Gregor Mendel


Mendel discovered the basic principals of heredity by breeding garden peas in carefully planned experiments.

Among his greatest achievements were the creation of the following theories:

  • By the law of segregation, the two alleles for a character are packed into separate gametes.
  • By the law of independent assortment, each pair of alleles segregates into gametes independently
  • Mendelian inheritance reflects rules of probability
  • The discovery of the particulate behavior in genes


I. Law of Segregation

  1. made hybrid off-spring of plants and allowed them to self-pollinate
  2. came to the conclusion that genes have alternative forms (alleles) and that each organism inherits one allele for each gene from each parent
  3. alleles separate during gamete formation: the sperm or an egg carries only one allele
  4. after fertilization, if the two alleles of a gene are different, one (dominant allele) is masked.
  5. homozygous individuals have identical alleles for a given character and are true-breeding
  6. heterozygous individuals have two different alleles for a given character

II. Law of Independent Assortment

  1. law based on hybrid crosses between plants that were heterozygous for two characters
  2. characters may include flower color and seed shape, as plants were his experimental group
  3. alleles for each character segregate into gametes independently of alleles for other characters

III. Mendelian Inheritance

  1. the use of the Punnet square can determine the probability of acquiring specific genotypes and phenotypes
  2. the rule of multiplication states that the probability of a compound event is equal to the product of the seperate probabilities of the independent single events
  3. the rule of addition states that the probability of an event that can occur in two or more independent ways in the sum of the separate probabilities
  4. many human disorders follow patterns of inheritance
  • genetic disorders are inherited as simple recessive traits from phentotypically normal, heterozygous carriers
  • some disorders are due to dominant alleles

IV. Phenotype and Genotype

  1. in co-dominance, a heterozygote exhibits phenotypes in both of its alleles
  2. pleiotropy is the ability of a single gene to affect multiple phenotypic traits
  3. epistasis: one gene affects the expression of another gene
  4. quantitative characters very continuously---indicate polygenic inheritance, an additive effect of two or more genes on a single phenotypic character
  5. quantitative characters that are influenced by the environment are said to be multi-factorial

  • Homozygous: This genotype is one in which the alleles on homologous chromosomes are the same.
example: YY is homozygous dominant
yy is homozygous recessive

  • Heterozygous: This genotype is one in which the alleles on the homologous chromosomes are different.
example: Yy = heterozygous
*This concludes that the "true breeding" plants must have been homozygous.*


V. The Experiments

Mendel used 34 "true-breeding" strains of the common garden pea for his experiments. These strains differed from each other in very pronounced (visible) ways so that there could be no doubt as the results of a given experiment. Pea plants were perfect for such experiments since their flowers had both male (anthers) and female (pistils) flower parts and the flower petals never open, therefore no foreign pollen could enter and self fertilization was easy.
Mendel used seven different pairs of traits:

1) Seed form - round or wrinkled.
2) color of seeds - yellow or green (contents).
3) color of seed coat white or gray.
4) color of unripe seedpods - green or yellow.
5) Shape of ripe seedpods - inflated or constricted between seeds.
6) Length of stem - short 9 - 18 inches of long 6 - 7 feet.
7) Position of flowers - axial (on stem) or terminal (at tip of stem)

VI. Pedigree Analysis
  • can be used to hypothesize the possible phenotypes of individuals
  • make predictions about future offspring
  • predictions are usually statistical probabilities rather than certainties

  • A Punnet Square for a dihybrid cross