Question Video: Identifying the Cross That Will Produce a Phenotypic Ratio of Nine to Three to Three to One Biology • First Year of Secondary School

Video Transcript

Assume that in guinea pigs, the allele for black fur, uppercase B, is dominant to white fur, lowercase b, and the allele for smooth fur, uppercase F, is dominant to the allele for rough fur, lowercase f. Which of the following crosses will produce a phenotypic ratio of nine to three to three to one? (A) All lowercase bbff crossed with all uppercase BBFF. (B) Uppercase B lowercase b uppercase F lowercase f crossed with uppercase B lowercase b uppercase F lowercase f. (C) Uppercase B uppercase B lowercase f lowercase f crossed with uppercase B uppercase B lowercase f lowercase f. Or (D) uppercase B lowercase b uppercase F lowercase f crossed with all lowercase bbff.

This question asks us to identify the parental genotypes that, when crossed, will produce a phenotypic ratio of nine to three to three to one. Let’s review some key terminology so that we can select the correct answer choice.

You might remember that a genetic cross is used to map all of the possible offspring combinations from parental genotypes. This is a process attributed to sexual reproduction involving two parents. During gamete production, which involves a process called meiosis, the parents’ genetic information is divided in half to produce haploid sex cells. So, while most other body cells will contain two copies of each chromosome and therefore have two alleles for each gene, their gametes will only contain one allele for each gene.

Gamete formation is a very simple concept when we’re considering a single gene. Here, we could say that I uppercase A is the allele that codes for the dominant trait and I lowercase a is the allele that codes for the recessive trait. Notice that the gametes only contain one of the parents’ alleles, as they are haploid. These gametes combine in fertilization to form a new individual with different characteristics than the parents. When considering more than one gene, we need to determine all of the possible combinations for resulting gametes that might be formed by each parent.

Let’s consider the two genes that we are presented with in the question. One gene, b, determines the color of fur, black or white, while the other gene, f, determines the texture of the fur, smooth or rough.

Now, let’s look at the different gametes that can be produced by the parental genotypes listed in the answer choices until we find those that result in the correct phenotypic ratio.

For answer choice (A), the first parent can only form gametes with one possible allele combination: lowercase b lowercase f. That is because they only possess lowercase recessive alleles for both fur color and texture. Similarly, the other parent in this answer choice will only produce gametes with the alleles uppercase B uppercase F. If these gametes were to combine in fertilization, all of the offspring will have the genotype uppercase B lowercase b uppercase F lowercase f. With this genotype, all of the offspring will have black, smooth fur as the presence of the dominant alleles will mask the expression of the recessive traits. As 100 percent of the offspring will display this phenotype, and they do not produce a phenotypic ratio of nine to three to three to one in the traits that are expressed, option (A) cannot be the correct answer to this question.

Answer choice (B) involves the mating between two individuals who are heterozygous for the two genes we are considering, which means that they each possess one dominant allele and one recessive allele. Heterozygous organisms can produce four different types of gametes. In this case, both parent organisms can produce the gametes uppercase B uppercase F, uppercase B lowercase f, lowercase b uppercase F, and lowercase b lowercase f.

Let’s cross each of these gametes in a Punnett square to see what the potential offspring genotypes they produce could be. When crossed, these organisms can produce offspring with 16 different potential genotype combinations. Phenotypically, only four different offspring are produced. Nine out of the 16 offspring should exhibit the dominant phenotype for both traits, represented here by the genotypes circled in blue. Three offspring are expected to be dominant for fur color but recessive for texture, represented here by the genotypes circled in orange. Another three will be recessive for fur color but dominant for texture, represented here by the genotypes circled in green. Only one offspring of the 16 is likely to be recessive for both, represented here by the genotype circled in pink. So, the offspring of this dihybrid cross exhibit a phenotypic ratio of nine to three to three to one.

This means that we have found the correct answer to this question. The cross which will produce a phenotypic ratio of nine to three to three to one is answer choice (B): uppercase B lowercase b uppercase F lowercase f crossed with uppercase B lowercase b uppercase F lowercase f.