Question about using Mendelian inheritance to reverse-guess the parent genotype

Question

I would like some help with a question about Mendelian inheritance, based on the following information (I assume this is autosomal and not sex linked):

-- Start of information --

"As flatfish (Pleuronectiformes) develop, they lose their bilateral symmetry. One of their eyes migrates across the top of the head, so that both eyes end up adjacent on the same side of the head. As a result, the adult fish is able to lie flat on its side on the sea floor with both eyes facing up. In some species of flatfish the right eye migrates to the left side of the head (they are left-eyed); in other species the left eye migrates to the right side of the head (they are right-eyed).
In one species of flatfish, the starry flounder (Platichthys stellatus), different populations have different proportions of left-eyed and right-eyed individuals. Off the west coast of the USA the population is evenly divided between left-eyed and right-eyed individuals. However, in the north Pacific, midway between the USA and Japan, 70% of the population is left-eyed and in Japanese waters all the starry flounder are left-eyed.

Consider the following hypotheses (I – III) about the starry flounder. The hypotheses assume that a single gene locus determines the final location of the eyes on the head.

I. The Japanese fish are homozygous for a dominant left-eyed allele (LL), whereas west coast fish are homozygous for a neutral allele (ll) and have an equal chance of developing into either left- or right-eyed individuals.

II. The Japanese fish are homozygous for the left-eyed allele (LL), whereas in the west coast fish both left-eyed (L) and right-eyed (R) alleles occur in equal proportions. Heterozygous individuals (LR) have an equal chance of developing into either the left- or right-eyed form.

III. There are three alleles of the controlling gene: the left-eyed allele (L) is dominant to both the right-eyed allele (l) and the neutral allele (l’). Further, the right-eyed allele is dominant to the neutral allele. Fish that are homozygous neutral (l’l’) have an equal chance of developing into right- or left-eyed individuals."

-- End of information --

Question:
The question says: When a left-eyed mid-Pacific female and a right-eyed west coast male were crossed and the offspring were all kept in standard laboratory conditions on the west coast of the USA, 50% of the thousands of offspring were right-eyed and 50% were left-eyed. Of the following this result completely rules out:
A hypothesis I.
B hypothesis II.
C hypothesis III.
D none of the hypotheses

The 3 hypotheses only talk about Japanese fish and west coast fish, there is no mention of mid Pacific fish, and, if it is talking about the North Pacific fish, the info state that "However, in the north Pacific, midway between the USA and Japan, 70% of the population is left-eyed and in Japanese waters all the starry flounder are left eyed."

From this we infer that:

1. the 70% part refer to the non-Japanese fish due to the word 'and'
2. the non-Japanese fish must be the north Pacific fish since there are no other fish mentioned.

So:

1. What are mid Pacific fish? And what are their genotypes/alleles?
2. Some fish have equal chances of being left or right eyed, so that specific genotype represents 12.5% and 12.5% where each square of a Punnett square is 25%. So you add 25% + 25% and + or - 12.5% or 25%. Therefore the closest thing to 70% is 50+12.5 = 62.5% or 50+25% = 75%, either under or over, when trying to determine how to get the mid Pacific fish.

So I can't get the 70% and still don't know what mid Pacific fish are.

I know how to do Punnett squares. But my issue is the wording of this question. It says "left-eyed mid-Pacific". None of the genotypes mentioned in any of the hypotheses mention Mid-Pacific. Unless it means midway between Japan and USA, which the question info state is NORTH Pacific.

So my question is, do we need to and if so how, to reverse-guess the female North Pacific fish' genotype. Or if we don't, how to do this question.

My reasoning is, the only way Hyp 1 can create a right eyed fish, as opposed to Hyp 2 (which say R is right eyed) is if I take each hyp 1 as it is (ie, the available alleles are LL for homozygous dominant left and ll for homozygous neutral where there is a 50:50 chance of being either left or right), without trying to figure out what a Mid Pacific fish is (I believe if a Mid Pacific fish is a cross between a West Coast fish and a North Pacific fish, then we need to look at all the possible combinations of this, in which the result is 70 left eyed. There are 2 problems with this, one this is a practise exam question where you have 1.5 minutes per question, and 2, there is no way to get 70 from adding or subtracting 12.5, 25, and 50.) then, Hyp 1 says LL and ll. A cross between that is 100% Ll, which is 100% Left, ruling it out instantly since we want 50% left and 50% right.

However, the answer was D, no hypothesis is ruled out.

This is a Punnett square I did for each combination listed in the 3 hypotheses, based on a West Cost male and a Japanese fish.

