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Tips on Reading Pedigree Charts?
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From reading and other sources, I found that if on the pedigree, the trait skips a generation, that means it is a recessive trait.
If the trait is dominant, wherever you see it occur, the one or two of the parents of the affected individual MUST have it.
The question now is...how to ID sex-linkeD?
If the trait is dominant, wherever you see it occur, the one or two of the parents of the affected individual MUST have it.
The question now is...how to ID sex-linkeD?
First: take a look at a few examples of each pattern of inheritance.
Second: Sex-linked vs. Autosomal is one of the most difficult to discern. All I can say is sex-linked (most commonly X-linked) has it so you see males affected more often then females.
But, that is pretty much the only indicator. I don't think there is anything that makes it obvious.
Second: Sex-linked vs. Autosomal is one of the most difficult to discern. All I can say is sex-linked (most commonly X-linked) has it so you see males affected more often then females.
But, that is pretty much the only indicator. I don't think there is anything that makes it obvious.
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Autosomal recessive: e.g., CF, Tay Sachs, Maple Syrup Urine Disease, Sickle cell, etc...
- theoretically the same number of males and females affected
- typically "skip" generations
X-linked recessive: e.g., G6PDH, hemophilia A/B, DMD/BMD, PMD, Androgen Insensitivity, etc...
- an affected male does not pass the trait to his sons, but produces all carrier daughters.
- many more males than females affected (since the former are hemizygous)
- may be partially penetrant in female carriers (e.g., patches of skin affected due to lyonization)
- typically "skip" generations
- theoretically the same number of males and females affected
- typically "skip" generations
X-linked recessive: e.g., G6PDH, hemophilia A/B, DMD/BMD, PMD, Androgen Insensitivity, etc...
- an affected male does not pass the trait to his sons, but produces all carrier daughters.
- many more males than females affected (since the former are hemizygous)
- may be partially penetrant in female carriers (e.g., patches of skin affected due to lyonization)
- typically "skip" generations
Now what do you mean partially penetrant in female carriers.
When you say penetrance, are you referring to the genetic term? Why would that be?
If anything, you'd think that carriers (an individual), would show partial expressivity.
Just curious 😳
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The idea that carrier females never show x-linked recessive trait is false because of dosage compensation. Describing lyonization with penetrance is appropriate, since it refers to the actual presence of trait. Expressivity would refer to the degree of expression for a given trait, and is conditional to penetrance.
Refer to Friedman et al., 1994 Nephrogenic Diabetes Insipidus: An X Chromosome-Linked Dominant Inheritance Pattern [but recessive gene] with a Vasopressin Type 2 Receptor Gene that is Structurally Normal: "The incomplete penetrance in female obligate carriers is most likely related to variable lyonization (X-inactivation) of the normal...."
Refer to Russell et al., 1995 Inherited restrictive cardiomyopathy in a 74-year-old woman: A case of Fabry's disease: "Most likely incomplete penetrance is the result of lyonization of the normal X chromosome...."
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While I'm impressed with your analysis here, am I wrong in assuming that for MCAT purposes, we should rely on the Bio I concept that an X-linked recessive trait is something that generally won't be expressed in a female unless both parents have it?
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From what my kaplan tutor has told me and from what i've seen in problems you can spot sex-link recessive if only males are affected.
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Don't worry too much about lyonization, I think it is beyond the scope of the MCAT. I was just responding to andafoo's question.
Here are my notes from my human heredity class. I know most of this has been said, but here it is anyway. 🙂
- Describe autosomal dominant inheritance
- A trait can appear in either sex (at least one parent has trait if a child has trait as well)
- Do not skip generations
- Describe autosomal recessive
- Can appear in either sex
- Homozygous recessive genotype
- Heterozygotes, or carriers, wild type allele masks expression of the mutant allele
- Can skip generations
- If both parents affected, then we will see all kids affected as well
- X-linked recessive
- Much more common in males, skips generations
- X-linked dominant
- Much more common in females because they are twice as likely to be affected
- Affected father will have all daughters affected
- Y-linked inheritance
- All sons affected and no daughters affected
- Mitochondrial inheritance
- Mother passes on disease to all children and males don't pass it on
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I do, but I would have to scan them.
I'll try to scan them either Sunday or Monday ... I am not going to even LOOK at MCAT stuff today or tomorrow. 
How come for Autosomal Recessive, if both parents have it, the kids will have it?
Autosomal recessive diseases only occur if both alleles are recessive. *Having* a disease is a phenotype. (Note, they could be CARRIERS with just one recessive allele.)
Let's use "D" for disease. If both parents have an autosomal recessive disease, they are both dd. Cross two dd people and all of the kids can only be dd.
Hope that helps.
