Dihybrid Crosses: Videos & Practice Problems

A dihybrid cross involves organisms that are heterozygous for two specific genes, resulting in a characteristic phenotypic ratio of 9:3:3:1. This ratio reflects the independent assortment of alleles during gamete formation. In a Punnett square for a dihybrid cross, each box represents a potential fertilization event, leading to diverse combinations of traits. Understanding dihybrid crosses is essential for grasping concepts like genetic variation and inheritance patterns in organisms, particularly in relation to dominant and recessive alleles.

Dihybrid Crosses

A dihybrid cross involves organisms that are heterozygous for two specific genes, indicated by the prefix "di-" meaning two. For example, consider a dihybrid genotype represented as RrYy, where "R" and "Y" are dominant alleles, and "r" and "y" are recessive alleles. In this case, the organism is heterozygous for both the shape gene (R for round and r for wrinkled) and the color gene (Y for yellow and y for green). This results in a phenotype of a round yellow pea, showcasing the dominant traits.

In genetic terms, the dominant allele masks the effect of the recessive allele. Therefore, in a dihybrid organism like RrYy, the presence of one dominant allele for each gene ensures the expression of the dominant traits. Understanding dihybrid crosses is essential for predicting the genetic variation in offspring when two heterozygous parents are crossed. This foundational knowledge sets the stage for exploring the outcomes of dihybrid crosses in future discussions.

Dihybrid Crosses & Punnett Squares

A dihybrid cross involves the fertilization between two organisms that are heterozygous for two specific genes, leading to a complex genetic interaction. In this context, dihybrid organisms possess two different alleles for each of the two genes being studied. The characteristic phenotypic ratio resulting from a dihybrid cross is 9:3:3:1, which reflects the various combinations of traits that can arise from the independent assortment of alleles during gamete formation.

When examining a dihybrid cross using a Punnett square, each parent contributes gametes that represent all possible combinations of their alleles. For instance, if one parent is heterozygous for both the shape and color genes, it can produce four types of gametes. The resulting Punnett square will be larger than those used for monohybrid crosses, as it accounts for the increased number of gamete combinations.

In a typical dihybrid cross, the outcomes can be categorized into four distinct phenotypic groups. The ratio of these phenotypes is as follows: 9 offspring exhibit the dominant traits for both genes (e.g., round and yellow), 3 offspring display the dominant trait for the first gene and the recessive trait for the second (e.g., round and green), another 3 show the recessive trait for the first gene and the dominant trait for the second (e.g., wrinkled and yellow), and finally, 1 offspring shows the recessive traits for both genes (e.g., wrinkled and green). This 9:3:3:1 ratio is a clear indication of the principle of independent assortment, which states that alleles for different traits segregate independently during gamete formation.

Understanding dihybrid crosses and their associated Punnett squares is essential for predicting the genetic outcomes of breeding experiments and illustrates fundamental concepts in Mendelian genetics.

Which of the following statements is correct in describing the terms monohybrid cross and dihybrid cross?

• A. A monohybrid cross involves a single parent, whereas a dihybrid cross involves two parents.
• B. A dihybrid cross involves organisms that are heterozygous for two characters that are being studied, and a monohybrid cross involves organisms that are heterozygous for only one character being studied.
• C. A monohybrid cross is performed for one generation, whereas a dihybrid cross is performed for two generations.
• D. A monohybrid cross results in a 9:3:3:1 phenotypic ratio, whereas a dihybrid cross gives a phenotypic 3:1 ratio.

Answer: (B)

Which of the following phenomena is a consequence of independent assortment?

• A. For any gene displaying complete dominance, heterozygous individuals exhibit the dominant phenotype.
• B. Pure breeding plants, when mated with each other, produce completely homozygous offspring.
• C. The phenotypic ratio produced from a F1 × F1 dihybrid cross is 9:3:3:1.
• D. Smooth seed trait is dominant to wrinkled seed trait in peas.

Answer: (C)

Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). What fraction of the progeny created by crossing BbTt × BBtt will be expected to have black fur and long tails?

• A. 1/16.
• B. 3/8.
• C. 1/2.
• D. 9/1.

Answer: (C)

In the dihybrid cross of AaBb x AaBb, what fraction of the offspring will be homozygous recessive for BOTH traits?

• A. 1/16.
• B. 1/8.
• C. 3/16.
• D. 1/4.
• E. 3/4.

Answer: (A)

Here’s what students ask on this topic:

What is a dihybrid cross and how does it differ from a monohybrid cross?

A dihybrid cross involves organisms that are heterozygous for two specific genes, whereas a monohybrid cross involves organisms that are heterozygous for only one gene. In a dihybrid cross, the Punnett square is larger and more complex because it accounts for the independent assortment of two different genes, leading to a characteristic phenotypic ratio of 9:3:3:1. In contrast, a monohybrid cross typically results in a 3:1 phenotypic ratio. Understanding dihybrid crosses is crucial for grasping genetic variation and inheritance patterns.

What is the phenotypic ratio of a dihybrid cross?

The phenotypic ratio of a dihybrid cross is 9:3:3:1. This ratio represents the likelihood of different combinations of traits appearing in the offspring. Specifically, 9 offspring will exhibit the dominant trait for both genes, 3 will exhibit the dominant trait for the first gene and the recessive trait for the second gene, another 3 will exhibit the recessive trait for the first gene and the dominant trait for the second gene, and 1 will exhibit the recessive trait for both genes. This ratio is a result of the independent assortment of alleles during gamete formation.

How do you set up a Punnett square for a dihybrid cross?

To set up a Punnett square for a dihybrid cross, follow these steps: 1) Determine the genotypes of the parent organisms, which should be heterozygous for two genes (e.g., RrYy). 2) List all possible gametes each parent can produce. For RrYy, the possible gametes are RY, Ry, rY, and ry. 3) Create a 4x4 grid and label the rows and columns with the possible gametes from each parent. 4) Fill in each box by combining the alleles from the corresponding row and column. This will give you the genotypes of the potential offspring. 5) Analyze the results to determine the phenotypic ratio.

What is the significance of the 9:3:3:1 ratio in a dihybrid cross?

The 9:3:3:1 ratio in a dihybrid cross is significant because it demonstrates the principle of independent assortment, one of Mendel's key genetic laws. This ratio shows that alleles for different traits segregate independently during gamete formation, leading to a variety of genetic combinations in the offspring. The 9:3:3:1 ratio specifically indicates that 9 offspring will have both dominant traits, 3 will have the first dominant and second recessive trait, another 3 will have the first recessive and second dominant trait, and 1 will have both recessive traits. This ratio is foundational for understanding genetic variation and inheritance patterns.

Can you explain the concept of independent assortment in the context of a dihybrid cross?

Independent assortment is a principle stating that alleles for different genes segregate independently of one another during gamete formation. In the context of a dihybrid cross, this means that the inheritance of one trait (e.g., seed shape) does not affect the inheritance of another trait (e.g., seed color). This principle is illustrated by the 9:3:3:1 phenotypic ratio observed in the offspring of a dihybrid cross. Each combination of alleles for the two traits is equally likely, leading to a variety of genetic outcomes and demonstrating the genetic diversity that results from sexual reproduction.