Mendelian and Non-Mendelian Heredity

• Mendelian Heredity

  Also known as Mendelian Inheritance and Mendelism, refers to a type of biological inheritance based on principles proposed by Gregor Mendel (Austrian monk and biologist; 1822 - 1884) in the 19th century
• Mendel's Experiments

  1. Conducted hybridization experiments with pea plants (Pisum sativum) in his monastery garden

  2. Cultivated and tested around 5,000 pea plants between 1856 and 1863

  3. Derived two generalizations known as Mendel's Principles (or Laws) of Heredity
• Mendel's Principles

  • Law of Dominance
  • Law of Segregation
  • Law of Independent Assortment
• Law of Dominance

  Every organism receives a trait from the mother and a trait from the father, one trait may have dominance over the other and mask the recessive trait to keep it from showing in the offspring, dominant traits are often the darker, heavier, or larger of the two genes, dominant alleles mask the expression of recessive alleles
• Law of Segregation

  The genes carried in the same place on homologous chromosomes will separate when gametes form, when the egg and sperm cell unite to form a new individual, the equal contribution of each parent is assured, each individual carries two alleles for a trait (one from each parent) that segregate during gamete formation
• Law of Independent Assortment

  The genes for one characteristic (alleles; denote a variant of a gene) are inherited independently of the genes from another characteristic (a second pair of alleles) except when gene linkage occurs (two genes are located on the same chromosome), genes on separate chromosomes are inherited independently, genes for different traits segregate independently during inheritance
• Non-Mendelian Heredity

  Refers to patterns of genetic inheritance that do not follow Mendel's classical laws
• Incomplete Dominance

  Neither allele dominates the other, a heterozygous individual displays an intermediate phenotype (observed traits), for instance, in snapdragons (Antirrhinum majus), the pigmentation of blossoms shows incomplete dominance
• Co-Dominance

  Both alleles of the same gene locus are expressed in the phenotype, for example, in chicken varieties: the allele for black feathers and the allele for white feathers are co-dominant, resulting in "erminette" chickens with speckled black and white feathers
• Sex-Linked Traits

  Refers to characteristics or traits that are influenced by genes carried on sex chromosomes, for example, hemophilia, red green color blindness, high blood pressure, DMD, and fragile X syndrome
• Multiple Alleles

  Unlike Mendelian inheritance (with only two alleles per gene), some genes have multiple alleles, for instance, blood type alleles (A, B, O) represent multiple forms of the same gene
• Polygenic Traits

  These traits result from the combined effects of multiple genes, for example, skin pigmentation is controlled by several genes working together