Session 221

Discretes can be easy points, but be sure to mentally let previous passages go. We tackle another discrete section on genetics and the respiratory system.

We’re joined by Alex from Blueprint MCAT. If you would like to follow along on YouTube, go to premed.tv.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[02:04] Mindset When Going into the Discretes

Alex says students often do really well and much better on discretes, on average, than they do on passages because they draw more purely on content knowledge rather than interpretation.

[03:20] Question 44

A researcher analyzes a nucleic acid sample. After looking only at its nucleotide composition, he decisively concludes that this sample represents single-stranded DNA and not another form of nucleic acid. Which of the following compositions most likely represents this sample?

1. 17% A, 17% T, 33% G, 33% C
2. 29% A, 14% U, 11% G, 46% C
3. 4% A, 4% U, 46% G, 46% C
4. 12% A, 12% T, 30% G, 46% C

Thought Process:

A – They’re connected to each other. There are equal amounts. It’s unlikely to be double-stranded. And so, we cross it out.

B & C – Now, the concept to bring in here is base pair composition. DNA consists of four base pairs, A-T, C-G. But U which refers to uracil is only present in RNA. So in this case, we can eliminate both B and C based on the fact that they have U in them. Because DNA doesn’t contain U.

So then that leaves that leaves us with D as the only option left. There’s 12% A and 12%. T. Those proportions must always be equal in a double-stranded DNA. And the fact they are not means that this must be single-stranded.

Correct Answer: D

[08:19] Question 45

In a population of Amish people, the frequency of the recessive autosomal allele for polydactyly is 1.2%. What percent of the population are heterozygotes for the polydactyly allele?

1. 0.0144%
2. 1.19%
3. 2.37%
4. 97.6%

Thought Process:

A & D are both extreme high and low while answer choices B and C are somewhat similar in terms of their excessiveness. There’s something about B and C that make them seem approximately right and something about A and D that’s off in either direction.

Rephrasing the question: Heterozygous means you have at least one of them. These are people who carry one copy of the recessive allele and one copy of the dominant allele.

The equation that we’re looking for that describes this relationship between recessive and dominant allele frequencies and percentages of the population that have various gene types is the Hardy-Weinberg equation:

p² + 2pq + q² = 1

Where:

p² = dominant homozygous frequency (AA)

2pq = heterozygous frequency (Aa)

q² = recessive homozygous frequency (aa)

Looking at 2pq, it’s the proportion that is heterozygous. The frequency of the alleles is 1.2%. That means the people who have the dominant allele is 98.8%

Then we can assume that people who are heterozygous, presumably there are more of them then there are people who have two copies of the recessive gene.

1.19% is almost exactly the same as 1.2%. So we pick C, which is 2.37%. This is how we can solve it with estimation where we have an intuitive understanding of how the proportions work. This is a great way to save a lot of time if you understand the concepts intuitively and conceptually.

Imagine a big pool of genes. Some percentage of them will be the dominant version, and some will be the recessive version. And if we look in the population, that’s what the equation describes. This isn’t in reference to people’s genotypes, it’s in reference to the proportion of genes in the population as a whole. So of all alleles in a population where everyone has two, in this case, 1.2% of them are the recessive version.

Using the equation, multiply the frequency of the recessive allele by itself. And that makes intuitive sense, because you would expect that, if 1.2% of all genes in this population are other recessive genes, well, then presumably, an even smaller percentage of those people will be lucky enough to get two of them.

Correct Answer: C

[17:08] Question 46

Those species that are capable of both sexual and asexual reproduction will typically prefer sexual reproduction because it:

1. increases the likelihood of each individual offspring surviving.
2. increases the likelihood of beneficial mutations.
3. creates more variation in the next generation.
4. takes less time to complete.

Thought Process:

From an evolutionary standpoint, we as a species tend to prefer more variation in our offspring. So we’re more likely to pick C here.

Sexual reproduction is a valuable mechanism by which you could combine the genes of two parents and use that to create more variation in the next generation. This variation is incredibly helpful because it creates a more interesting gene spread in the population. This allows them to better adapt to new and interesting situations.

A – In a lot of cases, that’s not necessarily true. Certainly, sexual reproduction is good for the whole species because it can create an improved variation across the entire species. But often, sexual reproduction leads to unfavorable genes being expressed in the next generation.

B – That’s not how sexual reproduction works. That’s separate from the process of mutations, which is species-specific, but is not inherently linked to the concept of sexual reproduction.

D –  This is definitely not true.

Correct Answer: C

[20:18] Question 47

Decreased number of alveoli in the lungs leads to respiratory distress because:

1. damage to the respiratory epithelium reduces the ability of the epithelial cells to actively transport O2 into the body and CO2 out of the body.
2. reduced surface area in the lungs reduces the rate at which O2 and CO2 can diffuse through the lung epithelium.
3. passive expiration depends on the inherent elasticity in the walls of the alveoli and reduced expiratory volume prevents or reduces subsequent inspiration.
4. the cilia lining the lungs which move mucus and other debris out of the lungs and keep the respiratory epithelium clear cannot function as well.

Thought Process:

A – We can eliminate A immediately since the question is not talking about damage here. We’re just talking about decreased numbers. Epithelial cells don’t actively transport O2 into the body and CO2 out of the body since the process is entirely passive.

D – this happens to be a true statement in the sense that if the cilia are damaged, then they can’t move debris out of the lungs. But again, that’s not what the question stem is about. So we can eliminate this because we’re not talking about cilia. We’re talking about alveoli.

So we’re left with B and C.

C – passive expiration happens because of elastic recoil. But it’s not the elasticity in the walls of the alveoli. The walls of the alveoli consist of just a single layer of epithelium. It’s very thin, and therefore, has almost no elastic recoil.

B – this is correct because you have less exchanging space. It reduces the rate at which O2 and CO2 can diffuse through the lung epithelium because they do diffuse passively. They’re not actively transported.

Correct Answer: B

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