Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

What does incomplete dominance refer to in genetic crosses?

1. When one allele completely masks another

2. When both alleles are fully expressed

3. When two different genes blend together to form a third phenotype

4. When there is no dominant or recessive allele

The correct answer is: When two different genes blend together to form a third phenotype

Incomplete dominance refers to a genetic scenario where the phenotypes of two different alleles blend together to create a third phenotype that is distinct from both parent alleles. This blending results in an intermediate trait that is a combination of the characteristics of each allele. A classic example of incomplete dominance is seen in the flower color of certain plants, where a cross between red-flowered and white-flowered varieties produces pink flowers. The concept of blending is essential in understanding how traits are passed on in incomplete dominance. Unlike complete dominance, where one allele would fully mask the effect of another, or co-dominance, where both alleles are fully expressed at the same time, incomplete dominance creates a unique phenotype that does not resemble either parent exactly but is a mix of both. This results in a range of phenotypes in the offspring that highlights the contribution of each allele without one overpowering the other.