Biology Test Preparation Practice

    Ls1.B: Growth And Development Of Organisms

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    Base your answers to questions 13 on the passage below and on your knowledge of biology. Biologists have been studying the genes present in newborn twins.

    Twins Don’t Share Everything

    …Chemicals called epigenetic markers can be attached to those [inherited] genes, like flags or balloons hanging off the sides of the DNA ladder. These don’t just change the look of the genes. Like pieces of tape stuck over a light switch, these markers can force a gene to remain turned on or off. The type of marker scientists studied in the twins generally sticks the switch in the off position so that some proteins don’t get made. And that means the proteins’ jobs won’t get done.

    Every time a cell divides, new epigenetic markers may form. Foods, pollutants, and stress may all contribute to the development of new markers. So throughout our lives we tend to accumulate more and more. But a few are there from the day we’re born.

    …His [Jeffrey Craig’s] team found that newborn twins have markers attached to different genes from the very start. It’s true in identical twins, which come from the same fertilized egg. It’s also true in fraternal twins, which come from different fertilized eggs. However, fraternal twins had more such differences than identical twins did.

    Source: Science News for Students; July 31, 2012

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    Base your answers to questions 18 on the information below and on your knowledge of biology.

    The testes of a human male produce gametes. The process that produces these gametes differs from the process that produces new skin cells in the same individual.

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