Course: Science Subtest II
Lesson: Genetics and Evolution
Topic: Inheritance of Traits      Page 13 of 18  

Dominant and Recessive Traits

In organisms that reproduce sexually, traits in offspring are determined by the combination of alleles they inherit. Alleles may be dominant, recessive or express incomplete dominance.

Dominant alleles are expressed in offspring regardless of whether both alleles are dominant not.

Recessive alleles are not expressed unless the organism is homogeneous (both allele are recessive). Dominant alleles are able to block the expression of recessive traits.

In this example, you can tell that the affected allele is dominant, because it is expressed, even when combined with an unaffected allele.