Mendelian Genetics 6.3 - 6.5

The groundwork of our understanding of genetics came from the Austrian monk known as Gregor Mendel in the mid 1800's.

He was fascinated with pea plants and used them in his experiments because they reproduced quickly.

He crossed two different color pea plants which he thought were "purebred" and observed their offspring traits (characteristics passed from parent to offspring).




He concluded that there was something causing the pea plant to appear different than the others.

This same concept can be used for our genetic makeup....

We have different forms of genes in our chromosomes called Allelesrepresented by letters. 



The terms Dominant (Strong allele) and Recessive (Weak allele) are used to determine the outcome of certain traits.

Dominant: Expressed when at least one strong allele is present.

Recessive: Only expressed when two identical weak alleles are present.

When 2 alleles are identical they are known as Homozygous. When they are different they are known as Heterozygous.




Using this information we are able to determine the Genotype (Genetic makeup=GG, gg, or Gg) and the Phenotype (Physical characteristic=brown, tall, round etc....)




Tutorial on Punnett Squares 

Monohybrid Cross: When one trait is used for example one letter T (TT x tt) or(Tt x tt) or (tt x tt). The capital letter is written first.


Remember This Handout?

Dihybrid Cross: When two traits are used for example two letters R and Y. RrYy x RrYy (Tall & Green).

Laws of Genetics:

Law of Independent Assortment - alleles separate independently from each other during meiosis. 

Lets try some Punnett Square Problems....

Practice