DAT Question of the Day

Correct Answer: C. a homozygous red haired cow

In genetics, a test cross is used to determine the genotype of an organism that expresses the dominant phenotype. The black cow could be BB or Bb for the black hair color. If you were to mate the cow with a homozygous black cow BB, all of the offspring would be black haired and this would not help the farmer determine the genotype of the original black cow. Likewise using a heterozygous black cow (Bb), could result in all black offspring, since there is only a 25% chance of getting a red haired cow if the original cow was heterozygous (Bb). A white cow nor an albino cow are not mentioned in the original question and would not be relevant in determining the genotype of the black cow. A test cross, uses a homozygous recessive individual, since this genotype is easily physically identified. If the 50% of the offspring are black and 50% are red, then the black cow was heterozygous (Bb). If all black offspring result, then the black cow was homozygous dominant (BB).

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Correct Answer: C. a nonsense mutation

A silent mutation often occurs in the wobble site of the codon on the mRNA strand. For most codons, if the third base pair is changed, the correct amino acid can still be made. A missense mutation results in the addition of an incorrect amino acid, but can still lead to a functional protein. Nonsense mutations result in a polypeptide chain that is shorter and is non-functional. For nonsense mutations, an insertion or deletion of a nucleotide base creates a premature stop codon (UGA, UAG, or UAA), terminating translation before the polypeptide can be completed. Transfer mutation is not an accurate term in biology.

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Correct Answer: A. Two squirrel populations separated by a river

In allopatric speciation, two species result due to the geographical barrier between the two species. Two birds living in different parts of the tree may result in two separate species over time, if the birds have different mating rituals or mating seasons. This would not be due to a geographical barrier and would be an example of sympatric speciation. Lion prides could still reproduce with one another especially since they inhabit the same area and climate. Insects with the same mating call would interbreed and this would not result in two different species over time. Similarly, two groups of dogs with different fur color should still be able to reproduce. However, two squirrel populations separated by a river may evolve into two different species over time, as the squirrels may be inhabiting different environments and are separated by the body of water.

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