Lois and Peter, who are each NOT trisomic for chromosome 21, are awaiting the birth of a 2nd child (Chris). Given their age, the probability of Down’s syndrome is 1/20. Down’s syndrome is seen individuals with 3 copies of chromosome 21. Peter and Lois want to know if their 2nd child has trisomy-21. Prenatal genetic screening was conducted by genotyping Lois, Peter, and the unborn child (Chris) with a RFLP marker for chromosome 21. Genotyping was also conducted on their first born (Meg): The results of the RFLP are shown below:     Are the test results indicative of a trisomy-21 in Chris? If no, explain your reasoning. If yes, explain your reasoning and describe in which parent and which meiosis did the nondisjunction occur.

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Inheritance and Mendelian Genetics

Inheritance means the passing of traits to offspring from parents. These traits could be passed either through asexual reproduction or sexual reproduction. The offspring receives the genetic material from the parents. Mendelian inheritance is a certain biological inheritance that follows the rules and principles laid down by Gregor Mendel in 1865. The initial hypothesis was considered wrong, but other inheritance theories modified the concept of inheritance and thus genetics.