Sickle cell anemia is a homozygous recessive disorder. this means, in order to have sickle cell anemia, you must have two copies of the recessive allele (aa). sickle cell anemia can be a lethal disease if proper treatment is not available. malaria is a disease that kills hundreds of thousands of people every year. more people die from malaria than sickle cell anemia worldwide. in malaria infested countries, those people who are carriers of the sickle cell allele (aa) are provided a resistance to malaria, and therefore, are more likely to survive. based on the information given, choose the best hypothesis.  a) over time, the frequency of the dominant (a) allele would increase in malaria infested populations.  b) in areas that do not have malaria, there would be an increase in the frequency of the sickle cell (a) allele.  c) due to natural selection, the frequency of the heterozygous (aa) genotype would increase over time in malaria infested populations.  d) due to natural selection against the recessive (a) allele in malaria infested areas, there would be a decrease in the number of people who have sickle cell anemia.

Environmental science studies the interactions between the physical, chemical, and biological components of the environment, including their effects on all types of organisms. earth science (also known as geoscience), is an inclusive term for all sciences related to earth (geology, meteorology, oceanogr maphy, etc).  although environmental and earth science cover essentially the same material, environmental science places greater emphasis on the biological realm, while earth science places greater emphasis on the physical realm.    environmental and earth science study the interactions of four major systems or “spheres” (figure 8.6).  the  geosphere  consists of the core, mantle and crust of the earth.the  atmosphere  contains all of the earth’s air and is divided into troposphere, stratosphere, mesosphere, thermosphere and ionosphere.the  hydrosphere  contains all of the solid, liquid and gaseous water on earth, extending from the depths of the sea to the upper reaches of the troposphere where water is found.  ninety-seven percent of the hydrosphere is found in salty oceans, and the remainder is found as vapor or droplets in the atmosphere and as liquid in ground water, lakes, rivers, glaciers and snowfields.  the  biosphere  is the collection of all earth’s life forms, distributed in major life zones known as biomes:   tundra, boreal forest, temperate deciduous forest, temperate grassland, desert, savannah, tropical rainforest, chaparral, freshwater, and marine.although the four systems have their unique identities, there is substantial interaction between them.  environmental scientists study the effects of events in one sphere on the other spheres. for example, a volcanic eruption in the geosphere may cause profound direct and indirect effects on the hydrosphere, atmosphere and biosphere as follows: example 1 (volcano)    on may 18, 1980, mount saint helens, in the state of washington, erupted.  this event altered the surrounding environment, and provided scientists with an opportunity to study the effects of volcanic eruptions on the geosphere, hydrosphere, atmosphere and biosphere.  such studies are vital because volcanic eruptions will continue to occur, and will have increasing impact on humans as people continue to settle lands closer to dormant volcanoes. the following are but a few of the myriad of interactions resulting from a volcanic