Incomplete Dominance & Codominance

There is no dominant or recessive, the heterozygous condition results in a "blending" of the two traits. Example: Snapdragons can be red, white, or pink (heterozygous)

Dihybrid Crosses: Crosses that involve 2 traits.

For these crosses your punnet square needs to be 4x4

In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio.

If you cross other combinations, you will need to do a square. Try RrYy x rryy. Click to see solution.

Sex Linked Traits

The genes for these traits are on the X chromosome, because boys only receive one X chromosome they are more likely to inherit disorders passed to them from their mother who would be a carrier.

Hemophilia and Colorblindness are sex linked traits, the punnet square below shows how a woman who is a carrier passes the trait to her son, but not her daughters.

Muliple Allele Traits

Traits that are controlled by more than two alleles. Blood type in humans is controlled by three alleles: A, B, and O

Examples of Blood type crosses

Blood Transfusions

Blood can only be transferred to a body of a person who's immune system will "recognize" the blood. A and B are antigens on the blood that will be recognized. If the antigen is unfamiliar to the body, your body will attack and destroy the transfused blood as if it were a hostile invader (which can cause death).

O is like a blank, it has no antigens. O is called the universal donor because a person can receive a transfusion from O blood without having an immune response

AB is the universal acceptor, because a person with AB blood has both the A and B antigens already in the body, A and B blood can be transfused to the person (as well as O) and the body will recognize it and not attack.

Polygenic Traits

Traits controlled by many genes: hair color, height weight, intelligence

Sex Influenced Traits

Traits are influenced by the environment. Pattern baldness affects men because testosterone activates the genes.

Environmentally Influenced Traits

Siamese cats have dark ears and feet due to the temperature. Height in humans is influenced by the environment (diet)

[ Human Genetics ]