Biology Unit 4 Questions Megathread

[simpak]

The question doesn't specify this exactly, so I'm going to assume this is what they mean - they have chosen to show you four sperm cells instead of four 'gametes' or 'eggs' because they want you to assume the gametes are the product of one meiotic event and not several.  This is important!

We know that we have a male who is heterozygous at both loci and that said loci are on separate chromosomes.  So he must have A, a, B and b chromosomes in the dividing cells.  After the chromosomes replicate, the cell will look something like that at the top below.  Let's first assume no crossing over occurs.  The chromosomes can line up in one of two ways, as shown in the picture below...

http://i55.tinypic.com/fdrqqu.jpg

There are two possibilities for one meiotic event.  The combinations on the right side of the red line can be produced, or the combinations on the left side of the line can be produced.  For one meiosis ONLY ONE SET OF COMBINATIONS IS POSSIBLE.  Both combinations would only be possible (if no crossing over) if we had a number of meioses.  So really, independent assortment is correct unless they specify that it's one meiotic event to you in the question, but because they're trying to trick people with the difference between meiosis and mitosis for one of the options they cannot explicitly state this.

Now let's consider what happens when crossing over does occur.  Now we have mixed genetic material between the chromatids, and one meiotic event can give us four combinations (as shown below).

http://i51.tinypic.com/vhq1kh.jpg

So the bottom line is - the sperm cells (if they all resulted from the one meiotic event) would only be produced if crossing over occurred.  If crossing over didn't occur, then you would still have four cells but only two combinations of chromosomes would occur.  So you'd be shown AB x 2 and ab x 2 OR Ab x 2 and aB x 2.  Independent assortment ensures we get equal proportions of gamete combinations across a number of meioses since either manner of lining up has an equal probability.  But the same isn't true for one event - there is a 50/50 chance of either metaphase I arrangement.

[LittleMunckin]

can anyone please link the neap exam for unit 4 2011 ? it would help me a great deal !

[Mr. Study]

This is from the 2010 VCAA Unit 4 Exam.

1. Why is Molly's chance of being heterozygous 66%? I know Molly's parents are Heterozygous but I don't understand why we don't include her sister, who is homozygous recessive? (I included a picture ).
2. Why is the chance of the sperm, containing the normal Chromosome 1 and 2, 25%? (I read the assessors report but I still don't understand).

3. If it says count the number of Autosomes, Would I say there are 22 autosome pairs or 44 autosomes?
4. What is the purpose of the Centromere?
5. Clarification: Diploids and Haploids are the number of chromosomes in a biological cell?
6. For DNA Replication.
    5-prime ------------------------------- 3-prime <--\ My DNA
    3-prime ------------------------------- 5-prime <--/

If that DNA double helix replicates, would it happen like this:

    5-prime--------------------------\
    3-prime-----------------------     \
                                                   \------------3-prime
                                                   /------------5-prime
                                                  /
     3-prime-------------------------/
     5-prime----------------------

Hopefully thats not a confusing diagram. xD

7. Not a question but I was wondering if their was a website that lists the differences of a Eukaryotic and Prokaryotic Cell, including chromosomes.

That is all! Thank you to anyone who can reply to my immense amount of questions!

[HERculina]

^ mavisgibbons, thanks for that indepth explanation!!!!! i understand why it's defs crossing over now x))))

can anyone please link the neap exam for unit 4 2011 ? it would help me a great deal !

sorry, i only have the hardcopy verion of it which i wrote all over .                                                                                           To mr study, centromeres connect the chromosomea to the spindle fibres during cell division

[WhoTookMyUsername]

thanks herc
anyone else able to double confirm and explain when the foramen magnum reached its current location:

In early hominins (Australopithecus) is the foramen magnum already completely centrally located (very close to modern humans? ) .

AKA

is it incorrect to say a more centrally located foramen magnum is a distinguishing feature between homo sapiens and australoptiehcus?

[lexitu]

Yep I agree - definitely more centrally located in homo species. I think you can confirm that with VCAA exam answers when you get up to that too.

[Russ]

This is from the 2010 VCAA Unit 4 Exam.

