Genetics recombination

[typicalindian]

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This is from TPR bio chapter 6
I just got really confused when doing this problem because I'm very weak in genetics, could someone please walk me through it?







The answer is C.

 

[chiddler]

Bones and hair color are linked genes. That means that, without recombination (crossing over), they are usually transmitted together. The child has brittle bones and dark hair which means that he is a recombinant. If he were not recombinant, he would either have blond hair and brittle bones, or sturdy bones and dark hair.

All their kids must have fragile bones is not true. Their first kid is an example of this, but in principle crossing over may occur during meiosis which causes recombinant phenotypes. This answers both II and III.

 

[milski]

Let's start with the grandparents. Each of them had one which was blond/fragile, which means genotype of ssbb. All other 3 grandparents are SSBB.

Tanya's grandparents:
SSBB SSBB SSBB ssbb
Jose's grandparents:
SSBB SSBB SSBB ssbb

When SSBB and SSBB mate, the kids are SSBB. When SSBB and ssbb mate, the kids are SsBb. That means that both Tanya and Jose have one SSBB and one SsBb parents.
Tanya's parents:
SSBB SsBb
Jose's parents:
SSBB SsBb

That means that both Tanya and Jose can be either SSBB or SsBb. Remember that S and B are on the same chromosome and close to each other, which makes SSBb or SsBB very very unlikely.

We know that one of their children has fragile bones and is brunette, which means that it is ssBb or ssBB. To get the ss, both Tanya and Jose have to be SsBb.

I is true since recombination is the only way that you can get an ssBb genotype.
II is not true Tanya and Jose can easily have SxBx kids who will not have fragile bones.
III is also true because Tanya and Jose can have ssbb kids who ill have fragile bones and blonde hair.

 

[Danlee07]

Um, first shrink your picture. This problem says the genes are on the same chromosome which makes this problem easier since now only the given genes are recombined with each other; also genes on the same chromosome are likely to be genetically linked since these genes tend to have a small distance between each other.

When doing any problem, write out the givens. The dominant and recessive genes of both brown, blond hair and strong, weak bones are given. For now, try it the common sense way. Since "one of their children has brown hair and fragile bones" this is another given. There's a dominant trait and a recessive trait. Now, look up the lineage to the grandparents (in this problem, the grandparents are considered F0). One grandparent is homozygous dominant with both brown hair and sturdy bones while the other grandparent is homozygous recessive with both blond hair and fragile bones. If a homozygous recessive and homozygous dominant bred, then what ratio do you expect? Now we look at the progeny, Jose and Tonya (one generation was skipped so F2). Jose and Tonya have the dominant characteristics (e.g. brown hair, sturdy bones) as well; the genes could be heterozygous dominant or homozygous dominant. But the child of Jose and Tonya (F3) has one dominant trait (e.g. brown hair) and one recessive trait (e.g. fragile bones) so we know both parents have heterozygous traits.

The goal is to find the parental and recombinant phenotype. In this case, the recombinant phenotype is the fragile bones. For II to be true, then I has to be false. If "All of the children..." had a homozygous recessive trait, then the child would have blond hair as well. And most of the time, statements with "All" or "never" or anything extreme is usually wrong. III is true because of the parental gametes of Jose and Tanya. You'll get a 1/4th chance of a child with both homozygous traits.

 

[MedPR]

.

 

[milski]

What if the recombination happened for Tanya or Jose? So they had genotypes like this:

T: SsBB
J: SsBb

Would not that mean that they will never have blonde children, making III false?

 

[MedPR]

Since both parents have dominant traits, they cannot be homozygous recessive for both traits.

If both parents have dominant traits, but have offspring with a recessive trait, you know that both parents must be heterozygous for that trait.. bone strength in this case.

So Jose is either BbSs or BBSs

Tanya is also either BbSs or BBSs.

