Mendelian Genetics

Overview – Mendelian Genetics

Mendelian genetics refers to the set of foundational principles that describe how traits are inherited from one generation to the next. These principles, first described by Gregor Mendel through his pea plant experiments, explain patterns of dominance, segregation, and independent assortment of alleles. Modern genetics builds upon these rules, adding complexity through mechanisms like incomplete dominance, codominance, and epistasis. Understanding Mendelian inheritance is essential for clinical genetics, predictive inheritance, and the interpretation of genetic disorders.

Genetic Nomenclature

• Gene: A unit of heredity at a specific locus (location on a chromosome) that contributes to phenotype
• Locus: The physical position of a gene within the genome
• Alleles: Variants of a gene that cause phenotypic differences
• Dominant allele: Expresses its effect even in heterozygous state
• Recessive allele: Only expressed when homozygous
• Homozygous: Two identical alleles at a locus
• Heterozygous: Two different alleles at a locus
• Genotype: The specific combination of alleles an individual carries
• Phenotype: The observable trait or characteristic expressed by the genotype

Mendel’s Experiments & Laws

Gregor Mendel crossed pea plants with distinct traits and deduced the foundational laws of inheritance.

Law of Segregation

• Each individual inherits two alleles for each trait (one from each parent)
• These alleles segregate during gamete formation
• Each gamete receives only one allele
• Upon fertilisation, the alleles recombine at random
• Explains the 3:1 ratio seen in monohybrid crosses (e.g. Tall:Short plants)

Law of Independent Assortment

• Genes for different traits assort independently of each other during gamete formation
• Exceptions:
  • Linked genes (located close together on the same chromosome)
  • Sex-linked genes (on X or Y chromosomes)

Mendelian Ratios in Experiments

• F1 generation (pure Tall × pure Short): All Tall

• F2 generation (Tall × Tall hybrids):
  • 3:1 Tall:Short
  • Demonstrated hidden inheritance of the recessive trait

• Self-pollinated F2 plants showed:
  • Some only produced Tall offspring
  • Others produced both Tall and Short, depending on genotype

Exceptions to Simple Mendelian Inheritance

Multiple Alleles

• More than two allele options exist in the population, but an individual can carry only two
• Example: ABO Blood Group
  • IA = A antigen
  • IB = B antigen
  • i = no antigen
  • IA and IB are codominant; i is recessive

Codominance

• Both alleles are expressed equally in the phenotype
• Example: Blood type AB (IAIB) expresses both A and B antigens

Incomplete Dominance

• The heterozygous phenotype is an intermediate blend
• Example:
  • Red × White = Pink flowers (heterozygous)
  • Pink × Pink = 1 Red : 2 Pink : 1 White

Epistasis

• One gene masks the expression of another at a different locus
• Example: Labrador coat colour
  • Locus 1: Black (B) > Brown (b)
  • Locus 2: Yellow (e) is epistatic to B/b
  • Homozygous ‘ee’ results in yellow regardless of B/b

Sex-Linked Inheritance

• Genes found on X or Y chromosomes, usually X-linked
• Males (XY) are more likely to express X-linked recessive traits (only one X chromosome)
• Females (XX) must inherit two copies to express the trait
• Carrier females (X’X): 50% chance sons will be affected
• Common in conditions like Haemophilia A and Duchenne muscular dystrophy

Summary – Mendelian Genetics

Mendelian genetics outlines how traits are inherited through dominant and recessive alleles, with principles such as segregation and independent assortment forming the core of classical genetics. Although real-world inheritance can be more complex—due to codominance, multiple alleles, or epistasis—Mendel’s laws remain fundamental. For a broader context, see our Genetics & Cancer Overview page.