Psyxwar's Biology 3/4 Question Thread

[Yacoubb]

I dunno, doubt they'll ask a question on it + when they same "true breeding organism" in a question they always mean homozygous for that allele.

Mendel crossed pure-breeding (also referred to as true-breeding) smooth-seeded plants with a variety that had always produced wrinkled seeds (60 fertilizations on 15 plants). All resulting seeds were smooth.

^That's another reason I thought my pure breeding definition would have been right. But oh well, I'll clarify it! But yeah, when the 'true breeding organism' is given, that's a homozygous genotype for a given trait.

[psyxwar]

Why is having a centered foramen magnum a sign of bipedalism? Is it due to greater balance?

Similarly, why are having small hip bones a sign of bipedalism?

[psyxwar]

http://imgur.com/WxJ7IpM

VCAA 2010, SA q11. Why is the answer B? How would you work out that is isn't C exactly? No idea how to do this lol

[alondouek]

lol I remember this question, what got me was the difficulty of reading the genotypes. Make sure you bring a couple of different colour highlighters to the exam.

MCQs in bio often want a "best answer", so sometimes more than one answer can be technically correct, but only one will be accepted as the right answer.

The question asks, "what is a reasonable assumption". From what you know about genotypes and phenotypic expression of those phenotypes, does the order in which I write alleles make any difference? Nope!

Alleles R₁⁺ and R₂⁺ have the same genetic effect, as do genes R₁^- and R₂^-.

Hence, the genotype R₁⁺R₁⁺R₂^-R₂^- (50% level of red colour) will have the same phenotype as R₁^-R₁^-R₂⁺R₂⁺ in that there are in both genotypes two red-colour-giving alleles.

[psyxwar]

But that's making the assumption both alleles code for the same level of redness, and is that ever the case? It seems rather odd that R₁⁺ can have the exact same effect on redness as R₂⁺.

[alondouek]

If the question doesn't state otherwise, always assume the simplest correct explanation. Plants often show redundant alleles where more than one allele can code for the same trait.

[psyxwar]

Thanks. What would be a good way to summarise the role of the Helper T cell? "activating the other cells cells of the specific immune system" doesn't sound sufficient.

How do I do dihybrid crosses using two monohydrids?

[psyxwar]

Supposedly the graph for Species 1 is representing 3 phenotypes only:

http://imgur.com/T4Jm8sh

wat. How? If it was just 3 phenotypes you should just have three straight lines going up.

[alondouek]

Supposedly the graph for Species 1 is representing 3 phenotypes only:

http://imgur.com/T4Jm8sh

wat. How? If it was just 3 phenotypes you should just have three straight lines going up.

Not quite. Remember that height is a continuous variable, so you have a significant number of minor variations throughout the population.

What the graph is saying is that there are three major height ranges that are favoured, with the intermediate height range being selected for the most.

[psyxwar]

Not quite. Remember that height is a continuous variable, so you have a significant number of minor variations throughout the population.

What the graph is saying is that there are three major height ranges that are favoured, with the intermediate height range being selected for the most.

Hmm... actually I see where they're getting at. Thanks.

Also: Chimpanzees are the closest living relative to modern humans (98% of our genomes are similar).
Explain, in terms of natural selection, how the two species diverged.

My answer was:

• There was variation in the ancestral species that diverged into humans and chimps
• Two populations of ancestral species were separated and exposed to different selection pressures
• The populations diverged over time due to different adaptations being selected for, eventually resulting in the two distinct species we have today (humans and chimps)

The suggested answer was:
Chimpanzees and humans diverged due to natural selection favouring the most fit for their respective
environments. [1] For example, as early humans began to walk through savannah grasses, being upright
became a selective advantage, as they could see across the grasses for potential predators or prey, and
thus those with a more upright stature had more reproductive success, and the allele frequency of alleles
coding for a more upright stance increased in the early human population. Similarly, as chimpanzees
remained predominantly tree dwellers, an upright stance did not infer any selective advantage, so this
was not a characteristic selected for. [1] As these different requirements of their respective environments
caused different selecting pressures and different traits that were deemed ‘fit’, the allele frequencies of
the two groups slowly diverged until speciation occurred, that is, two members of each group could not
reproduce and create viable (able to reproduce) offspring. [1]

I realise my answer was really broad. Is it necessary to provide examples like they did (it was a three mark question and my answer already used up all the lines given) in this kind of question?

[alondouek]

It's probably best to be as specific as possible, so as to show the examiner that you know what you're talking about.

A couple of suggestions:

• Instead of "ancestral species", refer to a "common ancestor" or "common ancestral species".
• If you're talking about isolation, note that it can be both geographical and behavioural.
• Talk about specific phenotypes that were selected for in both species, and why they were selected for.l
• Make note of how these selected advantages increase the fitness of both species in their respective niches.

[Yacoubb]

Thanks. What would be a good way to summarise the role of the Helper T cell? "activating the other cells cells of the specific immune system" doesn't sound sufficient.

How do I do dihybrid crosses using two monohydrids?

Helper T-cells are specialised T-lymphocytes involved in the initiation of an acquired (adaptive) immune response to an infection.

^^ May need a bit of tweeking, but it could work.

[Scooby]

Also mention that individuals at a selective advantage are more likely to pass their alleles on to the next generation

[psyxwar]

Discrete variation is when a population can be organised into a few non overlapping discrete groups. But what if you say there's a flower that is either blue or red, but the intensity of the color is dependent on environment factors. Is this still discrete?

[Scooby]

Discrete variation is when a population can be organised into a few non overlapping discrete groups. But what if you say there's a flower that is either blue or red, but the intensity of the color is dependent on environment factors. Is this still discrete?

I guess it depends how you classified it. If you're classifying the phenotype of the plants according to whether the petals are "red" or "blue", then it's definitely discrete. If you're actually considering all the different shades of those colours, I'd argue that it's continuous