Nondisjunction and genetic evidence
Meiosis I nondisjunction sends both homologs into one daughter cell; if meiosis II is otherwise normal, all four products are abnormal for that chromosome. Nondisjunction in one meiosis II cell can produce two normal and two abnormal products. A chi-square goodness-of-fit test compares observed counts with expected counts under a proposed model. A large deviation may reject that model, but it does not identify the biological cause by itself. Small pedigrees can be consistent with more than one inheritance model.
For 80 offspring expected in a 3:1 ratio, expected counts are 60 and 20. Chi-square sums (observed − expected)^2 / expected for the two categories.
Failure to reject a hypothesis does not prove it true. Also, an extra copy of one chromosome is not an extra complete chromosome set.