In a certain flower, a blue petal phenotype is dominant to a white petal phenotype....

Explanation

Dominant alleles are referred to with capital letters, so let's call the dominant blue-petal allele B. Recessive alleles are referred to using lower case letters, so we will call the recessive white-petal allele b. A heterozygous organism has one dominant and one recessive allele, so the heterozygous flower has one B allele and one b allele. Its genotype is Bb. Because B is dominant to b, its phenotype (the trait produced by its genotype) is blue petals. A homozygous organism has two of the same allele. The homozygous flower will either have two BB alleles or two bb alleles. The question states that the flower with white petals is homozygous recessive, so its genotype is bb and its phenotype is white petals. The only genotype that produces a white phenotype is bb, because you need two recessive alleles in order to express the recessive trait. When you cross the two flowers, each parent donates one of its two alleles for petal color to the offspring. Accounting for every possible combination of alleles from each parent, there are four possible outcomes from a cross between Bb and bb: Bb, Bb, bb, and bb. (It may also help to draw a punnet square to visualize the four possible combinations). As you can see, these outcomes lead to two possible genotypes: Bb and bb. The Bb genotype produces flowers with blue petals, and the bb genotype leads to flowers with white petals. Because two of the four possible outcomes are genotype bb, two of the four possible outcomes are for flowers with white petals. Two out of four is equal to 50%, so 50% is the correct answer.