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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:
--- Quote from: tiredandstressed on September 30, 2021, 08:50:48 pm ---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.
--- End quote ---
Thank you very much! Ill make sure I understand and can remember the trends of each. Already making more sense to me :)
Billuminati:
--- Quote from: sam777 on September 30, 2021, 09:00:16 pm ---Thank you very much! Ill make sure I understand and can remember the trends of each. Already making more sense to me :)
--- End quote ---
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:
--- Quote from: Billuminati on September 30, 2021, 09:49:50 pm ---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
--- End quote ---
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.
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