Lecture Notes on Comprehensive Genetics Chapter 8

Overview

• Focus on predicting outcomes of genetic crosses when the genetic map is known.
• Determining progeny outcomes and the frequency of each progeny type.

Predicting Progeny for Two Linked Genes

• Setup: Three total genes; two linked, one unlinked.
  • Example: Black body and purple eyes (autosomal recessive, 14 map units apart), held out wings (on chromosome 3).
• Cross: Heterozygous female test crossed with a male expressing all three traits.
• Outcome:
  • Linked genes: Two parentals and two recombinants (14% recombination, divided into 7% each for recombinants, 43% each for parentals).
  • Unlinked gene: Monohybrid test cross resulting in a 1:1 ratio.

Steps to Predict Genetic Outcomes

1. Map Setup:
   • Identify linked and unlinked genes.
   • Draw a genetic map indicating distances (map units).
2. Crossing Strategy:
   • Separate genes into different chromosomes.
   • Begin with the single unlinked gene (monohybrid test cross).
   • For linked genes, focus on crossover events during meiosis.
3. Calculation of Progeny Types:
   • Use given recombination percentages to determine the frequency of recombinant and parental types.
   • Combine frequencies with gametes produced by male parent.
4. Phenotypic Expression:
   • Calculate expected phenotypic outcomes by combining recombinants and parentals with unlinked gene results.

Examples and Practice

Example 1

• Traits: White eyes (sex-linked, recessive), dumpy wings, black body (autosomal, 20 map units apart).
• Cross:
  • Female: White eyes on X, dumpy wings (not black body).
  • Male: Expressing black only.
  • F1 cross to a male with all three traits: Calculate potential gamete combinations.
• Recombination:
  • 20% total recombination for linked genes (10% each for recombinants).
  • 40% for each parental type.
• Outcome: Calculate the frequency of wild-type progeny.

Example 2

• Traits: Black body, curved wings (12 map units apart), white eyes (sex-linked).
• Cross:
  • Female: Homozygous for white eyes, not black or curved.
  • Male: Homozygous for black and curved.
  • F1 progeny are interbred (self-cross).
• Calculation:
  • Consider possible gametes and their recombination frequencies.
  • Determine phenotypic outcomes for specified conditions (e.g., white, curved, not black).
• Outcome: Calculate frequency of wild-type progeny and other specified phenotypes.

Important Concepts

• Map Distance: Directly equated to percent recombination.
• Test Cross vs. Inbreeding: Different strategies affect outcome calculations.
• Gamete Production: Understand the implications of crossover on gamete diversity.

Next Steps

• Explore predicting outcomes involving three linked genes.
• Work backwards from the genetic map to determine outcomes in complex scenarios.

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