A recombinant phenotype is any phenotype in offspring that is not represented in the parents. Since both parents have strong bones and dark hair, a child with blond hair and/or fragile bones represents a recombinant phenotype. So I is true.

II is false because Jose and Tanya don't have fragile bones, and thus are not homozygous recessive. The only way all offspring can have a particular trait is if both parents are homozygous for that trait. In other words, all of their children COULD have fragile bones, but "must have..." is the reason why this answer is wrong.

For answer III, you need to look at the information about the grandparents. Since one of the grandparents was homozygous bb (blond hair), you know that Jose and Tanya cannot be homozygous BB (dark hair). Since they are heterozygous, they can produce homozygous recessive children.

 

[MedPR]

Let's start with the grandparents. Each of them had one which was blond/fragile, which means genotype of ssbb. All other 3 grandparents are SSBB.

Tanya's grandparents:
SSBB SSBB SSBB ssbb
Jose's grandparents:
SSBB SSBB SSBB ssbb

When SSBB and SSBB mate, the kids are SSBB. When SSBB and ssbb mate, the kids are SsBb. That means that both Tanya and Jose have one SSBB and one SsBb parents.
Tanya's parents:
SSBB SsBb
Jose's parents:
SSBB SsBb

That means that both Tanya and Jose can be either SSBB or SsBb. Remember that S and B are on the same chromosome and close to each other, which makes SSBb or SsBB very very unlikely.

We know that one of their children has fragile bones and is brunette, which means that it is ssBb or ssBB. To get the ss, both Tanya and Jose have to be SsBb.

I is true since recombination is the only way that you can get an ssBb genotype.
II is not true Tanya and Jose can easily have SxBx kids who will not have fragile bones.
III is also true because Tanya and Jose can have ssbb kids who ill have fragile bones and blonde hair.

Can you explain that? Why can't the parents be BBSs?

 

[milski]

Can you explain that? Why can't the parents be BBSs?

Since B and S are linked, the kids get them as pairs from the parents, unless crossing over occurs. From a SB|SB and SB|sb parent you can get only SB|SB or another SB|sb.

During normal myosis you will have the BS|bs first replicated as BS|BS|bs|bs and then split to four cells as BS, BS, bs and bs. Only in rare cases (the closer the genes are, the rarer) you will get some gene crossing over between the BS and bs chromosomes and you'll end up with bS, Bs, BS, bs.

So the parents can be BBSs but that's very unlikely. It's actually as likely as having a recombination happening for the kid and being Bbss.

What bothers me about the answer is that if one of the parents was BBSs, they would not have blonde kids. Crossing over can occur only in heterozygous parents.

 

[MedPR]

Since B and S are linked, the kids get them as pairs from the parents, unless crossing over occurs. From a SB|SB and SB|sb parent you can get only SB|SB or another SB|sb.

During normal myosis you will have the BS|bs first replicated as BS|BS|bs|bs and then split to four cells as BS, BS, bs and bs. Only in rare cases (the closer the genes are, the rarer) you will get some gene crossing over between the BS and bs chromosomes and you'll end up with bS, Bs, BS, bs.

So the parents can be BBSs but that's very unlikely. It's actually as likely as having a recombination happening for the kid and being Bbss.

What bothers me about the answer is that if one of the parents was BBSs, they would not have blonde kids. Crossing over can occur only in heterozygous parents.

I think that's why answer III says "may have..." instead of "will have...". We can't say for sure if Jose and Tanya are BBSs or BbSs. They could both be BbSs though, and since that is a possibility, blonde kids is also a possibility.

 

[milski]

I think that's why answer III says "may have..." instead of "will have...". We can't say for sure if Jose and Tanya are BBSs or BbSs. They could both be BbSs though, and since that is a possibility, blonde kids is also a possibility.

Yes, that makes sense. You're right that we have to consider all the possibilities.

 

[typicalindian]

wow...lots to read though thanks for all the help guys! I havn't gotten to read any of these posts yet but I'm sure you guys did a great job explaining. thanks again!!