Dihybrid Cross - Definition, Steps, Process, Punnett Square and Examples in Pea Plants and Drosophila

What is Dihybrid Cross?

Dihybrid cross is a type of genetic cross between two individuals with either homozygous or heterozygous genotypes of two characters or traits.

• Different from monohybrid cross which involves single genetic character.
• More complex - involves more than one genetic trait where parents can be homozygous or heterozygous.
• Used to prove law of independent assortment - proves two characters segregate independently.
• Works only for genes that are not linked and are present on different chromosomes.
• Number of gametes formed is more than monohybrid as two separate genes are involved.
• Number of offsprings is more with more phenotypic variation than monohybrid crosses.
• Phenotypic ratio in F1 after dihybrid cross is 9:3:3:1.
• Genotypic ratio is 1:2:2:4:1:2:1:2:1 - more diverse.

Steps / Process

Steps to conduct a dihybrid cross between two individuals:

1. Selection of Parents

• First step is selection of characters as well as parents to be crossed.
• Pure lines are obtained by selfing individuals for three generations which confirms pure lines.

2. Designation for the Characters

• Alleles designated with alphabet characters.
• Dominant alleles indicated by upper case letters, recessive by lower case letters.

3. Punnet Square

• Gametes are formed for fertilization - four distinct gametes formed while studying two characters.
• Punnet square is set up by listing phenotype and genotype of parents.
• Gametes will be haploid as result of meiotic division.
• Possible combinations filled into Punnet square - all equally possible as fertilization is random.

4. Determination of Ratios

• Once all combinations determined, phenotypic and genotypic ratios of offsprings are noted.

Examples of Dihybrid Cross

1. Dihybrid Cross in Pea Plants

Cross between homozygous pea plant with round yellow seeds and wrinkled green seeds - classic Mendelian example.

• Round yellow seeds represented by RRYY alleles, wrinkled green by rryy.
• Gametes formed from these alleles are RY and ry.
• When crossed, F1 hybrids with RrYy allele formed - round yellow seeds with dominant R for roundness and dominant Y for yellow color.
• Four alleles can combine in four different combinations: RY, Ry, rY, and ry.
• Four alleles assorted randomly to produce four types of gametes.
• Gametes unite at random during fertilization to produce sixteen types of individuals in F2 generation.
• Hybrids occur in ratio 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green.

2. Dihybrid Cross in Drosophila

• Example: homozygous long-winged and black-bodied Drosophila and vestigial-winged and grey-bodied fly.
• F1 generation hybrids obtained are long-winged grey-bodied Drosophila fly.
• Hybrids after mating produce F2 generation.
• F2 generation includes 9 long-winged grey bodies, 3 long-winged black-bodied, 3 vestigial-winged grey-bodied, 1 vestigial winged black-bodied Drosophila.

References and Sources

• Mendel, G. - Original experiments on pea plants - Law of independent assortment.
• Microbe Notes - Dihybrid Cross - Definition, Steps and Examples.
• Griffiths, A. J. F. et al. - Introduction to Genetic Analysis - Dihybrid cross and Punnett square.
• Alberts, B. et al. - Molecular Biology of the Cell - Meiosis and genetic variation.
• LibreTexts Biology - Mendelian Genetics and Dihybrid Crosses.