16.09 Dihybrid Inheritance

1. Introduction to Dihybrid Inheritance

Definition:

• Dihybrid Inheritance involves the study of inheritance patterns where two different genes, each with two alleles, are tracked simultaneously. This allows for the analysis of how these genes interact and assort during the formation of gametes.

Example:

• Rabbit Traits:
  • Coat Color:
    • B (Brown, dominant)
    • b (White, recessive)
  • Ear Length:
    • E (Long ears, dominant)
    • e (Short ears, recessive)

2. Key Concepts

a. Independent Assortment

• Definition: Genes located on different chromosomes assort independently during meiosis, meaning the inheritance of one trait does not influence the inheritance of another.
• Implication: This leads to the production of gametes with varied combinations of alleles, increasing genetic diversity.

b. Genotypes and Phenotypes

• Genotype: The genetic makeup of an organism, representing the alleles present.
• Phenotype: The observable traits resulting from the genotype.

Example Table: Genotypes and Phenotypes for Coat Color and Ear Length

3. Gamete Formation in Dihybrid Crosses

Example: Genotype BbEe

• Possible Gametes:
  • BE
  • Be
  • bE
  • be

Rules for Writing Gametes:

1. One Allele per Gene: Each gamete must contain one allele for each gene.
2. Consistent Order: Maintain the same gene order (e.g., coat color before ear length) for clarity.

Example:

• Genotype: BbEe
• Gametes Produced: BE, Be, bE, be

4. Performing Dihybrid Crosses

a. Example 1: Rabbit Cross (BbEe × Bbee)

Parental Phenotypes:

• BbEe: Brown coat, long ears
• Bbee: Brown coat, short ears

Parental Gametes:

• BbEe: BE, Be, bE, be
• Bbee: Be, be

Punnett Square:

Predicted Phenotype Ratio:

• 3 Brown coat, long ears
• 3 Brown coat, short ears
• 1 White coat, long ears
• 1 White coat, short ears

Simplified Ratio: 9:3:3:1 (when both parents are heterozygous)

b. Example 2: Dihybrid Test Cross (Tomato Example: AaDd × aadd)

Genes:

• Stem Color:
  • A (Purple, dominant)
  • a (Green, recessive)
• Leaf Shape:
  • D (Cut, dominant)
  • d (Potato, recessive)

Parental Phenotypes:

• AaDd: Purple stem, cut leaves
• aadd: Green stem, potato leaves

Parental Gametes:

• AaDd: AD, Ad, aD, ad
• aadd: ad

Punnett Square:

Predicted Phenotype Ratio:

• 1 Purple stem, cut leaves
• 1 Purple stem, potato leaves
• 1 Green stem, cut leaves
• 1 Green stem, potato leaves

c. Example 3: Dihybrid Cross with Both Parents Heterozygous (AaDd × AaDd)

Parental Phenotypes:

• AaDd: Purple stem, cut leaves
• AaDd: Purple stem, cut leaves

Parental Gametes: AD, Ad, aD, ad

Punnett Square Summary:

Predicted Phenotype Ratio:

• 9 Purple stem, cut leaves
• 3 Purple stem, potato leaves
• 3 Green stem, cut leaves
• 1 Green stem, potato leaves

Note: The 9:3:3:1 ratio is typical for dihybrid crosses where both parents are heterozygous for both traits.

5. Key Points to Remember for Dihybrid Crosses

1. Independent Assortment:
   • Ensures that each pair of alleles segregates independently from other pairs during gamete formation.
   • Leads to genetic variation in offspring.
2. Gamete Formation:
   • Each gamete receives one allele from each gene.
   • Maintain consistent gene order when listing alleles.
3. Punnett Squares:
   • Useful tool for predicting genotype and phenotype ratios.
   • Helps visualize possible genetic combinations.
4. Phenotypic Ratios:
   • 9:3:3:1 ratio is common in dihybrid crosses with independent assortment.
   • Variations occur based on parental genotypes.
5. Test Crosses:
   • Crossing an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual.
   • Helps determine the unknown genotype.

