What common condition is caused by the presence of three copies of chromosome 21?

75%

Explanation:

Tall stems are dominant over short stems. Let T represent tall and t represents short. The parents are Heterozygous tall plants, which means that their genotype would be Tt and their phenotype would be tall. In a Punnett square it would look like this:

      T    t

T   TT   Tt

t    Tt    tt

Notice that we have 3 possible genotype combinations. When the dominant allele T is present that means that the phenotype that will be expressed is the dominant trait. In this case we have the following genotype and phenotype:

TT      Tall         1/4

Tt       Tall         2/4

tt        Short      1/4

The question is asking the chance the pffspring phenotype is the same as the parent specifically so we can see that 3/4 are tall.

3/4 = 75%