11.2 Dihybrid Inheritance
Dihybrid inheritance involves the study of the simultaneous inheritance of two distinct characteristics, where each characteristic is determined by a separate gene located at a different locus. Through dihybrid crosses of pea plants, Gregor Mendel discovered the Law of Independent Assortment, which explains how different pairs of alleles segregate and combine independently during the formation of gametes.
Learning Objectives
- Define dihybrid inheritance and dihybrid cross.
- Construct and interpret schematic diagrams and Punnett squares for dihybrid crosses from parental generation to \(F_1\) and \(F_2\) generations.
- State and explain Mendel's Second Law (Law of Independent Assortment).
- Determine the genotypic and phenotypic ratios produced in the \(F_2\) generation of a dihybrid cross.
Punnett Square and Schematic Diagram of Dihybrid Cross
Illustrate how two pairs of alleles assort independently during meiosis to yield 4 gamete types, 16 genotypic combinations, and a 9:3:3:1 phenotypic ratio in the \(F_2\) generation.
Short Explanation
Definition
Dihybrid inheritance involves the inheritance of two characteristics, where each characteristic is controlled by a different gene located at a different locus on homologous chromosomes.
Mendel's pea plant characteristics
Seed shape is controlled by alleles \(B\) and \(b\). Dominant \(B\) produces round seeds; recessive \(b\) produces constricted seeds. Seed colour is controlled by alleles \(K\) and \(k\). Dominant \(K\) produces yellow seeds; recessive \(k\) produces green seeds.
Parental cross
Purebreed round and yellow seed parent \(BBKK\) is crossed with purebreed constricted and green seed parent \(bbkk\). Meiosis produces gametes \(BK\) and \(bk\). Fertilisation produces \(F_1\) generation: all \(BbKk\), round and yellow seeds.
\(F_1\) self-cross
Self-crossing \(BbKk \times BbKk\) produces four types of male and female gametes in equal frequencies: \(BK\), \(Bk\), \(bK\), and \(bk\).
Mendel's Second Law
The Law of Independent Assortment states that during gamete formation, each allele from a pair of alleles can combine randomly with any allele from another pair of alleles.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the definition of dihybrid inheritance?
What is the phenotypic ratio of the \(F_2\) generation obtained from the self-crossing of \(F_1\) heterozygous dihybrid pea plants?
What does Mendel's Second Law (Law of Independent Assortment) state?
Common Mistakes
- Writing a single allele instead of two alleles in a gamete (e.g., writing gamete as “\(B\)” instead of “\(BK\)”). In dihybrid inheritance, every gamete must carry one allele for each of the two genes.
- Confusing the \(F_2\) phenotypic ratio (\(9:3:3:1\)) with the genotypic ratio.
- Misinterpreting the order of phenotypes in the \(9:3:3:1\) ratio; 9 represents both dominant traits, 3 and 3 represent one dominant and one recessive trait, and 1 represents both recessive traits.
- Forgetting that purebreed parents must be homozygous for both traits (\(BBKK\) and \(bbkk\)).
Concept Misunderstandings
Alleles for different characteristics remain permanently linked together in the combinations received from parents.
According to Mendel's Law of Independent Assortment, alleles for different traits segregate independently during gamete formation and can combine randomly, creating new phenotypic combinations in the \(F_2\) generation.
The \(F_1\) generation shows variations of both traits.
Because both round shape and yellow colour are fully dominant, all \(F_1\) individuals display only the dominant phenotype (round and yellow seeds).
Summary
- Dihybrid inheritance involves two characteristics controlled by different genes at different loci.
- Purebreed cross: \(BBKK \times bbkk \rightarrow F_1\) generation is all \(BbKk\) (round and yellow seeds).
- Self-crossing \(F_1\) (\(BbKk \times BbKk\)) produces four types of gametes: \(BK\), \(Bk\), \(bK\), and \(bk\).
- Mendel's Second Law states that during gamete formation, each allele from a pair of alleles can combine randomly with any allele from another pair of alleles.
- \(F_2\) generation produces 16 genotype combinations with a phenotypic ratio of 9 round yellow : 3 round green : 3 constricted yellow : 1 constricted green.
Short Activity
Objective: check your understanding of dihybrid inheritance, Mendel's Second Law and the \(F_2\) phenotypic ratio.
A. Objective Quiz
Which of the following shows the correct phenotypic ratio produced in the \(F_2\) generation of Mendel's dihybrid cross between two heterozygous round and yellow pea plants (\(BbKk \times BbKk\))?
According to Mendel's Second Law, what occurs during gamete formation?
B. Fill in the Blanks
Dihybrid inheritance involves the inheritance of two characteristics, where each characteristic is controlled by a different gene located at a different .
Mendel's Second Law of Inheritance is also known as the Law of Independent .
C. Matching / Drag and Drop
Match each genotype with its corresponding seed phenotype from Mendel's dihybrid cross. Drag each genotype to its correct phenotype. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Genotype to Its Phenotype
Write the correct genotype next to each phenotype.
| Phenotype | Genotype |
|---|---|
| 1. Round and yellow seed (homozygous dominant for both traits) | |
| 2. Round and green seed | |
| 3. Constricted and yellow seed | |
| 4. Constricted and green seed (homozygous recessive for both traits) |
Answer choices: \(BBKK\) • \(BBkk\) • \(bbKK\) • \(bbkk\)
