In the karyotype of a person with Down syndrome, or trisomy 21, an extra copy of chromosome 21 is apparent. This abnormality affects roughly 1 of every 700 children born and is the most common serious birth defect in the United States
People affected by Down syndrome often have mental retardation and shortened life spans.
Based on this scenario and your understanding of meiosis, what is the most likely explanation for the extra chromosome in people with Down syndrome?
A failure of one pair of homologous chromosomes to separate during meiosis I could have resulted in an extra chromosome 21 being incorporated into a parental gamete. Then the extra chromosome would be inherited along with the normal maternal and paternal copies of chromosome 21.
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