Detailed Notes on Mendel's Principles of Inheritance

Mendel's Principles (Laws) of Inheritance

• Mendel postulated specific laws of heredity based on the results of his experimental work with garden peas. These are recorded as the Principles of Genetics.
• Mendel's original "factors" are now scientifically identified as genes.

Law of Segregation or Purity of Gametes

• This is defined as a universal law that applies during gamete formation in all sexually reproducing organisms.
• Mechanism of Segregation:
  • During the formation of gametes, the two chromosomes of each pair separate (segregate) into two different cells which then form the gametes.
  • The two factors of a pair pass into different gametes.
  • Result: Each gamete receives exactly one member of a pair of factors. Consequently, the gametes are considered "pure."

Seven Traits Studied by Mendel

• Mendel focused on seven specific traits in the garden pea (Fig. 22.2):
  • Flower colour: Purple ($pp$) or White ($pp$). (Note: While the diagram uses $pp$ for both, they represent alternative forms of the trait).
  • Stem length: Long ($TT$) vs. Short ($tt$).
  • Seed shape: Round ($RR$) vs. Wrinkled ($rr$).
  • Seed colour: Yellow ($YY$) vs. Green ($yy$).
  • Flower position: Axial vs. Terminal.
  • Pod shape: Inflated vs. Constricted.
  • Pod colour: Green vs. Yellow.

Law of Dominance

• Inheritance of many traits—such as eye colour, flower colour, or seed shape—is controlled by a single pair of genes.
• Homozygous Condition: Occurs when the two genes of a pair are of the same kind (e.g., both genes for brown eye colour or both for red flower colour).
• Heterozygous Condition: Occurs when a pair of chromosomes carries the gene controlling the same feature in two different forms (e.g., a red flower gene on one chromosome and a white flower gene on its homologue).
• Alleles: Defined as alternative forms of the same gene. For example, the factors for red flower colour and white flower colour are alleles of the flower colour gene.
• The Law of Dominance specifically maintains that when two genes of a pair represent contrasting characters, the expression of one is dominant over the other.
• Dominant Gene:
  • Expression occurs in both homozygous and heterozygous conditions.
  • Example: If both genes of an allele are for tallness ($TT$) or if one is for tallness and one for dwarfness ($Tt$), the pea plants will be tall.
• Recessive Gene:
  • This is the opposite of the dominant gene.
  • The recessive feature (e.g., dwarfness) is expressed only when both genes of the allele are in the homozygous condition ($tt$).
• Mendel found the law of dominance to be true in both monohybrid and dihybrid crosses for all seven characteristics studied in the garden pea.

Law of Independent Assortment

• This law refers to the inheritance of two features simultaneously, where each feature is controlled by a pair of genes.
• Definition: Genes for two different features are passed down into offspring independently.
• The segregation of one pair of factors is independent of the segregation of factors belonging to any other pair of factors or allelic pair.
• Dihybrid Cross Example (Fig. 22.3):
  • Labels: $R$ stands for red flower colour; $r$ stands for white flower colour; $T$ stands for tallness; $t$ stands for dwarfness.
  • Parents: Red Tall ($TTRR$) $\times$ White Dwarf ($ttrr$).
  • Gametes: ($TR$) and ($tr$).
  • $F_1$ Generation: All offspring are ($TtRr$) (Tall, red) and are subsequently self-pollinated.
  • $F_2$ Progeny Genes in Male and Female Gametes:
    • Possible gamete combinations: ($TR$), ($Tr$), ($tR$), ($tr$).
  • $F_2$ Genotypic Combinations:
    • ($TR$) $\times$ ($TR$) = ($TTRR$) (Tall red)
    • ($TR$) $\times$ ($Tr$) = ($TTRr$) (Tall red)
    • ($TR$) $\times$ ($tR$) = ($TtRR$) (Tall red)
    • ($TR$) $\times$ ($tr$) = ($TtRr$) (Tall red)
    • ($Tr$) $\times$ ($TR$) = ($TTRr$) (Tall red)
    • ($Tr$) $\times$ ($Tr$) = ($TTrr$) (Tall white)
    • ($Tr$) $\times$ ($tR$) = ($TtRr$) (Tall red)
    • ($Tr$) $\times$ ($tr$) = ($Ttrr$) (Tall white)
    • ($tR$) $\times$ ($TR$) = ($TtRR$) (Tall red)
    • ($tR$) $\times$ ($Tr$) = ($TtRr$) (Tall red)
    • ($tR$) $\times$ ($tR$) = ($ttRR$) (Dwarf red)
    • ($tR$) $\times$ ($tr$) = ($ttRr$) (Dwarf red)
    • ($tr$) $\times$ ($TR$) = ($TtRr$) (Tall red)
    • ($tr$) $\times$ ($Tr$) = ($Ttrr$) (Tall white)
    • ($tr$) $\times$ ($tR$) = ($ttRr$) (Dwarf red)
    • ($tr$) $\times$ ($tr$) = ($ttrr$) (Dwarf white)
  • Dihybrid Phenotypic Ratio: $9$ Tall Red : $3$ Tall White : $3$ Dwarf red : $1$ Dwarf white.

Genotype and Phenotype

• Genotype: The composition of genes which controls the outside expression.
• Phenotype: The outside expression which can be seen (visible traits).
• Phenotypic Ratio: The ratio of progeny observed in crosses based on their physical expression.

Deviations from Mendel's Laws

• As genetic research progressed, scientists discovered that Mendel's laws do not hold true in all cases.
• Known deviations from Mendelian inheritance include:
  • Incomplete dominance.
  • Codominance.
  • Polygenic inheritance.