Pedigree - Comprehension...

[grayfalcon89]

I'm having terrible time on the pedigree. I did amazingly well on the Punnett square but I just don't understand the pedigree well.

 

Here, help me out here.

 

I inserted a image as an example.

 

Problem:

 

If we're given that the trait we're dealing with is eye color, then determine whether the trait is dominant or recessive.

 

In my opinion, the trait is recessive because if it was dominant, more "shading" should've occurred. But my biology teacher said that appearing a lot doesn't necessarily means the trait is dominant. One of my classmate had a right answer for it but I don't quite understand it.

 

I can't belive I'm confusing on this simple looking pedigree. But I just don't understand this...

 

If I have more questions, I'll post more. But now, I'm stuck on the pedigree.

[ydoaPs]

it is just a fancy family tree.

it is reccessive, because neither of a carrier's parents had the phenotype for it, yet the child did.

[grayfalcon89]

I'm kind of confused even what I thought was right.

 

Basically, how does this dominant and recessive thing work in the pedigree system? I kind of don't really understand this.

 

Like how do you determine whether the trait is dominant or not? Is the more occurrence means it's dominant in the pedigree?

 

My teacher said that in humans, six finger is dominant yet it's not common.

 

Can anyone really help me?

[Auburngirl05]

Dominance has to do with expression, not frequency. This means that if the organism is heterozygous, the dominant allele will be expressed in the phenotype and the recessive allele will not "show up." This is how people are carriers of diseases without actually being afflicted by the disease themselves. If B is brown hair and b is blonde hair, and brown is dominant, and a person's genotype is Bb, they will have brown hair but can have blonde children if they mate with someone who also carries the recessive (b) allele.

[Auburngirl05]

I just reread your post and realized I didn't fully answer your question about frequency. The reason certain that things like polydactyly (extra fingers/toes) are rare and yet caused by a dominant gene is that not many people have the dominant allele, meaning the majority of the population is heterozygous recessive, so even a dominant gene can have a relatively rare phenotypic expression if not many people carry it. The type of dwarfism called achondroplasia (not sure if that's spelled right, sorry) is another example of a dominant allele that's pretty rare.

[grayfalcon89]

Thanks for the reply.

 

Then application question:

 

If the diagram below shows the pedigree of family of JackColson (cool name!) then is the trait that is shown there a dominant or recessive?

 

* means shading. + means no shading. ~ is used to fill out the empty spaces.

- is used to donate marriage. ________ is children.

