Lecture on Genetic Problems and Pedigrees

Introduction

• Focus on the review of general genetic problems:
  • Mendelian one-trait and two-trait crosses
  • Non-Mendelian traits (incomplete dominance, codominance, multiple alleles, sex-linked traits)
  • Pedigrees
• Five preliminary points:
  1. Use paper and pen/pencil for note-taking and problem-solving.
  2. Assumed familiarity with genetic vocabulary.
  3. Focus on concepts rather than symbols which can vary.
  4. Genetic problems determine probabilities, not certainties.
  5. Genetics is complex: traits can be polygenic or pleiotropic, epistasis and epigenetics may influence gene expression.

Mendelian Genetics

One-Trait Cross (Monohybrid)

• Example with guinea pigs using allele 'H' for hair:
  • Homozygous dominant: HH
  • Heterozygous: Hh
  • Homozygous recessive: hh
• Cross example: Hairless (hh) x Heterozygous (Hh):
  • Genotype ratio: 1:1 (Hh:hh)
  • Phenotype ratio: 1:1

Two-Trait Cross (Dihybrid)

• Example with cats and fictional "love for sinks" trait:
  • Dominant S allele means sink-loving.
  • Heterozygous genotype: HhSs
• Cross of two HhSs cats leads to phenotype ratio: 9:3:3:1.

Non-Mendelian Genetics

Incomplete Dominance

• Intermediate phenotypes (example: snapdragons)
  • Red (RR), White (rr), Pink (Rr)
  • Cross two pink: Genotype ratio: 1:2:1 (RR:Rr:rr); Phenotype ratio: 1:2:1 (Red:Pink:White).

Codominance

• Both traits fully expressed (example: chickens)
  • Black (BB), White (WW), Black & White speckled (BW)
  • Cross Black (BB) with speckled (BW): Genotype ratio: 1:1 (BB:BW); Phenotype ratio: 1:1.

Multiple Alleles

• Example: Blood types (A, B, AB, O)
  • Genotype for Type O: 25% chance from heterozygous type B and A parents.

Sex-Linked Traits

• Traits on sex chromosomes, often X-linked (example: hemophilia)
  • Male genotype for disorder: XhY
  • Female genotype to have disorder: XhXh
  • Example cross: Male with hemophilia x Female homozygous dominant -> 0% chance offspring with disorder.

Pedigrees

• Used to track traits across generations:
  • Shaded shape = trait of interest.
  • Circles = females; squares = males.
• Example: Autosomal recessive trait
  • Fill genotypes of shaded first; shaded individuals = recessive genotype (ee).
• Determining autosomal vs. sex-linked recessive:
  • Example shows not likely sex-linked due to impossible offspring genotypes.

Conclusion

• Importance of practicing genetic problems.
• Connect genetics to real-life applications and significance.
• Encouragement to stay curious and explore further.