Genetics Study Notes

Mendelian Genetics

Mendel’s Theory of Inheritance

• Background: Gregor Mendel, an Austrian monk in the 1860s, studied inheritance patterns of simple traits in the garden pea.
• Key Terms:
  • Traits: Characteristics identifiable by distinct forms or alleles.
  • Phenotype: The visible expression of a trait.

Dominant and Recessive Traits

• Traits include:
  • Flower Color: Purple, White
  • Flower Position: Axial, Terminal
  • Seed Color: Yellow, Green
  • Seed Shape: Round, Wrinkled
• Each trait is represented by its corresponding genes and alleles.

Monohybrid Cross

• Focus: Study the inheritance of one trait using true-breeding (homozygous) parents.
• Example: Cross between purple (PP) and white (pp) flowers.
  • Parental Generation (P): PP (Purple) x pp (White)
  • F1 Generation: All Pp (Purple)
  • F2 Generation: Pp x Pp produces a 3:1 phenotypic ratio (3 Purple: 1 White).
• Genotypic Ratio: 1 PP : 2 Pp : 1 pp
• Important Reminder: Memorize monohybrid cross ratios, as these are critical for advanced genetics comprehension.

Law of Segregation

• States that each organism is diploid, having two copies of each gene; alleles segregate during meiosis, forming gametes with one allele each.
  • Homozygous: Two of the same alleles (e.g., PP or pp).
  • Heterozygous: Two different alleles (e.g., Pp).

Genetic Terminology

• Phenotype: Visual appearance of an organism.
• Genotype: Genetic composition of an organism.

Punnett Squares

• Function: Visual tool for predicting the genotypes and phenotypes from genetic crosses.
• Example: Tt x Tt (T = tall, t = short)
  • Genotypic Ratio: 1 TT : 2 Tt : 1 tt
  • Phenotypic Ratio: 3 Tall : 1 Short

Testcross

• Function: Determine the genotype of an organism with a dominant phenotype by crossing it with a homozygous recessive organism.
  • Outcomes:
  • If all offspring display the dominant phenotype, the genotype of the dominant parent is homozygous (TT).
  • If there are both dominant and recessive phenotypes, the genotype is heterozygous (Tt).

Dihybrid Cross

• Focus: Study the inheritance of two traits simultaneously, maintaining the Law of Independent Assortment.
  • Example: Seed color (Yellow - Y, Green - y) and seed shape (Round - R, Wrinkled - r).
  • Parental Cross Example: YYRR x yyrr
  • All offspring produce phenotypes in a 9:3:3:1 ratio for two traits.

Modified Mendelian Inheritance

• Incomplete Dominance: Heterozygote shows a phenotype that is a blend of the dominant and recessive traits (e.g., red + white = pink).

Codominance

• Both alleles are expressed distinctively in a heterozygote (e.g., AB blood type shows both A and B antigens).

Multiple Alleles

• Traits controlled by one gene with more than two forms (e.g., ABO blood type).
  • Types:
    • IA (Type A): Functional A enzyme
    • IB (Type B): Functional B enzyme
    • i (Type O): Non-functional enzyme

Antigens & Antibodies in Blood Types

• Each blood type has specific antigens present on red blood cells and antibodies in serum against the opposite blood type antigens.
  • Type A: IAIA or IAi; anti-B antibodies present.
  • Type B: IBIB or IBi; anti-A antibodies present.
  • Type AB: IAIB; no antibodies present.
  • Type O: ii; both anti-A and anti-B antibodies present.

Rh Factor

• Another trait influencing blood types. Rh+ has Rh antigens (can be Rh+/Rh+ or Rh+/Rh-), while Rh- has no Rh antigens (Rh-/Rh-).

Blood Transfusions

• Compatibility is determined by the recipient’s antibodies and the donor’s antigens to prevent agglutination.
• Universal Donor: Type O (ii) with no antigens.
• Universal Recipient: Type AB (IAIB) with no antibodies against A or B antigens.