Question

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?

25%
50%
75%
100%
I think it's B, but I also have tis feeling that it could be A

Answer 1

A child, who is born with Down Syndrome will exhibit excessive amounts of chromosomes 21 in some or all of their cells. DS will also cause abnormally different characteristic in their physical traits and it will even cause development delays or retardation. The majority of people with DS will carry an extra copy of chromosomes in every cell of their body. There are three forms of DS- Non-disjunction, mosaicism, and trans-location. Non-disjunction will be the most common and the individuals, who do have it, will have an entire extra copy of chromosomes in all of their cells.

Now when the fertilization occurs, the embryo will have 46 chromosomes all together. 23 for the sperm cell and 23 from the egg cell. But things can go very wrong during meiosis. Things may run smoothly at first, but one pair of chromosomes may fail to separate and both of these chromosomes will go to the one side of the cell dividing. This cell-division mishap is know as non-disjunction.

Now it will result in two (23 single chromosomes) one (22 single chromosome) and one (24 single chromosome). Now when the non-disjunction occurs, the gametes will have an abnormal chromosome count. One single cell will have 22 chromosomes and one single cell will have 24 chromosomes.


You will likely find what percentage of their offspring, who will have the probability of Down Syndrome, by using a punnet square-shows alleles and crosses. The best method to determine and see Down Syndrome would be to look through a karyotype- it can show use the chromosomes.


If you did use a punnet square it would get your answer B. 50%
Great job! :)


Hope that help! n_n