Suppose pcr is used to amplify a single dna marker on human chromosome 21. this is a way to identify individual chromosomes, such as paternal and maternal chromosome 21 from each parent. further suppose that a couple who have a child with down syndrome (trisomy 21) is examined for this marker. the mother has marker alleles of 310 and 380 bp. her mate has marker alleles of 290 and 340 bp. (hint: these are alleles of the same gene. they are simply being designated by size – in bp – instead of a letter. you can think of them as a1, a2, a3 and a4 if you prefer, with the child inheriting one allele from each parent). what combinations of pcr bands would be expected in their child with down syndrome if nondisjunction occurred in