For a long time scientists havebelieved that mammals cannot survive when they ha

Question

For a long time scientists havebelieved that mammals cannot survive when they have twice as muchDNA as they should have. A rat found in Argentina is challengingthis belief.

Scientists know that somenon-mammals, such as fish and amphibians, can live with four copiesof each chromosome instead of the usual two. These extrachromosomes give the animals extra genes that may help them survivein changing environments. Many plants also have more than two sets.For example, bananas have three of each chromosome and wheat hassix of each chromosome.

In 1990, a biologist discoveredthat the red rat, a member of the rodent family, has 51 pairs ofchromosomes. This seemed to be unusual because other members ofthat rodent family usually have about 26 pairs of chromosomes. Thebiologist decided to use a special stain and estimate the amount ofDNA in the nucleus of the red rat’s cells. He determined thatthe red rat has about twice as much DNA as other types of rats.

The red rat has 51 pairs of chromosomes. It probably has many ofits chromosome segments repeated. In effect, this means that thered rat has the equivalent of four copies of most of its genes,instead of the normal two copies.

Describe how having four of each gene might be beneficialfor the survival of an organism. Remember that many geneticdiseases are recessive characteristics. Include in yourresponse

·        the effects of being heterozygous or homozygous on the survival ofan organism.

·        information about abnormal chromosome numbers beyond whatis stated in the reading.

Explanation / Answer

Introduction: Talk about genetics and heritable diseases inthe case of the status quo: two chromosomes to each gene. Talkabout survival and propagation of diseases and introduce theconcept of four chromosomes to a gene. Write your thesis afteryou've written the paper and decided whether it would be better orworse because you might not know right now.
Topic 1: Talk about how heritable diseases can be recessiveand how having extra DNA could be beneficial. People that might nototherwise survive could live on if they had extra dominant copiesof the necessary DNA. Later link to thesis with a "hook."
Topic 2: Talk about the negative effects of having extra DNA:more mutations, more room for propagation within a populationthrough recombination during sexual segregation and reproduction.Talk about whether this result is more significant than thatpresented in Topic 1 and why. Later connect to thesis with a"hook."
Topic 3: Talk about how one can make a decision about thecosts/benefits of having extra DNA, speaking for an entirepopulation (and how it is impossible to say that some people shouldlive because of recessive diseases and dominant copies while othersdie because of propagated mutations within a population, etc.).Here there is room for rhetoric and expansion on previous themesthrough the eye of philosophy. Link back to thesis loosely whencomplete.
Conclusion: This should be similar but more definitive thanTopic 3; restate points about pluses/minuses of the proposedalteration and bring argument to a head. Finally decide one way orthe other, but acknowledge that the alternative is interesting oroffers some obscure/inferior advantage. Do not leave the essayopen-ended, but point out that evolution selects for the mostviable life forms.
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