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What would the doctoral student say is his rationale for including the chicken DNA in his homology analysis?

The Coughing Frog

Use the passage below, as well as the original reading, to answer this question.

Scientists use a number of pieces of evidence to determine if an organism should be classified as its own species. These pieces of evidence almost always include making comparisons between the new organism and other species who are similar to the new organism. One such comparison can be made by looking at DNA sequences between organisms, scientists can not only determine species origin, but they can also determine how closely related species are and gather knowledge about an organisms evolutionary history.

The doctoral student has drawn blood samples from each of five organisms: The coughing frog, a Northern Leopard Frog, a Southern Leopard Frog, the American Bullfrog, and a chicken. Using the latest in automated techniques, he sequenced DNA from an area in the first chromosome from each organism. Using a sophisticated database, he was able to compare the sequences of each of the species to the others. The goal of the comparison was to show the per cent homology (likeness) between the sequences. The results of the comparison are shown in the chart below:

Per Cent Homology between DNA Sequences Taken From Chromosome #1 Blood Samples

Organism Coughing Frog NOrthern Leopard Frog Southern Leopard Frog Bullfrog Chicken
Coughing Frog 100 92 88 70 14
Northern Leopard Frog 92 100 93 74 16
Southern Leopard Frog 88 93 100 72 18
Bullfrog 70 74 72 100 21
Chicken 14 16 18 21 100

The chicken acts as a basis of comparison between Northern Leopard Frog and Southern Leopard frog DNA.


The chicken DNA acts as a basis of comparison between coughing frog and bullfrog DNA.


The chicken DNA acts as a positive control, showing how closely related it is to other frogs.


The chicken DNA acts as a negative control allowing a more clear picture of the homology between the frog species.

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