lecture recording on 07 February 2025 at 12.50.53 PM

Genetics and Flower Color

• Traits and Phenotypes

  • Focusing on flower color in plants (purple and white).

  • Phenotype: The visible traits (e.g., flower color) resulted from genotype.

  • Purple flowers arise from a functional gene that produces a pigment; non-functional results in white flowers.

• Dominance vs. Recessive

  • Dominant: Trait that is expressed (e.g., purple flower).

  • Recessive: Trait that is not expressed when a dominant allele is present (e.g., white flower).

  • Example of dominance in blood types, featuring multiple alleles—dominance, recessiveness, and codominance.

Mendel's Principles

• True Breeding

  • Purple and white flowers are true breeding, meaning they consistently produce offspring with the same phenotype.

  • True breeding traits are homozygous (same alleles).

• Mendel's Experiments

  • Cross-pollination using closed flowers to control breeding.

  • Results: F1 generation consistently yielded purple flowers when crossing purple and white.

  • Subsequent F2 generation displayed a phenotypic ratio: 3 purple flower plants to 1 white flower plant.

• Genotypes and Gametes

  • Homozygous (same alleles) vs. Heterozygous (different alleles).

  • A purple flower could be homozygous dominant (PP) or heterozygous (Pp), while a white flower is homozygous recessive (pp).

  • Cross of heterozygous individuals leads to a range of genotypes in F2 generation.

Law of Segregation

• Alleles segregate independently during gamete formation.

  • Example of monohybrid cross:

    • Cross true breeding purple (PP) with true breeding white (pp) → F1 (Pp) → F2 has genotypic ratio 1:2:1 (PP:Pp:pp) and phenotypic ratio 3:1.

Dihybrid Crosses

• Principle of Independent Assortment

  • Two traits (flower color and seed color) can be inherited independently.

  • Example: Purple flowers (dominant) and yellow seeds (dominant) crossed with white flowers (recessive) and green seeds (recessive).

  • Gametes from each parent combine in a Punnett square, leading to a 9:3:3:1 phenotypic ratio for the offspring.

Incomplete Dominance

• Example of Incomplete Dominance

  • Red flower crossed with white flower producing pink flowers.

  • Genotypic ratio: 1 red (RR), 2 pink (RW), 1 white (WW).

  • Pink flowers result from one dose of red protein (from red allele) and no protein from the white allele.

Blood Typing in Humans

• Blood Group Alleles

  • Type A (dominant allele for A antigen), Type B (dominant for B antigen), Type O (recessive; no antigens).

  • Type AB individuals lack antibodies against both types of antigens, allowing for universal recipient status.

  • Type A individuals can accept A and O but risk reactions with B blood due to antibodies.

• Rh Factor

  • Rh+ and Rh- factors are inherited as alleles.

  • Pregnancies involving Rh- mother and Rh+ father can lead to immune conflict in subsequent pregnancies without treatment (RhoGAM injection).

Summary of Key Concepts

• Dominance: Describes which trait is expressed in the presence of another.

• Recessiveness: Refers to traits that are masked unless two recessive alleles are present.

• Mendelian Ratios: Classic ratios observed in pedigree analysis (e.g., 3:1 in monohybrid crosses, 9:3:3:1 in dihybrid crosses).

• Genotype and Phenotype: The genotype underpins the phenotype; understanding both is key in genetics.