VCE Biology Question Thread

[soNasty]

Huntington's Disease does not affect individuals until they've reached a certain age (40-50), and possibly after they've had a child. It is therefore considered 'late-onset lethal'

[Reus]

Huntington's Disease does not affect individuals until they've reached a certain age (40-50), and possibly after they've had a child. It is therefore considered 'late-onset lethal'

Thanks heaps!

[RazzMeTazz]

When working backwards to find out the genotype of the parents, from the phenotype ratio of the offspring, are we expected to be able to do this in the case of not only monohybrid inheritance but dihybrid inheritance too?

Because for dihybrid inheritance there are just so many different parental genotype crosses.... and so I'm assuming so many different offspring phenotype ratios..

[RazzMeTazz]

Can anyone please give me an example of how the internal environment factors affect phenotype?

Thanks!

[Reus]

Can anyone please give me an example of how the internal environment factors affect phenotype?

Thanks!

As we know the expression of genes in an organism can be influenced by the environment, an example of that are the Hydrangea flowers of the same genotype can range from blue, violet to pink, depending the on soil acidity, however that example would be considered a external factor. In terms of internal environmental factors in an organism which include its hormones and metabolism. One major internal environmental influence that affects gene expression is gender. This is because it is revolved around with sex-influenced and sex-limited traits.

[RazzMeTazz]

What is the definition of pure breeding?
does it refer to a type of organism (homozygous ) or a cross of organisms which are both homozygous?

[grannysmith]

Crossing two homozygous organisms that share identical alleles for a particular gene.

[soNasty]

Can someone tell me exactly how many chromosomes / chromatids there are at each stage of meiosis? This is the only thing that confuses me. Are there ever 92 chromosomes (X-shaped) at one point, that splits into 46+46, that splits unto 23,23,23,23?

[ravi2]

Can someone tell me exactly how many chromosomes / chromatids there are at each stage of meiosis? This is the only thing that confuses me. Are there ever 92 chromosomes (X-shaped) at one point, that splits into 46+46, that splits unto 23,23,23,23?

Assuming you are referring to a human germline cell:
- There are 46 chromosome early interface. At the completion of interface, (after the S phase), there are 92  chromosomes, therefore 184 chromatids.
- At the end of the first cytokinesis, each cell has 46 chromsomes, and therefore 92 chromatids.
- At the end of the second cytokonises, each of the four cells will have 23 chromosome and 46 chromatids.

Therefore, the diploid cell has become a haploid, from 2n=46 ---> n=23

[dankfrank420]

Can someone tell me exactly how many chromosomes / chromatids there are at each stage of meiosis? This is the only thing that confuses me. Are there ever 92 chromosomes (X-shaped) at one point, that splits into 46+46, that splits unto 23,23,23,23?

Here's how I remember it:

1 x H H --> 2 x H --> 4 x I

At the start, there are 23 homologous pairs of chromosomes. So one cell has 46 chromosomes and 92 sister chromatids.

At the end of meiosis I, there are two cells. Each cell still has 46 chromosomes, this is a confusing point for alot of people.

At the end of meiosis II, there are 4 cells. Each cell has 23 chromosomes, so they can fuse to form a diploid zygote.

There are never 92 chromosomes in one cell, you just have to remember that the chromosomes in the original cell and the cell after meiosis I diploid. Even though homologous chromosomes split, each of the offspring still have 46 chromosomes.

[grannysmith]

So in S phase of Interphase, the 46 individual chromosomes replicate so that each has two chromatids joined at the centromere. So here we have 46 chromosomes but 92 chromatids. Still diploid. We never have 92 chromosomes (never 4n).

Meiosis I is the reductive division (diploid to haploid). Homologous chromosomes separate during anaphase I into two daughter cells. Each daughter cell is now haploid (23 chromosomes) with each still composed of two chromatids.

In meiosis II, these chromatids separate into different daughter cells (gametes) that are haploid. It's just that now the chromosomes are only composed (what would be an equivalent) of a single chromatid.

Edit: oh wow beaten twice over

[RazzMeTazz]

When the term 'parental generation' is used does it mean these organisms are always pure-breeding?

And so when the term 'F1 generation' is used, these individuals are always heterozygous?

or does it just mean the parents and their offspring (F1) with no specifications on their genotype?

[RazzMeTazz]

Consider the following pedigree (Image attached below)

What is the chance that Molly is heterozygous for the trait? Show your working out including the genotype of Molly's parents.

For this question I thought the answer would be 1/2
but the VCAA suggested solution is:

Molly's parents are heterozygous as seen from the pedigree. Molly is unaffected; therefore, she has a two in three chance of being heterozygous.

Note: When determining the chance that Molly is unaffected, offspring who show the trait are not counted.

Award 1 mark for stating that Molly's parents are heterozygous.

Award 1 mark for stating the chance (2/3 or 66%) that Molly is heterozygous.

VCAA Assessment Report note:
Students needed to give 2/3 or another suitable way of expressing the chance, for example, 66 per cent. They also needed to indicate that Molly's parents were heterozygous.

Some students gave good examples of chance in the form of ratios. A correct ratio for this question would be 2 (heterozygotes): 1 (homozygote)."


But I didn't understand why it would be 2/3 and not 1/2. Why does it say not to count the offspring who show the trait?

[Reus]

But I didn't understand why it would be 2/3 and not 1/2. Why does it say not to count the offspring who show the trait?

Quite interesting :/ I would like to know how and why too 

[grannysmith]

Because the trait is recessive, their genotype is certain. Therefore we don't count them