Genetics Inheritance Patterns

Incomplete Dominance

• Definition: Incomplete dominance occurs when the phenotype of a heterozygous individual is intermediate between the two homozygous phenotypes.

  • Example: Flower color in plants.
  • Red Parent: Homozygous dominant ($RR$)
  • White Parent: Homozygous recessive ($ww$)
  • Offspring: Heterozygous ($Rw$) = Pink flowers, an intermediate phenotype.

• Comparison to Mendelian Traits:

  • Mendelian Traits: Follow a distinct dominant/recessive pattern (e.g., Purple $PP$ vs White $pp$ in pea plants).
  • Incomplete Dominance: Results in a 1:2:1 ratio (1 Red: 2 Pink: 1 White) rather than 3:1.

Codominance

• Definition: Codominance occurs when both alleles in a heterozygous individual are fully expressed, resulting in distinct phenotypes.

  • Example: Roan cattle.
  • Red Parent: $RR$
  • White Parent: $WW$
  • Offspring: Heterozygous ($RW$) express both red and white hair (roan phenotype).

• Human Blood Types:

  • ABO blood system: Involves codominance.
  • Type A: $I^A I^A$ or $I^A i$ (A antigen present).
  • Type B: $I^B I^B$ or $I^B i$ (B antigen present).
  • Type AB: $I^A I^B$ (Both A and B antigens present).
  • Type O: $ii$ (No antigens present).
  • Importance of blood type compatibility in transfusions (e.g., type A can receive type A or O blood).

Environmental Effects on Phenotype

• Hydrangea Color:
  • The color is determined by soil pH; acidic soil ($pH < 6$) results in blue flowers, neutral or alkaline soil ($pH > 6$) results in pink flowers.
• Sex Determination in Leatherback Sea Turtles:
  • Environmental temperature affects sex determination; cooler temperatures yield males, and warmer temperatures yield females.
• Human Skin Color:
  • Polygenic inheritance influenced by environmental factors leads to a wide range of skin colors, which can change based on sun exposure.

Quantitative Genetics

• Involves studying traits that are influenced by multiple genes and environmental factors, affecting how traits are expressed (e.g., skin color, height).

Sex-Linked Traits

• X-linked Traits: Genes located on the X chromosome.
  • Example: Color blindness, Duchenne muscular dystrophy, hemophilia.
  • X-linked traits are of particular interest because males have only one X chromosome (hemizygous), making them more susceptible to express recessive X-linked traits.
  • Genetic inheritance patterns differ based on sex; males inherit Y from their father, and X from their mother only contributing X-linked traits.

Mendelian Inheritance Patterns

• Dominance relationships and allelic interactions play crucial roles in predicting phenotypes based on parental genotypes, utilizing Punnett squares for calculations.
• Punnett Square: Diagram used to predict genotype and phenotype combinations in offspring from a genetic cross.

Key Points

• Incomplete dominance results in intermediates, while codominance results in the expression of both traits.
• Environmental influences can modify phenotypes significantly.
• Understanding sex-linked traits is essential for predicting genetic disorders and inheritance patterns.