AAMC 5 Bio Question 112

[regeneration]

Hi,

In the passage this is given:

The ciliate protozoan Tetrahymena contains two nuclei: a diploid, germ-line micronucleus and a 45-ploid macronucleus that is the site of gene expression during the vegetative state. Sexual reproduction in Tetrahymena occurs by the process of conjugation.

In question 112, it is given that the micronucleus is heterozygous for a gene, and the macronucleus is homozygous, and the question asks what will the F1 generation be. The answer is that we ignore the macronucleus, and give the F1 a 1:2:1 genotypic distribution. Apparently, this is because only the micronucleus undergoes sexual reproduction (which is shown in a diagram). I got confused here because the passage said that the macronucleus is the site of gene expression, so I thought only the genes in the macronucleus would be expressed (and thus we'd have a homozygous F1). Why is this incorrect, and what does the "gene expression during the vegetative state" mean?

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[Jepstein30]

Hi,

In the passage this is given:

The ciliate protozoan Tetrahymena contains two nuclei: a diploid, germ-line micronucleus and a 45-ploid macronucleus that is the site of gene expression during the vegetative state. Sexual reproduction in Tetrahymena occurs by the process of conjugation.

In question 112, it is given that the micronucleus is heterozygous for a gene, and the macronucleus is homozygous, and the question asks what will the F1 generation be. The answer is that we ignore the macronucleus, and give the F1 a 1:2:1 genotypic distribution. Apparently, this is because only the micronucleus undergoes sexual reproduction (which is shown in a diagram). I got confused here because the passage said that the macronucleus is the site of gene expression, so I thought only the genes in the macronucleus would be expressed (and thus we'd have a homozygous F1). Why is this incorrect, and what does the "gene expression during the vegetative state" mean?

The micronucleus is the germline, while the macronucleus is used for gene expression. Basically, for daily life, the ciliate uses the macronucleus genome to carry out various functions. This macronucleus isn't passed on to future generations though, only the germline micronucleus is. None of the genes in the micronucleus are ever expressed, they are only there to be inherited by offspring (which then use the micronucleus to create a macronucleus which they use for gene expression).

So if the micronucleus is heterozygote and ONLY the micronucleus is inherited by the F1 generation, then you will get the typical 1:2:1 distribution.

Gene expression: 'daily life' functions
Germline: what's passed on

 

[regeneration]

The micronucleus is the germline, while the macronucleus is used for gene expression. Basically, for daily life, the ciliate uses the macronucleus genome to carry out various functions. This macronucleus isn't passed on to future generations though, only the germline micronucleus is. None of the genes in the micronucleus are ever expressed, they are only there to be inherited by offspring (which then use the micronucleus to create a macronucleus which they use for gene expression).

So if the micronucleus is heterozygote and ONLY the micronucleus is inherited by the F1 generation, then you will get the typical 1:2:1 distribution.

Gene expression: 'daily life' functions
Germline: what's passed on

Thanks! Didn't realize the old macronuclei were being destroyed