Dominant allele and recessive allele
Now we know that an allele is best thought of as one letter, it’s easy to see which is the dominant letter and which is the recessive letter.
The capital B dominates (dominant) the lower case b, so the lower case b retires to a small recess (recessive).
Dominant and recessive alleles can be shown in a punnet square.
In this example the mother has brown eyes but carries one brown eye allele and one blue eye allele. The father also has brown eyes but again carries one brown eye allele and one blue eye allele.
If you have two different alleles, one will be stronger/ more dominant than the other. The stronger allele will overrule the recessive allele. When it comes to eye genotypes, the dominant brown allele will overrule the recessive blue allele:
BB = brown eyes (Both alleles are the same so no overruling here)
Bb = brown eyes (B dominates b to give brown eyes)
bB = brown eyes (B dominates b to give brown eyes)
bb = blue eyes (Both alleles are the same so no overruling here)
Cystic fibrosis is a common inherited disorder of cell membranes. It is caused by inheriting two recessive alleles (ff). People who have the heterozygous genotype (Ff) are said to be carriers, with no ill effects. Only people who have the homozygous (ff) are affected. The recessive allele must be inherited from both parents.