Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal co

Question

Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal complement of 46 chromosomes plus one (extra) chromosome #21. Although fertility is reduced in both sexes, females have higher fertility than males. Given the fact that fetuses with 48 chromosomes (four chromosome 21's) are not likely to survive early development, what proportion of surviving offspring would be expected to have Down syndrome if both parents have Down syndrome?

PLEASE PROVIDE A STEP BY STEP EXPLANATION! THANK YOU!

Explanation / Answer

If both parents have Down syndrome i.e. one (extra) chromosome #21 and given the fact that fetuses with 48 chromosomes (four chromosome 21's) are not likely to survive early development, the proportion of surviving offspring that would be expected to have Down syndrome if both parents have Down syndrome is based on the fact that; there are 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), making a total of 46 per cell in a normal cell. The possible situations in this condition are:

XY (47)      XY (47)

22 X + 23 Y   X   22 X + 23 Y

22 X + 23 Y X    22 X + 23X

22 X + 23X   X    22 X + 23 Y

22 X + 23X   X     22 X + 23X

Taking the example of one of this situation:

22 X + 23 Y   X   22 X + 23 Y      

22 X 22 X

22 X 23 Y

23 Y 22 X

23 Y 23 Y   

Only one out of four is having 48 chromosomes so it would not survive. Thus each situation has four possibility and only one is having 48 chromosomes so a total of 16 outcomes only 4 are having 48 chromosomes so the proportion of surviving offspring that would be expected to have Down syndrome if both parents have Down syndrome are

12/16 = 0.75

= 75%

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