The haploid chromosome number for humans is ____; individuals with Downs' syndrome have ____ somatic chromosomes.   A) 47; 23. B) 23; 47. C) 32; 47. D) 64; 65. E) 92; 45

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For this question, choice B is correct. The haploid number (n)of chromosomes is the result of meiosis--a reduction division. It reduces the amount of chromosomes found in an organism to half the amount, which is found in a gamete--sperm or egg cell. For a human, the diploid(2n) amount of chromosomes in body cells is 46 and therefore, the haploid amount is 23. A person with Down Syndrome has an additional chromosome--they are actually 2n+1. They have an additional chromosome at position 21 and this is known as trisomy 21, or Down Syndrome.  There is a link to the age of the mother and the incidence of Down Syndrome. In older mothers after the age of 35, the number of cases goes up at a rapid rate due to nondisjunction within the egg cell. However, other reasons such as genetic mosaicism and translocation can cause Down Syndrome.

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