16.11 Autosomal Linkage and Crossing Over

1. Definition of Autosomal Linkage

• Autosomal Linkage: Occurs when two or more genes are situated close together on the same autosome. These genes do not assort independently during meiosis and are more likely to be inherited together.
  • Key Points:
    • Involves non-sex chromosomes (autosomes).
    • Genes are physically linked due to their proximity on the chromosome.
    • Reduces genetic variation compared to independent assortment.

2. Effects of Autosomal Linkage

• Non-Independent Assortment: Linked genes tend to be inherited together, maintaining the parental gene combinations in the offspring.
• Reduced Recombinant Types: Fewer new allele combinations appear in the offspring compared to unlinked genes.
  • Implications:
    • Predictable inheritance patterns.
    • Challenges the Mendelian principle of independent assortment when genes are linked.

3. Example: Autosomal Linkage in Drosophila (Fruit Fly)

• To illustrate autosomal linkage, consider a classic example involving two traits in Drosophila:

• Linked Traits:
  1. Body Color Gene:
     • E (Dominant): Striped body.
     • e (Recessive): Ebony (black) body.
  2. Antenna Shape Gene:
     • A (Dominant): Normal antennae.
     • a (Recessive): Aristopedia antennae (antenna resembles a leg with two claws).

Cross 1: Homozygous Striped, Normal Antennae × Homozygous Ebony, Aristopedia

• Parental Genotypes:
  • Male: ((EA)(EA)) – Homozygous dominant for both traits.
  • Female: ((ea)(ea)) – Homozygous recessive for both traits.
• Parental Gametes:
  • Male: Only (EA) gametes.
  • Female: Only (ea) gametes.
• Punnett Square:

• F₁ Generation:
  • Genotype: All offspring are ((EA)(ea)).
  • Phenotype: Striped bodies and normal antennae.

Cross 2: F₁ Generation Male ((EA)(ea)) × Homozygous Recessive Female ((ea)(ea))

• Parental Genotypes:
  • Male (F₁): ((EA)(ea)) – Heterozygous for both traits.
  • Female: ((ea)(ea)) – Homozygous recessive.
• Parental Gametes:
  • Male: Produces (EA) and (ea) gametes.
  • Female: Only (ea) gametes.
• Punnett Square:

• F₂ Generation Phenotype Ratio:
  • Striped body, normal antennae: 50%
  • Ebony body, aristopedia antennae: 50%
• Conclusion:
  • Due to linkage, only parental phenotypes appear in the F₂ generation.
  • No recombinant phenotypes are observed as no crossing over occurs between the linked genes.

4. Comparison: Linked vs. Unlinked Genes

• If Genes Were Unlinked:
  • Independent Assortment: Genes would segregate independently, allowing for more phenotype combinations.
• Expected F₂ Phenotypic Ratio (Unlinked):
  • 1 Striped, Normal : 1 Striped, Aristopedia : 1 Ebony, Normal : 1 Ebony, Aristopedia
• Punnett Square for Unlinked Genes:

• Result:
  • Four distinct phenotypes with equal probability, reflecting independent assortment.

5. Autosomal Linkage with Crossing Over

• While linked genes are typically inherited together, crossing over during meiosis can separate them, creating recombinant types.

• Crossing Over:
  • Occurs during Prophase I of meiosis.
  • Homologous chromosomes exchange segments, potentially breaking the linkage between genes.
• Recombinant Types:
  • Parental Types: Offspring exhibiting the same trait combinations as the parents.
  • Recombinant Types: Offspring exhibiting new trait combinations due to crossing over.
• Factors Influencing Recombinant Frequency:
  • Gene Distance: Genes that are farther apart are more likely to undergo crossing over.
  • Chromosome Size: Larger chromosomes may have more opportunities for crossing over.

Autosomal Linkage Example with Crossing Over in Drosophila

• Genetic Setup:
  • Female (EA)(ea): Striped body, normal antennae.
  • Male (ea)(ea): Ebony body, aristopedia antennae.
• Gamete Formation:
  • Female Gametes:
    • Parental Types: (EA) and (ea).
    • Recombinant Types: (Ea) and (eA) – Resulting from crossing over.
• Offspring Genotypes and Phenotypes:

• Observed Phenotypic Ratios:
  • Parental Types: 44% Striped & Normal, 44% Ebony & Aristopedia.
  • Recombinants: 6% Striped & Aristopedia, 6% Ebony & Normal.
• Recombinant Frequency:
  • Calculation: (Number of Recombinants / Total Offspring) × 100
  • Example: (30 + 29) / 287 × 100 ≈ 20.6%
  • Interpretation: The recombinant frequency indicates the genetic distance between the genes; higher frequency suggests genes are farther apart.

6. Key Terms

• Autosome: Any chromosome that is not a sex chromosome.
• Linkage: The tendency of genes located close to each other on a chromosome to be inherited together.
• Recombinant: An offspring with a combination of traits different from both parents due to crossing over.
• Parental Type: An offspring exhibiting the same combination of traits as one of the parents.
• Crossing Over: The exchange of genetic material between homologous chromosomes during meiosis, leading to genetic variation.
• Recombinant Frequency: A measure of the proportion of recombinant offspring, used to estimate the distance between genes on a chromosome.

7. Key Points to Remember

• Autosomal Linkage: Genes on the same autosome are inherited together unless crossing over occurs.
• Drosophila Exception: In Drosophila males, no crossing over occurs, making it easier to study linkage in females where crossing over does take place.