Complex Inheritance Notes

Complex Inheritance

Do Now (January 28th, 2025)

• Huntington’s Disease Affected Family Members:
  • I-1, II-2, II-3, II-7, III-3
• Huntington’s Disease Inheritance:
  • Dominant trait.
  • Reasoning: No carriers exist, and the illness does not skip generations.
• Number of Children of Individuals I-1 and I-2: 6
• Number of Daughters of Individuals II-1 and II-2: 2
• Relationship Between Individuals III-2 and II-4: Uncle (II-2) to Niece (III-2)

Incomplete Dominance

• Basics:
  • Occurs when one allele is not completely dominant over another.
  • Results in an intermediate phenotype.
  • Example: Snapdragon flower color.

Incomplete Dominance Examples

• Snapdragon Flower Colors:
  • Red-flowered snapdragon ($C^RC^R$) crossed with white-flowered snapdragon ($C^WC^W$) yields pink-flowered offspring ($C^RC^W$).
  • Traits can separate in the next generation.
  • Using 'C' for the COLOR gene; red and white are both DOMINANT alleles, hence capital 'C'.
  • Notation distinguishes incomplete dominance from complete dominance, indicating alleles don’t dominate each other.
  • Incomplete dominance causes a blending of traits.

Punnett Square Example

• Cross between two pink snapdragon flowers ($C^RC^W × C^RC^W$):

  	$C^R$	$C^W$
  $C^R$	$C^RC^R$	$C^RC^W$
  $C^W$	$C^RC^W$	$C^WC^W$

• Phenotype Ratio: 1:2:1 (Red:Pink:White)

More Incomplete Dominance Examples

• Four O'clock flower colors
• Humans:
  • Hair Texture:
    • Homozygous straight hair.
    • Heterozygous wavy hair.
    • Homozygous curly hair.

Incomplete Dominance Example: Palomino Horses

• Parent phenotypes:
  • CBCB
  • CWCW
• Parent genotypes:
  • CBCB
  • CWCW
• Gametes:
  • CB
  • CW
• Offspring genotype:
  • CBCW
• Offspring phenotype:
  • Palomino

Practice Questions Set 1

1. If a red flower ($C^RC^R$) is crossed with white ($C^WC^W$), what percentage will be pink?
   • 100% of offspring will be pink ($C^RC^W$).
2. What is the phenotype ratio in incomplete dominance crosses (Pink and Pink)?
   • 1:2:1 phenotype ratio
3. How does incomplete dominance differ from complete dominance?
   • Incomplete dominance shows blending; complete dominance shows a fully dominant trait only.
4. A pink snapdragon is crossed with a white snapdragon. What proportion will be red?
   • 0% will be red - needs $C^RC^R$ genotype which isn't possible in this cross.

Codominance

• Introduction:
  • Both alleles are expressed equally in heterozygous condition.
  • No blending occurs.
  • Both traits appear in their full form.
  • Example: Roan cattle color pattern ($C^WC^R$).
  • Genotypes:
    • CWCW
    • CRCR
    • CWCR

Codominance – Sickle Cell

• Sickle-shaped cells are less flexible than normal red blood cells, making it difficult to pass through small blood vessels.
• This can cause blockages in blood flow, leading to pain and tissue damage.
• Sickle-cell disease affects red blood cells and their ability to transport oxygen.
• Changes in hemoglobin-the protein in red blood cells-cause those blood cells to change to a sickle, or C-shape.

Sickle Cell Alleles

• $H^N$ for the normal hemoglobin allele.
• $H^S$ for the sickle cell allele.
• Parent 1 – normal ($H^NH^N$)
• Parent 2 – sickle cell ($H^SH^S$)
• All offspring will have half normal blood cells and half sickle cell (Co Dominance) with no symptoms of the disease but still carry the sickle cell trait ($H^NH^S$).

Heterozygous Cross

• Parent 1 – heterozygous ($H^NH^S$)
• Parent 2 – heterozygous ($H^NH^S$)
• Genotypes of offspring:
  • Healthy/normal cells ($H^NH^N$).
  • Half normal half sickle cells ($H^NH^S$).
  • Has sickle cell, won’t last long ($H^SH^S$).
• Genotype ratio: 1:2:1
• Phenotype ratio: 1:2:1
  • 1 Normal ($H^NH^N$) 25%
  • 2 some sickle cell some normal ($H^NH^S$) 50%
  • 1 Sickle Cell Disease ($H^SH^S$) 25%

Practice Questions Set 2

1. What's the difference between codominance and incomplete dominance?
   • Codominance shows both traits fully, incomplete shows blending
2. In cattle, what color results from red × white?
   • Roan pattern (both colors visible)
3. Why is Sickle cell anemia an example of codominance?
   • because both alleles (normal hemoglobin $H^N$ and sickle cell hemoglobin $H^S$) are expressed equally in the phenotype of a heterozygous individual.
4. What would be the genotype ratio in a cross between two heterozygous codominant traits?
   • 1:2:1 genotype ratio (like Mendel's ratios)

Codominance vs. Incomplete Dominance Explained

• Codominance:
  • Both alleles contribute equally and visibly to the phenotype in a heterozygous individual.
• Incomplete Dominance:
  • The heterozygous phenotype is an intermediate between the two parental phenotypes.
• Example for Codominance:
  • In cattle, the allele for red coat color (R) and white coat color (W) are codominant; heterozygotes ($C^RC^W$) have a roan coat, where both red and white hairs are present.
• Example for Incomplete Dominance:
  • In snapdragons, the allele for red flowers (R) and white flowers (W) show incomplete dominance; heterozygotes ($C^RC^W$) produce pink flowers.
• Phenotypic Ratios:
  • Codominance: In a cross between two heterozygotes, the phenotypic ratio would be 1 red : 2 roan : 1 white.
  • Incomplete Dominance: In a cross between two heterozygotes, the phenotypic ratio would be 1 red : 2 pink : 1 white.