Foundations of Genetics



A scientist and monk, Gregor Mendel, in the 1800s was the first to actually count numbers of offspring in crosses.

father of genetics.

Mendel's Experimental Design (image)  The garden pea Fig 11.2

true-breeding varieties Fig 11.3

P or parental generation


F1 generation or first filial generation.

F2 generation.

Used seven pairs of contrasting traits. Table 11.1

dominant trait


recessive. trait


3:1 dominant to recessive phenotype in the F2 generation. Fig 11.5

When F2 plants were allowed to self-fertilize, Mendel found a 1:2:1 ratio of true-breeding dominant to not true-breeding dominant to true-breeding recessive.

Today we use these terms:

Each individual has two factors (we now call these genes) for each character.  These are located on homologous chromosomes. Fig 11.6

character - a heritable feature.

trait - a variant for a character.






Analyzing Mendel's Results

Punnett Squares
Fig. 11.7

How Mendel analyzed flower color Fig 11.8

The Testcross - When Mendel did not know the genotype of an individual expressing a dominant trait, he did a test cross by crossing the individual with a homozygous recessive for the character. Fig 11.9

Mendel's Laws

Mendel's First Law: Law of Segregation

Mendel's Second Law: Law of Independent Assortment was determined when he worked with two traits at a time in dihybrid crosses. Fig 11.10 Crossing two individuals that are heterozygous for both characters yields a phenotypic ratio of 9:3:3:1. If the genes are located on the same chromosome, they would be linked.

Exceptions to Mendel's Laws

Continuous Variation - (polygenic) Fig. 11.12. Human skin color image and eye color. Eye color.

Pleiotropic Effects - Ex: cystic fibrosis. Fig 11.13

Incomplete Dominance -
Fig 11.14.

Environmental Effects - Fig 11.15

Fig 11.16    Fig 11.17
and  image

Codominance - Fig 11.18

Multiple Alleles with codominance

Human ABO blood group.  Fig. 11.19 and image and image


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