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Pedigrees

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Pedigrees

Postby pimchy » Mon Oct 17, 2011 11:48 am

Hi all, just some clarification needed, I'm really struggling to grasp pedigrees and modes of inheritance even though I've spent the last 4 hours reading the textbook, and attempting question but I seem to get every second one wrong still.

Here are a couple that I'm not sure of and what I think each of the modes of inheritance are:

1) Image
Autosomal dominant.


2) Image
Autosomal recessive


3) Image
Is the above mode of inheritance autosomal due to III-2 lacking the trait?
Also the question on this pedigree is: 'Females II3, II4 and II5 are...' is it necessary to know whether the trait is autosomal dominant or recessive to answer the question?, because I can't for the life of me figure it out :[


4) Image
Mode of inheritance is Xlinked dominant since male I-2 passes trait to daughters but not sons and some female/male offpsring of female II-7 have the trait.
Initially I thought this was autosomal dominant, but I'm not sure how to disprove this?

Any help, corrections or pointers in the right direction would be greatly appreciated. Thanks! :]
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Postby JackBean » Mon Oct 17, 2011 12:17 pm

1 and 2 seem OK

3) it's autosomal mainly because of the second generation.
'Females II3, II4 and II5 are...' are what? Sisters? Affected? I don't know what are you looking for.
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
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Postby Cat » Mon Oct 17, 2011 6:49 pm

1, 2, and 4 are correct.

3) is X-linked recessive, NOT autosomal...
II-2 passes on wild type X allele to all progeny, so none will have recessive phenotype. I-1 and II-8 females are heterozygous...
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Re:

Postby JackBean » Mon Oct 17, 2011 6:57 pm

Cat wrote:3) is X-linked recessive, NOT autosomal...

why?
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
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Postby Cat » Mon Oct 17, 2011 7:05 pm

Please, disregard my previous post. I was wrong…
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Postby JackBean » Mon Oct 17, 2011 7:25 pm

no, I won't. I want to discuss, which one is correct, I'm not able to see difference between autosomal or X-linked.
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
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Postby Cat » Mon Oct 17, 2011 8:54 pm

O.K. Problem 3:

Reasoning:

F0 generation - wt female and affected male produce 3 affected females and 2 wt males and 1 affected male.
Without looking at any further generations, I would say that this scenario is more consistent with X-linked phenotype because ALL females are affected.
Now, if you look at possible genotypes to fit this scenario, you will see that to produce F1 affected male (see II-7 male) the F0 female need to have the gene responsible for this condition. So, if she carries the gene but is unaffected herself, she must be heterozygous and the gene responsible has to be recessive.
However, if you concede that affected female is homozygous recessive for the X chromosome (II-1 female), than ALL male progeny should be affected and that is NOT the case (see III-2 male).
Therefore, the condition must be autosomal.
I don't think you can tell if it's dominant or recessive. In order to produce these results one of the F0 parents need to be heterozygous and one homozygous recessive. Regardless of which genotype you assign to each, the phenotypic results would not change...
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Postby pimchy » Mon Oct 17, 2011 11:40 pm

Hey sorry for not elaborating, for 3) it asks are the females ii3,4,5 are a) homozygous b) heterozygous or c) eithery homozygous or heterozygous...for the trait
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Postby Cat » Tue Oct 18, 2011 12:07 am

c) eithery homozygous or heterozygous...for the trait
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Re: Pedigrees

Postby pimchy » Tue Oct 18, 2011 6:51 am

The initial question for pedigree 3 is:

The trait must be autosomal because:
a) all daughters of I2 have the trait.
b) III3 lacks the trait.
c) III2 lacks the trait.

Is the answer c) because if it was X-linked then male III-3 would have the trait since the mother exhibits the trait?

and this question is also regarding pedigree 3
The chance that an additional child of II1 and II2 will show the trait is:
a) 0
b)1/4
c)1/2
d)3/4

I'm unsure of how to work this out without knowing whether the trait is dominant or recessive, since if the trait is recessive then II-2 must be heterozygous and II-1 is homozygous for the trait which would make the probability of their child having the trait 1/2.

But if it is dominant then II-2 is homozygous for not having the trait and II-1 is either heterozygous or homozygous for the trait. And that's where I'm lost :(

(also thankyou for your replies so far, i just needed someone to confirm that my answers so that i know my logic isn't completely made up :? )
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Postby pimchy » Tue Oct 18, 2011 7:14 am

And the final question to do with pedigree 3 is:
Males I2, II7 and III5 are all:
a) heterozygous for the trait.
b) homozygous for the trait.
c) could be either homozygous or heterozygous for the trait.

Similar to the question about the females, I'm not sure how you can tell unless you know whether the trait is dominant or not. Or is c) the correct answer because we do not need to specify whether the trait is dominant or not.
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Postby Cat » Tue Oct 18, 2011 5:48 pm

"Is the answer c) because if it was X-linked then male III-3 would have the trait since the mother exhibits the trait?"

C is correct. You made a type-o here - it's III-2 male.

"The chance that an additional child of II1 and II2 will show the trait is:
a) 0
b)1/4
c)1/2
d)3/4

I'm unsure of how to work this out without knowing whether the trait is dominant or recessive, since if the trait is recessive then II-2 must be heterozygous and II-1 is homozygous for the trait which would make the probability of their child having the trait 1/2.

But if it is dominant then II-2 is homozygous for not having the trait and II-1 is either heterozygous or homozygous for the trait. And that's where I'm lost."

Part 1 is correct, but you need to think through the 2nd part of your logic. If it's dominant, than II-2 is homozygous recessive and II-1 MUST be heterozygous because if it was homozygous dominant ALL of the offspring would be affected since all would be heterozygous for the trait, but NONE are affected so far. So, in either case you have a cross of heterozygous to homozygous that results in 50% chance...

For your final question c is also correct...
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