1. Why is Molly's chance of being heterozygous 66%? I know Molly's parents are Heterozygous but I don't understand why we don't include her sister, who is homozygous recessive? (I included a picture ).

Her parents are both heterozygotes so Molly has 4 possibilities
AA / Aa / aA / aa
We know she cannot be aa, because otherwise she would have the trait. Therefore she has a 2/3 chance of being a heterozygote
Confused by 2, they don't undergo recombination?


WIll get to rest tomorrow, must sleep/write lab report now

[simpak]

^ mavisgibbons, thanks for that indepth explanation!!!!! i understand why it's defs crossing over now x))))

Np, just doing my best not to learn this really boring stuff in my own Biology subject.  Procrastinating with other biology reduces the guilt.

[Reckoner]

Hey guys, just the one question here, what is the difference between co-dominance, incomplete dominance and partial dominance? I know that both alleles are expressed in an individual who is heterozygous, but is one of them a blend (eg red+white=pink) and one both (eg red and white patches)?
Thanks in advance 

[Mr. Study]

This is from the 2010 VCAA Unit 4 Exam.

1. Why is Molly's chance of being heterozygous 66%? I know Molly's parents are Heterozygous but I don't understand why we don't include her sister, who is homozygous recessive? (I included a picture ).
2. Why is the chance of the sperm, containing the normal Chromosome 1 and 2, 25%? (I read the assessors report but I still don't understand).

3. If it says count the number of Autosomes, Would I say there are 22 autosome pairs or 44 autosomes?
4. What is the purpose of the Centromere?
5. Clarification: Diploids and Haploids are the number of chromosomes in a biological cell?
6. For DNA Replication.
    5-prime ------------------------------- 3-prime <--\ My DNA
    3-prime ------------------------------- 5-prime <--/

If that DNA double helix replicates, would it happen like this:

    5-prime--------------------------\
    3-prime-----------------------     \
                                                   \------------3-prime
                                                   /------------5-prime
                                                  /
     3-prime-------------------------/
     5-prime----------------------

Hopefully thats not a confusing diagram. xD

7. Not a question but I was wondering if their was a website that lists the differences of a Eukaryotic and Prokaryotic Cell, including chromosomes.

That is all! Thank you to anyone who can reply to my immense amount of questions!

Adding on from this,

8. Clarification: Reverse Transcriptase converts mRNA to cDNA. True or False? (I read this in the 2011 NEAP Exam but is seems odd).

9. Two different species, who were once one, reproduced, why is the resultant child sterile? (I am 80% sure that is has something to do with gamete production but I am unsure).

Hopefully that is all my questions!

Thank you Russ for the very good explanation on why it's 2/3.

[HERculina]

Hey guys, just the one question here, what is the difference between co-dominance, incomplete dominance and partial dominance? I know that both alleles are expressed in an individual who is heterozygous, but is one of them a blend (eg red+white=pink) and one both (eg red and white patches)?
Thanks in advance 

partial dominance is the same as incomplete dominance which is the blend thing and co-dominance is the one where in a heterozygous organism both alleles are independently and equally expressed in the phenotype (the patches or better example A and B blood groups)

[WhoTookMyUsername]

1) is it possible for recombination between 2 homologous chromosomes in any one meiotic division to occur simulatenously between more than one chromatid? E.g. Chromatids 1 2 |
3 4

Can recombination occur between 1 and 3; 2 and 4 or only between two "middle" chromatids



2) when referring to sources of variation due to sexual reproduction,
Is the term random mating accurate? As due to sexual selection, nat selection, it is not really random... Does the random refer to "theoretically random" or ....


Thanks

[Russ]

Yes it can, but a double is less likely than a single for obvious reasons

[lexitu]

1) It has to be between two chromatids that are next to each other (i.e. the middle ones). But of course, the way the chromosomes (and hence chromatids) orient themselves is random so the middle chromatids will vary.

2) Yeah, i know what you're saying, but you still call it random unless there is an obvious reason why it's not.

[gen.mayu]

Would anyone be able to provide a good definition of the following:

Alternative Splicing

Gene Regulation

Reverse Transcriptase

Non-disjuction

Polyploidy?

Thank-you!