TBR biology down syndrome

[batista_123]

i have a question about #100 in the genetics section of TBR. They show a gel electrophoresis or whatever that thing is called and they ask did the mother or father have non-disjunction?
then the answer says from the figure, its clear that it was the mother. I have no idea why. The figure is weird because the bands are not all in the same columns they are in 3 or 4 columns. how do you tell if it was the mother or father?

by the way, if you dont understand dna sequencing, check this out:
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter15/animation_quiz_1.html

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[dragon529]

Both mother and father have 2 columns for the 2 chromosome 21 that they have. Now, the reason that the child has down syndrome is because the child has 3 columns (3 chromosome 21). Take a look at the child's box, the left band comes from mother, the middle band comes from father, and the right bands come from mother. The mother contributed both sets of chromosomes and resulted in a kid with down syndrome.

 

[batista_123]

Both mother and father have 2 columns for the 2 chromosome 21 that they have. Now, the reason that the child has down syndrome is because the child has 3 columns (3 chromosome 21). Take a look at the child's box, the left band comes from mother, the middle band comes from father, and the right bands come from mother. The mother contributed both sets of chromosomes and resulted in a kid with down syndrome.

i dont understand why the mother and father have 3 bands. If a person has 2 chromosomes, shouldnt they have 2 bands?

 

[dragon529]

Both mother and father have 2 COLUMNS. Each COLUMN is 1 CHROMOSOME. You see 1 column has 1 band because the specific restrictive enzyme didn't cut the DNA. The other column has 2 bands because the enzyme chopped the DNA. Focus on the column and not on the number of bands.