Dihybrid Cross: Definition & Example

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  • 0:00 Reviewing Definitions
  • 2:03 A Dihybrid Cross
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

In this lesson, learn what a dihybrid cross is and see an example. Also, find out about phenotypic ratio and how it's the same for each dihybrid cross. After the lesson, test your knowledge with a quiz.

Reviewing Definitions

What happens if you breed plants with different traits? For example, if you bred two pea plants with purple flowers and long stems, would you be surprised to get an offspring with white flowers and a short stem? It can certainly happen.

Let's take a look at how we can predict the possible offspring of two individuals. But first, let's go over some terms.

Genes are the units of heredity of all organisms. Different versions of the same gene are called alleles. A gene usually has two alleles: one from the mother and one from the father. Each allele is assigned a letter, either uppercase or lowercase, indicating whether it's recessive or dominant for a particular trait.

For example, an allele for patterning might be represented by a 'C' if it's dominant or a 'c' if it's recessive. Let's say the 'C' stands for a striped pattern while the 'c' is for a spotted pattern. If an organism inherits two 'C's (CC), it will have a striped pattern.

But what happens if the organism gets one dominant allele and one recessive allele? The dominant allele will mask the effect of the recessive allele, and the organism will still have a striped pattern. However, if the organism inherits two recessive alleles, or two 'c's (cc), it will have a spotted pattern.

If an individual has the same two alleles for a gene (CC or cc, in this case), this is called homozygous. If an individual has two different alleles for a gene, (Cc), this is called heterozygous. You can remember this by knowing that homo- means the same, and hetero- means different.

The combination of alleles an individual inherits is its genotype, while the physical manifestation of a gene, or what an individual looks like, is its phenotype. To determine all the possible genotypes and phenotypes of an organism's offspring, scientists perform crosses.

A Dihybrid Cross

A dihybrid cross is a cross between two individuals that are both heterozygous for two different traits. As an example, let's look at pea plants and say the two different traits we're examining are color and height. In other words, each plant has:

  • One dominant allele F for purple color and one recessive allele f for white color and
  • One dominant allele H for height and one recessive allele h, which produces a dwarf pea plant

Remember that dominant alleles hide the effects of recessive alleles, which means the phenotype associated with our heterozygous plants is tall with purple flowers.

In a cross, each parent plant contributes one allele for each gene, and every parental allele has an equal chance of being given to the offspring. So, a plant with the genotype FfHh can give its offspring one of four possible allele combinations:

  1. Dominant alleles for both traits, FH
  2. Dominant allele for trait one (flower color) and recessive allele for trait two (height): Fh
  3. Recessive allele for trait one and dominant allele for trait two: fH
  4. Recessive alleles for both traits: fh

To determine what all the possible offspring of this cross will look like, we can draw a diagram called a Punnett square.

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