Brachydactyly type D is a human autosomal dominant condition in which the thumbs are abnormally short and broad. In most cases, both thumbs are affected, but occasionally just one thumb is involved. The accompanying pedigree shows a family in which brachydactyly type D is segregating. Filled circles and squares represent females and males who have involvement of both thumbs. Half-filled symbols represent family members with just one thumb affected.
Is there evidence of incomplete penetrance in this family? Explain.

Question

Brachydactyly type D is a human autosomal dominant condition in which the thumbs are abnormally short and broad. In most cases, both thumbs are affected, but occasionally just one thumb is involved. The accompanying pedigree shows a family in which brachydactyly type D is segregating. Filled circles and squares represent females and males who have involvement of both thumbs. Half-filled symbols represent family members with just one thumb affected.

Is there evidence of incomplete penetrance in this family? Explain.

Pedigree chart showing inheritance of brachydactyly type D in a family, indicating affected members.

Is there evidence of incomplete penetrance in this family? Explain.

Accepted Answer

Hello, everyone. Let's look at our next problem. It says Wartenberg syndrome is a human aal dominant disorder. Displaying blank as individuals with Borden Brook syndrome may have any or all of the four principal features of the syndrome which include hearing loss, different colored eyes, a white forelock of hair and premature gray of hair. And our answer choices are a complete penetrance. B, variable expressivity, C epitasis and D codominance. Well, here we're talking about a syndrome where individuals with the genetic disorder. Now we know it's an autism dominant disorder. So these individuals who have inherited an alley for the disorder can have any or all of the four principal features. So they are affected in different ways or their phenotype varies. And in this case, when you see a variable phenotype and you see individuals with a certain gene expressing that gene to different degrees. That phenomenon is choice B variable expressivity. So we look for that word express and expressivity. So let's look at our other answer choices to understand why they're not correct, which was a complete penetrance and complete penetrance refers to the idea that if you inherit the gene, you express the phenotype. So every single person who has the gene shows the phenotype. And in this case, we actually don't know specifically that that's the case as it doesn't say every individual who inherits the allele shows one of these features. It just tells us individuals with the syndrome. So if you had a case of incomplete penetrance, you might have someone who carries the gene for Wartenberg syndrome. And even though it's an autosomal dominant disorder, they don't actually have a syndrome because it's not completely penetrate. So that's why complete penetrance is not the answer to describe this situation here. We're just looking at the different expressions of the phenotype and people that have the syndrome, not whether everyone who has the alley is expressing the syndrome. Now we move on to choice C epitasis epitasis refers to having two different genes where the expression of one interferes with the expression of the other. But in this case, we have no mention of two different genes. So that's not our answer choice. And finally choice D codominance is when two different alleys are expressed together in the phenotype. But that's not what's happening here. We have a variable expression of a gene on the phenotype, not two different alleys being expressed together. So that's why choice D is not correct. So again, in the case of Denberg syndrome, where individuals who have the syndrome may have any or all of the four principal features of it that refers to choice B variable expressivity. See you in the next video.

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Key Concepts

Autosomal Dominant Inheritance

Incomplete Penetrance

Pedigree Analysis