Down syndrome is caused by trisomy 21, the presence of three copies of chromosome 21. the extra copy usually results from nondisjunction during meiosis. in some cases, however, the extra copy results from a translocation of most of chromosome 21 onto chromosome 14. a person who has had such a translocation in his or her gamete-producing cells is a carrier of familial down syndrome. the carrier is normal because he or she still has two copies of all the essential genes on chromosome 21, despite the translocation. however, the same may not be true for the carrier’s offspring.

The carrier’s offspring would suffer Down syndrome due to the presence of XXY chromosome.

Explanation:

Down syndrome is a condition caused by a chromosomal excess e.g XXY instead of XX-chromosome found in normal females or XY-chromosomes found in normal males

Translocation is the transfer of a chromosomal segment to a new position, especially on a nonhomologous chromosome.

After translocation, the carrier is normal because he or she still has two copies of all the essential genes on chromosome 21. However, on production of sex gametes, the transferred segment follows Mendel law of segregation and makes it way as XXY to the newly formed zygote - putting the carrier offspring at risk of Down syndrome.

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