Question

Three copies of chromosome 21 in a child with Downs syndrome have been analyzed using molecular biology techniques to detect any possible DNA polymorphism with reference to different alleles located on chromosome - 21. Results showed that out of three copies two of the chromosomes of the child contain the same alleles as one of the mother's alleles. Based on this when did the non-disjunction event most likely occur?
(A) Maternal meiosis - I
(B) Maternal meiosis - II
(C) Paternal meiosis - I
(D) Paternal meiosis - II

Answer 1

The answer to this question is itself hidden in the question. The parent which contributes to this disorder is that which has the majority of alleles similar to the offspring and takes place in that meiosis in which crossing over takes place.

Complete answer:
The non-disjunction event most likely occurred at Maternal meiosis – I, as the child contains two of the chromosomes i.e majority as one of the mother's alleles and takes place in meiosis – I phase of the cell cycle.
So, the correct answer is ‘Maternal meiosis - I’.

Additional Information: Down syndrome or trisomy 21, is a genetic disease caused by the presence of all or a part of the 3rd copy of chromosome 21. It is usually related to physical growth delays, mild to moderate intellectual disability, and characteristic features of the face.. the common IQ of a young adult with trisomy 21 is 50, as the capacity of an 8- or 9-year-old child, but this will vary widely.
In many cases, it was seen that the parents of the affected individual are usually genetically normal. The probability increases from but $0.1\%$ in 20-year-old mothers to three in those aged 45. The extra chromosome is believed to occur accidentally, with no known behavioral activity or environmental factor that changes the probability. Trisomy 21 is often identified during pregnancy by prenatal screening followed by diagnostic testing or after birth by direct observation and genetic testing. After the screening, pregnancies with the diagnosis are often terminated via abortion.

Note: The individual with down syndrome has chances of suffering from many diseases like The rate of congenital heart disease in newborns with Down syndrome is around 40%, Hearing and vision disorders occur in more than half of people with Down syndrome, and many more.