In humans, females inherit an X chromosome from each parent; males inherit their X chromosome from their mother and their Y chromosome from their father. The same is true of all other placental mammals: males produce X and Y gametes, and females produce only X gametes. In this system, referred to as the XX-XY system, maleness is determined by sperm cells that carry the Y chromosome. Yet the XX-XY sex determination system does not occur in all animal species. It is only one of a variety of systems within the animal kingdom. Above are Punnett squares illustrating three examples of gender inheritance patterns: humans, insects and birds.

Arational expression has been simplified below. (x-5) (x+1)/3(x+1) = x-5/3 for what values of x are the two expressions equal

Over the past 60 years, many amphibian species have experienced significant population declines and some species have become extinct. scientists suspected that local human activities such as the destruction of wetlands, regional pollution, and deforestation were the main reasons for these losses. however, research over the past 20 years reveals significant amphibian population declines in protected areas of the world, such as nature preserves and parks. these global declines suggest widespread problems including increased ultraviolet radiation, acid rain, and disease. in switzerland, for example, 14 of the 20 native amphibian species are threatened with extinction. chytridiomycosis is a fungal disease first identified in 1998 as a cause of massive amphibian deaths. in some severely impacted populations, a few individuals have survived, perhaps because of some natural resistance. if these resistant individuals continue to survive and prosper, new resistant populations might emerge. this would be an example of the founder effect artificial selection genetic drift natural selection sexual selection

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