(So, in total we have 4 fish types, the West Cost male and a Japanese fish mentioned in the hypotheses, the North Pacific fish mentioned in the information and the Mid Pacific asked for in the question.)

Can someone help explain this?

Thank you.

Answer 1

First of all, let's distinguish between the three populations. The Japanese fishes are all left-eyed, midway between the US and Japan the population is 70% left-eyed, and in US (or the west coast) the population is 50% left-eyed.

Let's take a look at the US population. I want to emphasize on the point that this trait is genetically determined. That is, it does not have environment factors influencing the phenotype. Next, let's look at this logically. In order for the entire population to be 50% left-eyed, the trait needs to be 50%. That is, every fish needs to have (roughly) equal chances of producing an offspring with left eyes as well as an offspring with right eyes. This does not strike me as following the dominant-recessive pattern, since that will ensure 75%-25% phenotypes. On the contrary, I'm thinking off a heterozygous genotype which will result in a random phenotype. However, we don't have proof for this yet. All we know is that the population is 50% left-eyed.

Moving on, the Japanese population needs to all be left-eyed, so any newborn fish will be left-eyed (or so we can assume, for simplicity's sake). The genotype of this population ensures left-eyed fish.

The population midway between the US and Japan is trickier. Since they are 70% left-eyed, the eye position is not controlled by chance. However, if they were like the US population, then they would all be left-eyed. Hence, we can safely assume that this population is a mix between the US population and the Japanese population. This assumption is not only based on the location (midway between both the populations), but also based on the information that they give us. Since a single gene determines the location, you cannot have a population with 70% having one phenotype. You rightly mention this when you say:

So I can't get the 70% and still don't know what mid Pacific fish are.

This is because only genetic factors determine the phenotype (given or assumed from the question). Hence, I'm thinking of an approximately 50-50 mix between the Japanese population.

50 Japanese fish + 50 US fish = 100 fish
Since 50% of the US fish are left-eyed - 25% of the total fish are left-eyed (50% of 50%)
Since all the Japanese fish are left-eyed - 50% of the total fish are left-eyed (100% of 50%)
Adding this gives us 25 + 50 = 75% of the 100 fishes are left-eyed.
This is very close to the 70% that we have been given.

Now let's look at each hypothesis in turn.

Hypothesis I

If the Japanese fish are homozygous dominant LL, that means that they will all be left-eyed. On the other hand, if the US population is neutral ll, then they have an equal chance of being left-eyed or right-eyed. Since this is what we have observed, this hypothesis could be true.

Hypothesis II

This hypothesis states that LL individuals are left-eyed, RR individuals are right-eyed, and LR individuals have an equal chance of being left or right eyed. The Japanese fish are LL, so all of them are left-eyed. The west coast fish are LR, and hence the population is 50% left-eyed. Again, this agrees with our observations, so this hypothesis can be true.

Hypothesis III

Since the L gene is dominant to both l and l', fishes with at least one copy of the L gene must be left-eyed. Hence the Japanese fish must be LL, Ll or Ll'. However, since all newborn fishes are left-eyed, the population most likely carries the LL gene. Moving on, the US population must have the l'l' genotype since they are 50% left-eyed. If they were Ll' or Ll, they will be almost 75% left-eyed (or even more because the l' gene gives a random phenotype). On the other hand, if they were ll' then they will be 75% or more right-eyed. However, the l'l' genotype works without a problem. So this hypothesis is plausible too.

Notice that I said that these hypotheses could be true. We don't know for sure that they are true, since as you noted, the hypotheses do not talk about the midway population. There are also multiple possibilities, and we don't know for sure which one is correct. However, none of these are incorrect based on the information given. Since the question asks for the false hypothesis, the answer is None of the above. Hope I made this clear!

Answer 2

Re: 1: "mid Pacific" is just a short way of referring to "in the north Pacific, midway between the USA and Japan". Re: "what are their genotypes/alleles" Yes, that is what they want you to consider. For each of the 3 stated hypotheses, you should be able to come up with possible genotypes and rough genotype frequencies for the mid Pacific fish of different eye phenotypes.

Re: 2: Consider the genotype frequencies of the mid Pacific parental fish under each of the 3 hypotheses. A Punnett square only represents 2 specific parental genotypes. If multiple parental genotypes are present in a population, you will have to make a Punnett square for each different combination of 2 parental genotypes, and adjust overall resulting offspring genotype frequencies according to the proportion of that combination.

From comment: "Ultimately my question is how do you determine the genotype of that "70% left eyed J-WC mid-Pacific hybrid"?" Since the 70%... is a population, there may be more than one genotype among them.