Comprehensive Biology Notes: Introduction to Genetics and Meiosis

Gregor Mendel and the Foundation of Genetics

• Gregor Mendel (the ‘Father of Genetics’): A scientist and mathematician who taught in a monastery and conducted pioneering genetic research using the ordinary garden pea plant.
• Pea Plant Biology and Reproduction:
  • Mendel understood that the male part of each flower produces pollen containing sperm, and the female part of the flower produces eggs.
  • In sexual reproduction, sperm and egg cells join through the process of fertilization.
  • Pea flowers are naturally self-pollinating, meaning sperm cells in the pollen fertilize egg cells within the same flower.
• Mendel’s Experimental Foundation: He utilized purebred pea plants that, if allowed to self-pollinate, produced offspring identical to themselves. He specifically studied traits controlled by a single gene.

Mendel’s Cross-Pollination Experiments

• Methodology: To breed plants from two different parents, Mendel used cross-pollination. This involved cutting off the male parts of one plant and dusting the female parts with pollen from a different plant.
• Generations and Outcomes:
  • P (Parent) Generation: Mendel crossed a White Flower plant with a Purple Flower plant.
  • F1 (First Filial) Generation: The offspring were $100\%$ purple. Mendel then allowed the $F1$ generation to self-pollinate.
  • F2 (Second Filial) Generation: This generation produced a ratio of roughly $75\%$ Purple to $25\%$ White ($3:1$ ratio).
• Repetition of Traits: Mendel repeated these experiments for various traits (e.g., tall/short plants, green/yellow seeds). In every case, the $F1$ generation displayed only one of the P traits, while the $F2$ generation consistently showed a $3:1$ ratio.
• Findings: These results proved that the P generation was homozygous (purebred) and that the $F1$ plants were heterozygous (hybrids) carrying recessive alleles.

Genetic Vocabulary and Fundamental Laws

• Key Definitions:
  • Gene: A general location on a chromosome that codes for a specific trait.
  • Allele: A specific version of a gene (e.g., $H$ vs. $h$).
  • Homologous Chromosomes: A pair containing one maternal and one paternal chromosome.
  • Homozygous (Purebred): Possessing two of the same alleles (e.g., $RR$ or $rr$).
  • Heterozygous (Hybrid): Possessing two different alleles (e.g., $Rr$).
  • Genotype: The genetic information or allele combination.
  • Phenotype: The physical appearance or observable look.
• Mendel’s Core Conclusions:
  • Characteristics are determined by factors (genes) passed from one generation to the next.
  • Alleles can be dominant or recessive; dominant alleles mask the presence of recessive alleles.
  • Law of Independent Assortment: The inheritance of one gene does not affect the inheritance of another (e.g., pea color does not influence flower color).
  • Law of Segregation: During the creation of gametes (sperm or eggs), the two alleles for a trait separate so that each gamete carries only one allele.

Monohybrid and Dihybrid Crosses

• Monohybrid Cross (One-Trait Punnett Square):
  • Used for traits controlled by a single gene. Uppercase letters denote dominant alleles (e.g., $P$), while lowercase letters denote recessive alleles (e.g., $p$).
  • Example (Eye Color): $R$ (Red, dominant) and $r$ (White, recessive).
    • Homozygous dominant genotype: $RR$.
    • Heterozygous cross ($Rr \times Rr$) results: $25\%$ chance of white eyes ($rr$), $75\%$ chance of red eyes ($RR$ or $Rr$), $50\%$ chance of being heterozygous.
• Dihybrid Cross (Two-Trait Punnett Square):
  • Predicts the outcome of two traits inherited simultaneously. It demonstrates independent segregation.
  • Example (Seed Shape and Color): $R$ (Round) is dominant to $r$ (Wrinkled); $Y$ (Yellow) is dominant to $y$ (Green).
    • Crossing two double-heterozygotes ($RrYy \times RrYy$) yields a $9:3:3:1$ phenotypic ratio.
    • Chance of homozygous recessive for both ($rryy$, wrinkled and green): $1/16$.
    • Chance of heterozygous for both ($RrYy$): $4/16$.

Complex Patterns of Inheritance (Non-Mendelian Genetics)

• Incomplete Dominance: Neither allele is completely dominant. The heterozygote displays a blended phenotype (e.g., $Red (RR) \times White (WW) = Pink (RW)$).
• Codominance: Both alleles are expressed equally in the phenotype (e.g., $Black \times White = Speckled$ chickens with both black and white feathers).
• Multiple Alleles: More than two versions of a gene exist within a population (e.g., rabbit coat color or human blood type alleles $A, B, O$).
• Polygenic Traits: Traits produced by the interaction of two or more genes (e.g., human skin color controlled by four genes, or eye color controlled by at least three genes).
• Maternal Inheritance: Traits controlled solely by mitochondrial or chloroplast DNA provided by the mother’s egg cell.
  • Example: Leaf color in Four O'Clock plants (Green, White, or Variegated) is determined by the mother's phenotype because the egg contains the chloroplasts.
  • Mitochondrial Disorders: Even if a father has the disorder, he cannot pass it to his offspring.
• Sex-Linked Disorders: Traits carried on sex chromosomes ($X$ or $Y$). Color-blindness is more common in males because they inherit the $X$ chromosome from their mothers; a female needs the gene on both $X$ chromosomes to be color-blind.
• Genomic Imprinting: Gene expression depends on which parent the gene was inherited from due to chemical (epigenetic) changes during gametogenesis. This occurs on nine known chromosomes and is a reversible inactivation, not a mutation.

Environmental Influence on Gene Expression

• Phenotypes are not determined solely by genes; environmental factors play a significant role.
• Sea Turtles: Sex is determined by nest temperature ($Warm = Female$, $Cold = Male$).
• Buckeye Butterflies: Individuals born in the fall are darker than those born in the summer to help increase body temperature for flight.
• Human Traits: Height can be stunted by poor nutrition. Skin tone can change based on sun exposure.

Human Blood Types

• Inheritance Patterns: Blood type combines multiple alleles, polygenetics, and codominance.
• ABO System: Alleles $A$ and $B$ are codominant, while $O$ is recessive.
  • Type A: Genotypes $AA$ or $AO$.
  • Type B: Genotypes $BB$ or $BO$.
  • Type AB: Genotype $AB$ (Codominant).
  • Type O: Genotype $OO$.
• Rh Factor: $Rh+$ is dominant over $Rh-$.
  • Homozygous Rh+: $++$.
  • Heterozygous Rh+: $+-$ .
  • Homozygous Rh-: $--$ .

Meiosis and Cell Division

• Somatic Cells vs. Gametes:
  • Somatic Cells: Diploid ($2N$) body cells produced via Mitosis (Humans: $2N = 46$).
  • Gametes: Haploid ($N$) sex cells (sperm or egg) produced via Meiosis (Humans: $N = 23$).
• Meiosis Process:
  • Purpose: To produce $4$ genetically unique haploid gamete cells.
  • Meiosis I: Chromosomes replicate and the cell divides. Includes Crossing Over (homologous chromosomes form tetrads and swap genetic material), which increases genetic diversity.
  • Meiosis II: The two cells divide again, resulting in $4$ total cells.
• Gamete Formation Differences:
  • Males: Produces four equal-sized sperm cells.
  • Females: Produces one large egg and three small polar bodies. Polar bodies result from uneven division, carry the same DNA, but usually disintegrate via apoptosis.
• Gene Linkage: Alleles of different genes located close together on the same chromosome tend to be inherited together. This can violate the Law of Independent Assortment. Mendel missed this because the traits he studied were on different chromosomes or very far apart on the same one.