Question

Dihybirds involve the inheritance of two traits, two pairs of alleles.

In humans, brown eye color (B) is dominant to blue eyes color (b) and dark hair color (D) is dominant to blond hair color (d).

1. What is the genotype of a person heterozygous for both traits?

2. What are the possible genotypes of a blue-eye, dark haired individual?

3. What are the possible genotypes of a blue-eye, dark haired individual?

4. What is the genotype of a blue-eyed blond?

5. For an individual who is heterozygous for both traits, what would be the genotypes for the FOUR possible gametes they can produce?

Construct a Punnett square for two parents who are heterozygous for both traits: BbDd x BbDd to do this, place the possible gametes of each parent on the left side and top side, above column and row, and fill in the boxes with the offspring genotypes.

Answer 1

1) What is the genotype of a person heterozygous for both traits?

Ans : BbDd.

Reason : Heterozygous means they will have both dominant and recessive allele in their genotype. Phenotype will exhibit the dominant trait or character.

2 & 3) What are the possible genotypes of a blue-eye, dark haired individual?

Ans : Two posible genotypes are possible for blue-eye, dark haired individual.

(i) bbDD and (ii) bbDd

Reason : Since blue eye is a recessive trait it will be homozygous recessive allele and dark hair is a dominant trait it can be homozygous dominant or heterozygous allele.

4) What is the genotype of a blue-eyed blond?

Ans : bbdd

Reason : Since both blue-eye and blond hair are recessive trait, their genotype will be a homozygous recessive. Recessive trait only exhibit when both the allele is recessive.

5) For an individual who is heterozygous for both traits, what would be the genotypes for the FOUR possible gametes they can produce?

Ans : Since the individual is heterozygous for both traits (BbDd), there will be four possible gametes. Their genotype will be

(i) BD, (ii) Bd, (iii) bD and (iv) bd

Construct a Punnett square for two parents who are heterozygous for both traits: BbDd x BbDd

Ans : This is a heterozygous Dihybrid cross. Both parents have four gametes : BD, Bd, bD and bd

BbDd x BbDd

Gametes	BD	Bd	bD	bd
BD	BBDD	BBDd	BbDD	BbDd
Bd	BBDd	BBdd	BbDd	Bbdd
bD	BbDD	BbDd	bbDD	bbDd
bd	BbDb	Bbdd	bbDd	bbdd

Note :

Parent 1 gametes are highlighted as red color.

Parent 2 gametes are highlighted as blue color.

Offspring genotypes are highlighted as violet color.

Four different offsprings are produced.

The Phenotypic ratio = 9 : 3 : 3 : 1

Individual with Brown eye, Dark hair (Dominant, Dominant) = 9

Individual with Brown eye, Blond hair (Dominant, Recessive) = 3

Individual with Blue eye, Dark hair (Recessive, Dominant) = 3

Individual with Blue eye, Blond hair (Recessive, Recessive) = 1