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This question is from a genetics session in my university:

The test cross between a plant with smooth and yellow seeds and a plant with wrinkled and green seeds gives the following results: 140 plants with smooth and green seeds, and 142 plants with wrinkled and yellow seeds. These results are consistent with which case of the following?

  1. Linked genes
  2. Epistatic genes
  3. Genes with intermediate dominance
  4. Genes with complete dominance and independent assortment

In my answer key the correct choice is linked genes.

What I know is that in the case of linked genes, the outcomes are: either all parental (complete linkage) or the majority of offspring have parental traits and we have few recombinants(partial linkage).

But here it seems like all offspring are recombinants?! How can this be? Is there a mistake in the question, in the answer, or in my information?

Any help is appreciated. Thank you.

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There is nothing wrong with the question and it can be explained with not much more than mendelian genetics + linkage.

What do you think are the genotypes of parents?

First hint: If there is more than one phenotype (for the trait) of the offspring at least one of parents must be heterozygote. So the idea of pure lineages goes out of the window here.

Ok, there is at least one heterozygote for each color and smoothness of seeds. What is the genotype of the other parent?

Hint 2: Test cross is a term for cross between recessive homozygote and individual of unknown genome (with dominant phenotype). The test cross reveals genotype of the parent with unknown genome.

Hint 3: 1:1 ratio of phenotypes is typical of cross between a heterozygote and recessive homozygote.

Ok so we have heterozygote and recessive homozygote for both traits. Which traits are dominant and which are recessive?

Hint 4: This question seems to be inspired by original Mendel work. Feel free to read it here to find out which traits are recessive and which are dominant. It is quite a good read, clear in its presentation of evidence and conclusions and I recommend it to everyone.

However, this knowledge is not necessary to answer the question. How are dominant genes organized in parents? Principally we have two options:

(AaBb x aabb) or (Aabb x aaBb)

If we consider organization on chromosomes it expands to basically 3 options:

Option 1:

chr ---A---B---
chr ---a---b---
         X
chr ---a---b---
chr ---a---b---

Option 2:

chr ---A---b---
chr ---a---B---
         X
chr ---a---b---
chr ---a---b---

Option 3:

chr ---A---b---
chr ---a---b---
         X
chr ---a---B---
chr ---a---b---

I leave it to you to figure out which of the tree options leads to result offspring described in the question.

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I aggree with you. I think there is a mistake in the question.

Note that the question does not specify whether the parental lineages are pure lineages (fully homozygotes) or not. In all cases, answer 'B' would not explain the observed pattern for the reason you explained. Such pattern would be possible with some complex epistatic interactions if the parental lineages were pure and with complete linkage.

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