 

~~~~~~~~~~[*]-(+)~~~~~~~~

~~~~~~~~~~~~|~~~~~~~~~~

~~~~~~~~~______________~~~

~~~~~~~~(+)-[*]~(+)~~~[+]-(+)~~

~~~~~~~~~~|~~~~~~~~~~|~~~

~~~~~~~~_________~~~~______

~~~~~~~~[*]~(*)~[+]~~~[+]~(+)

 

I think this case is impossible. The reason is that if it's dominant, the nonshading is recessive. On the right generation of the third level, that makes sense pretty okay. recessive-recessive has offsprint of recessive. But comparing the third generation on the left and one on the top, there occurs some weird thing. Since the first generation has two recessive as the offspring, it automatically states that the one of alleles of the parent on the top has recessive trait and since the totally, it's dominant, the parent is heterozygous. Because another parent is recessive and to be recessive, you have to have recessive only, it became obvious that the offspring on the second generation that's dominant is also heterozygous. Now, here comes the part that doesn't make sense. The one that's marrying with the second generation dominant is recessive and this is exactly same case with the one on the top where the guy is dominant and woman is recessive that guy is heterozygous. On the top, however, there were only one offspring with dominant trait. But on the bottom, there occur two recessive trait. So, this proves that this is impossible case.

 

Let's look at the case where the shading is recessive. If it's recessive, then the other parent that it's marrying is heterozygous dominant (nonshaded) because there are one recessive offpring. That explains that the offspring is recessive. Now, this offspring who is recessive marries with the woman who is dominant. Since there is at least one recessive offspring, the genotype is heterozygous (for woman). Yet, here comes the nonmaking sense part. This case is again same as one above. But on the top, there were only one recessive. On the bottom, there were two recessive which can't be possible.

 

Thus, this case is impossible.

 

I'm getting this pedigree now.. But I need more practices I think..

[grayfalcon89]

Another question.

 

Tell me if it's wrong.

 

This trait (the shading) is recessive. Here is why. On the bottom, if the parents who are nonshading are recessive, they can't have children who are dominant because recessive-recessive only gives you dominant.

 

I can go for saying recessive is answer and explaining but I think that's enough to declare dominant nonanswer is enough since there are only two choices.

 

[Drug addict]

Another question.

 

Tell me if it's wrong.

 

This trait (the shading) is recessive. Here is why. On the bottom' date=' if the parents who are nonshading are recessive, they can't have children who are dominant because recessive-recessive only gives you dominant.

 

I can go for saying recessive is answer and explaining but I think that's enough to declare dominant nonanswer is enough since there are only two choices.

 

[/quote']

 

It's a while since I studied any genetics, but I think in the answer above you should be talking about expression rather than dominance (the phenotype can only be expressed if the genotype is d-d.) The parents on the third line must both have the genotype Dd

 

Your conclusion seems correct. If a trait is dominant, it is impossible for it to pass through a generation without being expressed, by definition. The only way a trait can skip generations is if it is recessive.

 

 

With regards to post #6, I think you need more information to be able to decide whether the trait is dominant or recessive. If we use D and d, then:

 

If the trait is dominant, the male in the first generation must be Dd, otherwise all the offspring would express the trait. The female doesn't express the trait, and so must be dd. One of the second generation expresses the trait (Dd) and marries someone who doesn't (dd.) If you do a punnet square (Dd x dd), you would expect 50% of the offspring (3rd generation) to express the trait, but that is statistics, it is entirely possible to have 2 out of 3 children expressing a dominant trait following a Dd x dd cross.

 

 

If the trait was recessive, the first generation expressor would have to be dd and their partner Dd as one of the offspring expresses the trait (= dd.) This person marries someone who doesn't express the trait, but two of three offspring do. This indicates the person joining the family in the second generation must be Dd (if DD, all offspring would express the trait.)

 

A pedigree is really just an extended series of punnet squares. Though your conclusion for post #6 seems correct, your arguement is wrong. What you need to remember is that statistics tell you what is likely to happen, not what will happen. If both parents are heterozygous for a dominant trait (Dd x Dd) and have four children, there is a 6.25% probability (0.5^4) that all four children will not express the trait. Unlikely, but possible.

 

Like I say, it's been a long time since I have done genetics, so if I am wrong please correct me (or if I am correct, confirm my answer.)

[grayfalcon89]

Oh.

 

Yea, I always have that probability in my head. I shouldn't look that way on pedigree.

 

Punnett Square only gives a probabliity, not necessarily what the true will be.

[grayfalcon89]

Ok. This isn't too good question but here, how about this case?

 

It can be dominant because dominant and recessive will have dominant offspring on no matter what. But those can also be recessive if the parent of the no trait is heterozygous and then, there will be a chance of offspring being recessive.

 

That's what I put for the answer. I know the dominant is a "sure" answer and recessive is "only in a special occasion" answer but still, I'm sort of confuse when I'm determing the trait is dominant or recessive.

[ecoli]

Dominant and recessive traits are not black and white. You have to keep in mind that a heterozygous dominant male can mate with homozygous recesive female. You won't know that the male is heterozygous until they have children. If any children express the recessive trait, then the male has to be heterozygous.

 

 

In this picture a homozygous dominant male mates with homozygous recessive female. All there children are phenotypically dominant, although they are actually heterozygous dominant.

 

 

In this picture a phenotypically dominant male mate with a recessive female. Becasue it expresses the dominant trait, there is no way to tell whether it is heterozygous or homozygous until it's children are born. Once you get a phenotypically recesive offspring, then the father has to be heterozygous, in order for it's children to recieve two recessive alleles.

 

When creating a pedigree, its important to know what the children are in order to discover the genotype of the parent.

 

 

In this picture the genotype of the mother is not known. But because on of the children is recessive (not shaded) and the father is phenotypically dominant, we know that the mother is hommozygous recesive, and the father is heterozygous dominant.

The punent square would look like this:

 

 

 

Or, both parents could be heterozygous Dominant, creating an offspring with recessive traits.

 

 

 

The only way to know which situation is true, is to mate the recessive offspring and see what phenotypes occur in it's children.

 

Creating a pedigree can be challenging, but interesting. Keep in mind, however that this form of punnet sqaure can only be used with looking at simple genes. Most phenotypes, in humans especially, are controlled by many genes. Its hard to create a genotype for a trait that is controlled by many different factors. Often times a dominant gene won't be expressed because of another gene controlling it.

 

Hope I helped!

[grayfalcon89]

THANK YOU!

 

But just that on the last part, both parents CAN"T be heterozygous because one of them in nonshading, meaning no traits so it has to be homozygous recessive or it would look like its husband.

[ecoli]

Technically you're right, but it's my fault I should have made the picture clearer. By the question mark, I meant you didn't know the phenotype or genotype of the mother. I'm sorry, that doesn't make to much sense. How could you see the phenotype of one and not the other? I just creating a hypothetical situation. I should have been more specific. Sorry