BackDihybrid Crosses and Mendelian Genetics
Study Guide - Smart Notes
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Dihybrid Crosses
Definition and Overview
A dihybrid cross is a genetic cross between two individuals that are both heterozygous for two different traits (genes). This type of cross is used to study the inheritance patterns of two characteristics simultaneously.
Dihybrid organism: Heterozygous for two specific genes (e.g., RrYy).
Example: Heterozygous round yellow pea plant (RrYy).
Punnett Square for Dihybrid Crosses
The Punnett square for a dihybrid cross (e.g., RrYy x RrYy) is a 4x4 grid that shows all possible combinations of alleles from the two parents.
Each parent produces four types of gametes: RY, Ry, rY, ry.
The resulting offspring genotypes and phenotypes can be predicted by filling in the Punnett square.
Phenotype Ratio in Dihybrid Crosses
The classic Mendelian dihybrid cross produces a 9:3:3:1 phenotypic ratio among the offspring:
Phenotype | Ratio |
|---|---|
Round, Yellow | 9 |
Round, Green | 3 |
Wrinkled, Yellow | 3 |
Wrinkled, Green | 1 |
Additional info: This ratio assumes independent assortment and complete dominance for both traits.
Key Terms
Allele: Different forms of a gene (e.g., R and r for seed shape).
Dominant allele: Expressed in the phenotype when present (e.g., R for round).
Recessive allele: Expressed only when two copies are present (e.g., r for wrinkled).
Genotype: Genetic makeup (e.g., RrYy).
Phenotype: Observable traits (e.g., round yellow seeds).
Independent Assortment
Mendel's Second Law
The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome.
Explains why new combinations of traits can appear in offspring.
Demonstrated by the 9:3:3:1 ratio in dihybrid crosses.
Practice Questions and Applications
Monohybrid vs. Dihybrid Crosses
Monohybrid cross: Involves one gene (e.g., Rr x Rr).
Dihybrid cross: Involves two genes (e.g., RrYy x RrYy).
Sample Practice Problems
In a dihybrid cross of AaBb x AaBb, the fraction of offspring homozygous recessive for both traits (aabb) is 1/16.
For a cross involving two traits, the probability of an offspring being homozygous recessive for both is calculated as:
Example: Black Fur (B) is Dominant to Brown (b); Short Tail (S) is Dominant to Long (s)
Cross: BbSs x BbSs
Fraction of offspring expected to have black fur and long tail: Calculate probability for B_ (black) and ss (long tail).
Probability of B_ = 3/4; Probability of ss = 1/4; Combined probability = 3/4 x 1/4 = 3/16.
Summary Table: Dihybrid Cross Outcomes
Genotype | Phenotype | Fraction |
|---|---|---|
R_Y_ | Round, Yellow | 9/16 |
R_yy | Round, Green | 3/16 |
rrY_ | Wrinkled, Yellow | 3/16 |
rryy | Wrinkled, Green | 1/16 |
Additional info: These ratios are based on Mendel's experiments with pea plants and are foundational to classical genetics.
