EVOLUTION
SOURCES OF VARIATION IN A POPULATION
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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Genetics only
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Environmental only
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Both
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Neither
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Detailed explanation-1: -Down syndrome (trisomy 21) Down syndrome, a trisomy of chromosome 21, is the most common anomaly of chromosome number in humans. The majority of cases result from nondisjunction during maternal meiosis I.
Detailed explanation-2: -Down syndrome is a genetic disorder caused when abnormal cell division results in an extra full or partial copy of chromosome 21. This extra genetic material causes the developmental changes and physical features of Down syndrome.
Detailed explanation-3: -Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.
Detailed explanation-4: -Answer and Explanation: The result of a nondisjunction of chromosome 21 in a zygote that has 46, XY would be cells with 47 chromosomes. It occurs as a result of an extra chromosome (chromsome 21) that originated in either the egg or the sperm. During fertilization, three instead of two are present.