Using your knowledge of meiosis and the formation of gametes, consider the following information. Down syndrome is caused by nondisjunction of chromosome pair 21. If a person with Down syndrome has a child with a person who does not have Down syndrome, what is the probability that the child will have Down syndrome?

50 percent

0 percent

100 percent

75 percent

Down's syndrome occurs due to the trisomy in 21st chromosomal pair. In normal scenario, there are two copies of chromosomes for chromosome number 21. But in this syndrome, due to the non-disjunction of chromosomes of 21st pair during meiosis, one of the gametes receive 2 copies instead of one and when this gamete fuses with the sperm which already has a haploid chromosome, the zygote ultimately receives 3 copies of chromosomes for chromosome number 21.

When a person with Down's syndrome forms gametes, at that time due to trisomy, one of the gametes receive 2 copies of the chromosome number 21 while another one receives only 1 copy. Therefore during fertilization, if the gamete which has 2 copies of chromosome number 21 fuses with the sperm then the progeny will also have Down's syndrome. So, there is 50% probability that the child will have Down's syndrome.

The cross is shown as under:

Parentals: XXX XX

↓ ↓ ↓ ↓

Gametes produced: XX X X X

Progenies: XXX XX XXX XX

So, it is clear from the above cross that out of the 4 probable progenies, 2 of the progenies will be born with trisomy (XXX) while 2 will be normal (XX). So, the probability is 50%.