6. Practice Questions

a. Animal Cross with Grey Coat and Dark Eyes

Alleles:

• Coat Color:
  • G (Grey, dominant)
  • g (White, recessive)
• Eye Color:
  • D (Dark, dominant)
  • d (Pale, recessive)

Question: Draw a genetic diagram to show genotypes and phenotypes for offspring from a cross between a homozygous grey, dark-eyed animal (GGDD) and a homozygous white, pale-eyed animal (ggdd).

Solution:

• Parental Genotypes: GGDD × ggdd
• Gametes:
  • GGDD: GD
  • ggdd: gd
• Punnett Square:

Offspring Genotype: All GgDd (Grey coat, Dark eyes)

Phenotype Ratio: 100% Grey coat, Dark eyes

b. Plant Cross with Tall Stem and Leaf Color Codominance

Alleles:

• Stem Height:
  • T (Tall, dominant)
  • t (Short, recessive)
• Leaf Color:
  • G (Green)
  • g (White)
  • Note: G and g exhibit codominance, resulting in variegated leaves when heterozygous (Gg).

Question: Explain the phenotype ratio from a cross between a tall green plant (TTGG) and a short variegated plant (ttGg).

Solution:

• Parental Genotypes: TTGG × ttGg
• Gametes:
  • TTGG: TG
  • ttGg: tG, tg
• Punnett Square:

Offspring Genotypes:

• TtGG: Tall, Green
• TtGg: Tall, Variegated

Phenotype Ratio:

• 50% Tall, Green
• 50% Tall, Variegated

c. Determining Mammal Genotype for Black Eyes and Long Fur

Alleles:

• Eye Color:
  • B (Black, dominant)
  • b (Red, recessive)
• Fur Length:
  • F (Long, dominant)
  • f (Short, recessive)

Question: List possible genotypes for an animal with black eyes and long fur, and explain how a test cross can identify whether the animal is homozygous or heterozygous for each trait.

Solution:

Possible Genotypes for Black Eyes and Long Fur:

1. Homozygous Dominant for Both Traits: BBFF
2. Homozygous Dominant for One Trait and Heterozygous for the Other:
   • BBFf
   • BbFF
3. Heterozygous for Both Traits: BbFf

Test Cross Explanation:

• Purpose: Determine if the black-eyed, long-furred animal is homozygous dominant (BBFF, BBFf, BbFF) or heterozygous (BbFf) for one or both traits.
• Procedure: Cross the animal with a homozygous recessive individual (bbff).

Possible Offspring Outcomes:

1. If the animal is Homozygous Dominant for Both Traits (BBFF):
   • All offspring will be BbFf (Black eyes, Long fur).
2. If the animal is Homozygous Dominant for One Trait and Heterozygous for the Other (BBFf or BbFF):
   • BBFf × bbff:
     • All offspring: BbFf (Black eyes, Long fur) or Bbff (Black eyes, Short fur)
   • BbFF × bbff:
     • All offspring: BbFf (Black eyes, Long fur) or bbFf (Red eyes, Long fur)
3. If the animal is Heterozygous for Both Traits (BbFf):
   • Offspring ratios will include:
     • Black eyes, Long fur (BbFf)
     • Black eyes, Short fur (Bbff)
     • Red eyes, Long fur (bbFf)
     • Red eyes, Short fur (bbff)
   • Phenotype Ratio: 1 Black, Long : 1 Black, Short : 1 Red, Long : 1 Red, Short

Conclusion:

• By analyzing the phenotypic ratios of the offspring, you can determine the genotype of the parent animal:
  • Only Black and Long Fur Offspring: Homozygous dominant for both traits.
  • Black and Long with some variations: Heterozygous for one or both traits.

7. Key Terms

• Dihybrid Cross: A cross between two individuals that are both heterozygous for two traits.
• Allele: Different forms of a gene (e.g., B and b).
• Homozygous: Having two identical alleles for a trait (e.g., BB or bb).
• Heterozygous: Having two different alleles for a trait (e.g., Bb).
• Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.
• Independent Assortment: The principle that genes for different traits can segregate independently during the formation of gametes.
• Test Cross: A cross between an individual with a dominant phenotype but unknown genotype and a homozygous recessive individual.