Dihybrid Cross: Definition, Example & Quiz
In this lesson, learn how to predict what offspring will look like when mating occurs between dihybrid individuals, or those that have two different alleles for two different genes. After the lesson, test your knowledge with a quiz.
Genes are the units of heredity of all organisms. A single gene can have many different versions called alleles.
A gene usually has two alleles: one allele comes from your mother and one comes from your father. If an individual has the same two alleles for a gene, this is called homozygous. If an individual has two different alleles for a gene, this is called heterozygous. The combination of alleles an individual inherits is called the genotype.
Scientists like to do crosses, or matings, between different individuals to determine all the possible genotypes and phenotypes of their offspring. A phenotype is the physical manifestation of a gene, or what an individual looks like. (For example, one phenotype of a pea plant is having purple flowers.) A dihybrid cross is a cross between two individuals that are both heterozygous for two different genes. A hybrid is just the offspring produced when two genetically different parents are crossed. This usually results in a heterozygous individual. The 'di' in dihybrid means two and is referring to there being two genes or traits involved.
Flower Color and Height in Pea Plants
Let's look at an example of a dihybrid cross and see how it can be used to predict the genotype and phenotype chances of offspring. As previously stated, pea plants have two alleles for flower color. Purple color (F) is dominant to white color (f). A second trait of pea plants is height. The dominant allele (H) makes a pea plant tall, and the recessive allele (h) produces a dwarf pea plant.
If we take two pea plants that are both heterozygous for each of these two traits (FfHh) and cross them, then we have done a dihybrid cross (FfHh x FfHh). The phenotype associated with FfHh is tall with purple flowers because dominant alleles hide the effects of recessive alleles. During a cross, each parent plant only contributes one of its alleles for each gene, and every parental allele has an equal chance of being given to its offspring. So a plant with the genotype FfHh can give its offspring one of four possible allele combinations:
- Dominant alleles for both traits: FH
- Dominant allele for trait one (flower color) and recessive allele for trait two (height): Fh
- Recessive allele for trait one and dominant allele for trait two: fH
- Recessive alleles for both traits: fh
In order to determine what all the possible offspring of this cross will look like, we can draw a diagram called a Punnett square. In a dihybrid cross, there are 16 possible ways for parental alleles to combine.
The image below is a 4 by 4 box - a Punnett square for the pea plant dihybrid cross (FfHh X FfHh). It shows all the possible genotypes and phenotypes that the offspring can have (one in each of the 16 boxes). The 4 possible allele combinations each parent can contribute are shown across the top for one parent plant and down the side for the other parent plant. Remember the parent individuals have the same genotype, which is heterozygous for both flower color and height (FfHh). They also have the same phenotype, which is tall with purple flowers. These tall purple parents can produce offspring with 4 different phenotypes. Offspring are produced in a 9:3:3:1 ratio for phenotype, which is illustrated under the Punnett square.
While there are many different genotypes possible, there are only 4 different phenotypes possible. Remember a phenotype is what an individual looks like. The phenotypic ratio, or the relative numbers of offspring with each phenotype, for a dihybrid cross is always the same. It is 9:3:3:1. Each number can be determined by counting up boxes with the same phenotype in the Punnett square. Then, if you add up all four of these numbers, you should get 16. This phenotypic ratio tells you that:
- 9 out of the 16 offspring will show the dominant phenotype for both traits
- 3 out of 16 will show the dominant phenotype for trait one and the recessive phenotype for trait two
- 3 out of 16 will show the recessive phenotype for trait one and the dominant phenotype for trait two
- 1 out of 16 will have both recessive phenotypes
In this pea plant example, the phenotypic ratio tells us that when a dihybrid cross is performed, offspring are most likely to be tall and have purple flowers. More specifically, there is a 9 in 16 chance that offspring will have both the dominant phenotypes. You should be able to confirm this by counting 9 boxes in the Punnett square that have the tall purple phenotype. This phenotypic ratio of 9:3:3:1 also tells us that short offspring with white flowers should be very rare, because there is only a 1 in 16 chance that offspring will have both recessive phenotypes. Again, you can confirm this by seeing that there is only one box in the Punnett square with a plant that is dwarf with white flowers.
A dihybrid is an individual gene that is heterozygous (has two different alleles) at two different genes. When two dihybrid individuals are mated, this is called a dihybrid cross. Even though the parents of a dihybrid cross have the same genotype and phenotype, their offspring could show any one of 9 different genotypes and 4 different phenotypes. The phenotypic ratio of offspring produced from a dihybrid cross is 9:3:3:1.
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