2. You decide to further study the ZmERT locus in maize by comparing 4 alleles (

Question

2. You decide to further study the ZmERT locus in maize by comparing 4 alleles (A through D. Sp 1 below) from the maize population to I allele from a different Zea species (called Sp 2 below), using a McDonald-Kreitman test for polymorphism vs. divergence. You observe the following pattern for part of the locus (nucleotides grouped into codons). Note that these sequences are the same as the mRNA codons except that T is substituted for U. This is the conventional way of writing coding gene sequences. 2a. Translate each strand. 2b. Translate the sequences above (write the amino acid sequence on the line below using amino acid abbreviations (see your text book, or find the genetic code online) and fill out the following table based on these data. Divergence refers to fixed differences (all Species 1 sequences differ from the Species 2 sequence). 4 points. 2c. Assuming that this pattern holds for the rest of the gene, what conclusion from the perspective of evolution by natural (artificial in this case) selection do these numbers support and why? (please give a qualitative answer). 2 points.

Explanation / Answer

2a.

Species

mRNA sequence

Amino acid sequence

Sp 1- A

Atg acc cat gat tta tca tgg gta cgt cga gat gta

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

Sp 1- B

Atg acc cac gat tta tca tgg gtg cgt cga gat gta

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

Sp 1- C

Atg acc cat gat tta tct tgg gtg cgt cga gat gta

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

Sp 1- D

Atg acc cat gat tta tca tgg gtg cgt cga gat gtt

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

2b.

Polymorphism

Divergence

Nonsyn

12

1

Syn

0

35

2c. Polymorphism is not found in proteins encoded by all the above species because the aminoacid sequences of all the proteins are identical due to degeneracy of genetic code.

Divergences of only one nucleotide in the coding sequences (highlighted in red) are found among all these species. But the amino acid coded by them is same.

Species

mRNA sequence

Amino acid sequence

Sp 1- A

Atg acc cat gat tta tca tgg gta cgt cga gat gta

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

Sp 1- B

Atg acc cac gat tta tca tgg gtg cgt cga gat gta

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

Sp 1- C

Atg acc cat gat tta tct tgg gtg cgt cga gat gta

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

Sp 1- D

Atg acc cat gat tta tca tgg gtg cgt cga gat gtt

Met-Thr-His-Asp-Leu-Ser-Trp-Val-Arg-Arg-Asp-Val

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