Multiple Choice
True or False:Aneuploids are more abnormal that polyploids
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6. Chromosomal Variation
Chromosomal Mutations: Aneuploidy
1:08 minutesProblem 1a
Textbook Question
How do we know that the extra chromosome causing Down syndrome is usually maternal in origin?
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Understand the context: Down syndrome, also known as trisomy 21, is caused by the presence of an extra copy of chromosome 21. This condition is often the result of nondisjunction during meiosis, which leads to an unequal distribution of chromosomes in gametes.
Review the concept of nondisjunction: Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis I or meiosis II. This can occur in either the maternal or paternal gametes, but studies have shown that it is more commonly maternal in origin.
Examine the evidence: Cytogenetic and molecular studies have been conducted to trace the origin of the extra chromosome. By analyzing DNA markers on chromosome 21, researchers can determine whether the extra chromosome came from the mother or the father. These studies have revealed that in approximately 90% of cases, the extra chromosome is maternal in origin.
Consider maternal age as a factor: The risk of nondisjunction increases with maternal age. This is because oocytes are arrested in prophase I of meiosis from fetal development until ovulation, which can lead to errors in chromosome segregation over time. This age-related factor supports the conclusion that the extra chromosome is usually maternal in origin.
Summarize the reasoning: The conclusion that the extra chromosome in Down syndrome is usually maternal in origin is based on molecular evidence (DNA marker analysis) and the observed correlation between maternal age and the incidence of Down syndrome. These findings provide strong support for the maternal origin of the extra chromosome in most cases.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nondisjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This error can lead to an abnormal number of chromosomes in the resulting gametes. In the case of Down syndrome, which is caused by an extra copy of chromosome 21, nondisjunction often occurs during meiosis in the mother, resulting in a gamete with an additional chromosome.
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Allopolyploidy
Maternal Age Factor
Maternal age is a significant risk factor for chromosomal abnormalities, including Down syndrome. As women age, the likelihood of nondisjunction events increases, particularly after the age of 35. This correlation suggests that the extra chromosome in Down syndrome cases is often derived from the maternal side, as older eggs are more prone to errors during division.
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Genetic Studies and Pedigree Analysis
Genetic studies, including pedigree analysis, help trace the inheritance patterns of traits and conditions like Down syndrome. By examining family histories and the occurrence of the condition across generations, researchers can determine the likelihood of maternal origin for the extra chromosome. Such studies have shown that the majority of Down syndrome cases arise from maternal nondisjunction, reinforcing the understanding of its genetic basis.
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