance
Autosomal dominant: all affected individuals have at least one parent that is affected (equal sex ratio)
Autosomal recessive: two unaffected parents have an affected child OR two affected parents have all affected children (automatically has to be autosomal recessive)
X-linked dominant: all affected daughters, have affected fathers
X-lined recessive: all affected sons, have affected mothers

These are the clear rules that can help you EXCLUDE a mode of inheritance
Here are some steps to answer the question
1. Is there an equal or unequal sex ratio (equal = autosomal, unequal = x-linked)
In your case, it is unequal so I would SUSPECT (but not confirm) it is x-linked
2. Notice trends, based on the rules above ^
Look at that all affected fathers have affected daughters.
This is likely x-linked dominant.

Thank you very much! Ill make sure I understand and can remember the trends of each. Already making more sense to me

[Billuminati]

Thank you very much! Ill make sure I understand and can remember the trends of each. Already making more sense to me

I'll just add a few more hacks (I'm doing 2nd year uni biomed genetics). I do it via the flowchart method, which is kind of like a choose-your-own-adventure book.

1. Is there an approximately equal number of affected males and females? (yes = 2, no = 3)

2. The trait is autosomal. Does it skip generations? (yes = 4, no = 5)

3. The trait is sex linked. Are only males affected and all affected males have affected sons? (yes = 6, no = 7)

4. Autosomal recessive

5. Autosomal dominant

6. Y-linked (this usually never comes up)

7. The trait is X-linked. Important: X-linked traits DO NOT have male-male transmission. Are there more males or females affected? (males = 8, females = 9)

8. X-linked recessive

9. X-linked dominant

Note that for seemingly autosomal disorders (approximately equal number of affected males and females), if you see an affected mother giving birth to all affected offsprings, then the trait isn't autosomal, it's carried on mitochondrial DNA.

I agree with tiredandstressed that this trait is X-linked dominant if you follow the steps in my choose-your-own-adventure style flowchart

[sam777]

 

I'll just add a few more hacks (I'm doing 2nd year uni biomed genetics). I do it via the flowchart method, which is kind of like a choose-your-own-adventure book.

1. Is there an approximately equal number of affected males and females? (yes = 2, no = 3)

2. The trait is autosomal. Does it skip generations? (yes = 4, no = 5)

3. The trait is sex linked. Are only males affected and all affected males have affected sons? (yes = 6, no = 7)

4. Autosomal recessive

5. Autosomal dominant

6. Y-linked (this usually never comes up)

7. The trait is X-linked. Important: X-linked traits DO NOT have male-male transmission. Are there more males or females affected? (males = 8, females = 9)

8. X-linked recessive

9. X-linked dominant

Note that for seemingly autosomal disorders (approximately equal number of affected males and females), if you see an affected mother giving birth to all affected offsprings, then the trait isn't autosomal, it's carried on mitochondrial DNA.

I agree with tiredandstressed that this trait is X-linked dominant if you follow the steps in my choose-your-own-adventure style flowchart

Wow! That's a great way of determining the inheritance pattern. Could I also just confirm that a trait that has an autosomal recessive inheritance doesn't always skip generations, but has the ability to? So if you get no for: "Does it skip generations?" you can't be 100% certain that it is autosomal dominant? Would you then look at the prevalence in general? Such as whether most offspring with affected parents are also affected? But then this also can't prove it for certain, given that there is an element of chance involved, and that the phenotypic ratios of the offspring only indicate what is likely.

[sam777]

Given, the helpful responses, I think for this question, the answer would be B:
Because if the trait expressed an X-linked recessive inheritance, then an affected mother would pass on the trait to all sons (Because all sons get their X chromosome from the mother and their Y from the father). However, II6 is unaffected, and not possibly a carrier, given that the trait is being considered as X-linked.
Though correct me if I'm wrong 

[Billuminati]

Wow! That's a great way of determining the inheritance pattern. Could I also just confirm that a trait that has an autosomal recessive inheritance doesn't always skip generations, but has the ability to? So if you get no for: "Does it skip generations?" you can't be 100% certain that it is autosomal dominant? Would you then look at the prevalence in general? Such as whether most offspring with affected parents are also affected? But then this also can't prove it for certain, given that there is an element of chance involved, and that the phenotypic ratios of the offspring only indicate what is likely.

A more accurate way of phrasing "dominant traits don't skip generations" is that if they disappear from a lineage, they don't reappear. Autosomal recessive traits don't skip generations only if both parents are affected, it won't skip generations as all of their children will be affected too. Not many people with genetic diseases decide to have kids with each other. They often seek genetic counselling to determine their risk of transmitting the disease to their potential children, and the job of a genetic counsellor is to help them make an informed decision without judging them (a huge no-no would be telling them that "you can't reproduce because your kids will be affected", that's essentially Hitler and communist Chinese behaviour).

[Billuminati]

Given, the helpful responses, I think for this question, the answer would be B:
Because if the trait expressed an X-linked recessive inheritance, then an affected mother would pass on the trait to all sons (Because all sons get their X chromosome from the mother and their Y from the father). However, II6 is unaffected, and not possibly a carrier, given that the trait is being considered as X-linked.
Though correct me if I'm wrong 

And yes, you are correct that you can only rule out X-linked recessive here, if it were the case, all sons of an affected mother will also be affected, but II-6 is unaffected