Dominant, Recessive alleles and using Punnet Squares.

Alleles come in pairs and a GENOTYPE IS a pair of alleles which code for a PHENOTYPE.  Phenotype refers to the observed characteristic.
For example:

B= brown eyed allele   and    b = blue eyed allele
Genotype   Bb will give the PHENOTYPE of brown eyes because B is dominant over b.

GENOTYPE BB is homozygous dominant = brown eyes
GENOTYPE Bb is heterozygous = brown eyes
bb is homozygous recessive = blue eyes.

This is a case of COMPLETE DOMINANCE and it results in grandparents passing traits to grandchildren:

If a BB person mates with a bb person, we can calculate probability of their offspring traits using a punnet square:

When Baby grows up, she mates with this guy who happens to have the same GENOTYPE!

Note that a recessive allele can be hidden inside a HETEROZYGOUS genotype.