MCAT Biology Question — Incomplete Dominance | Next Step Test Prep MCAT Biology Question — Incomplete Dominance | Next Step Test Prep

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The snapdragon is a plant demonstrating incomplete dominance in the flower color phenotype. If homozygous snapdragons have either red or white flowers, what color are the heterozygotes?

 

A) red, because the white coloring reflects all incoming light wavelengths in flowers expressing both red and white alleles

 

B) some flowers red and some white, because only one gene can be expressed at a time

 

C) white, because the white coloring will reflect all incoming light wavelengths in flowers expressing both red and white alleles

 

D) pink, because it is in-between red and white

 

 

Explanation

 

Incomplete dominance, or partial dominance, refers to a heterozygous phenotype that is in-between the two homozygous phenotypes. One of the most common examples is snapdragon flowers, which may have one allele for white flowers and one allele for red flowers expressed as pink flowers in heterozygotes. If two heterozygotes are crossed, the offspring will have red:pink:white flowers in a ratio 1:2:1.

 

A) red, because the white coloring reflects all incoming light wavelengths in flowers expressing both red and white alleles, incorrect, Red is not the dominant phenotype, and white objects reflecting light has nothing to do with gene expression.

 

B) some flowers red and some white, because only one gene can be expressed at a time, incorrect, This is true for dominant and recessive genes, but incomplete dominant genes express an intermediate phenotype.

 

C) white, because the white coloring will reflect all incoming light wavelengths in flowers expressing both red and white alleles, incorrect, White coloring reflecting light has nothing to do with phenotype expression of genes.

 

D) pink, because it is in-between red and white, correct.

 

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