1.2 Basic Principles of Inheritance

Inheritance is the foundation of genetics. It explains how traits are passed from parents to offspring through genes and alleles. Understanding these basic principles helps us predict and analyze genetic outcomes in various organisms.

Mendel's laws of segregation and independent assortment form the core of inheritance. These laws, along with concepts like dominance and recessiveness, allow us to solve genetic problems and understand more complex inheritance patterns beyond simple Mendelian genetics.

Basic Principles of Inheritance

Key terms in inheritance

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• Gene
  • Segment of DNA coding for a specific trait or characteristic
  • Located at a specific position (locus) on a chromosome
• Allele
  • Alternative forms of a gene
  • Can be dominant or recessive determining variations in a trait
• Genotype
  • Genetic makeup of an organism
  • Represented by the combination of alleles for a particular trait determining the phenotype
• Phenotype
  • Observable characteristics or traits of an organism
  • Influenced by the genotype and environmental factors (temperature, nutrition)

Mendel's laws of inheritance

• Law of Segregation
  • Each individual possesses two alleles for each trait
  • During gamete formation, alleles segregate and each gamete receives only one allele
  • Explains the 3:1 phenotypic ratio in monohybrid crosses (Aa x Aa → 3 dominant : 1 recessive)
• Law of Independent Assortment
  • Alleles for different traits are inherited independently of each other
  • Allows for prediction of outcomes in dihybrid crosses
  • Explains the 9:3:3:1 phenotypic ratio in dihybrid crosses (AaBb x AaBb → 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb)
• Significance of Mendel's laws
  • Provide a foundation for understanding basic principles of inheritance
  • Allow for prediction of offspring phenotypes and genotypes
  • Form the basis for more complex genetic concepts and inheritance patterns (epistasis, pleiotropy)

Genetic problem-solving

• Monohybrid cross
  • Involves a single trait controlled by one gene with two alleles
  • Use Punnett squares to determine probability of offspring genotypes and phenotypes
  • Example: Cross between heterozygous individuals (Aa x Aa) results in
    1. 1:2:1 genotypic ratio (1 AA : 2 Aa : 1 aa)
    2. 3:1 phenotypic ratio (3 dominant : 1 recessive)
• Dihybrid cross
  • Involves two traits controlled by two separate genes
  • Use Punnett squares to determine probability of offspring genotypes and phenotypes
  • Example: Cross between two heterozygous individuals (AaBb x AaBb) results in 9:3:3:1 phenotypic ratio
    1. 9 A_B_ (dominant for both traits)
    2. 3 A_bb (dominant for trait A, recessive for trait B)
    3. 3 aaB_ (recessive for trait A, dominant for trait B)
    4. 1 aabb (recessive for both traits)

Non-Mendelian inheritance patterns

• Incomplete dominance
  • Neither allele is completely dominant over the other
  • Heterozygous individuals display an intermediate phenotype
  • Example: Red (RR) and white (rr) flower cross results in pink (Rr) flowers
• Codominance
  • Both alleles are expressed equally in the heterozygous state
  • Heterozygous individuals display both phenotypes simultaneously
  • Example: Red (R) and white (W) cattle cross results in roan (RW) cattle with both red and white hairs
• Multiple alleles
  • A gene has more than two alleles
  • Inheritance follows the same principles as Mendelian genetics
  • Example: ABO blood group system in humans with alleles $I^A$, $I^B$, and $i$
    • $I^A$ and $I^B$ are codominant
    • $i$ is recessive to both $I^A$ and $I^B$