Hereditary characteristics are determined by pairs of genes. A gene pair for a particular characteristic is transmitted from parents to offspring by choosing randomly one gene from the mother’s pair and one genera from the father’s pair. Each gene may have several types or alleles. For example human beings have one allele denoted by B for brown eyes and one allele denoted by b for blue eyes. A person with the allele pair BB has brown eyes, with bb has blue eyes. A person with the pair Bb ( or bB which is the same thing) will also have brown eyes and the allele B is said to be dominant.
A brown eyed woman has brown eyed parents, both known to be Bb. She and a blue-eyed man have a child. Given the child is brown eyed what is the chance the woman carries the allele b?