11.2 - Dihybrid Inheritance

Definition of Dihybrid Inheritance

• Dihybrid inheritance involves the inheritance of two characteristics; each characteristic is controlled by a different gene at a different locus.

Mendel’s Dihybrid Cross Experiment

• Characteristics studied: seed shape and seed colour.
• Alleles:
  • $B$: dominant allele for round seed; $b$: recessive allele for constricted seed.
  • $K$: dominant allele for yellow seed; $k$: recessive allele for green seed.

The $F_1$ Generation

• Parental phenotype: Purebreed round yellow seed $\times$ purebreed constricted green seed.
• Parental genotype: $BBKK\times bbkk$.
• Meiosis and gametes: $BK$ and $bk$.
• Fertilisation produces $BbKk$.
• $F_1$ genotype: All $BbKk$.
• $F_1$ phenotype: All round yellow seeds; only the dominant traits are observed.

The $F_2$ Generation

• $F_1$ cross: $BbKk\times BbKk$.
• During gamete formation, any allele for seed shape can pair with any allele for seed colour.
• Each parent produces four gametes: $BK$, $Bk$, $bK$ and $bk$.

Punnett Square for the $F_2$ Generation

Analysis of the $F_2$ Generation

• Phenotypic ratio: $9$ round yellow : $3$ round green : $3$ constricted yellow : $1$ constricted green.
• New combinations of characteristics: constricted yellow seeds and round green seeds.
• The two characteristics combine in $F_1$, then separate and act independently in $F_2$.

Mendel’s Second Law (Law of Independent Assortment)

• During gamete formation, each allele from a pair of alleles can combine randomly with any allele from another pair of alleles.