1. Beyond Dominant and Recessive Alleles

2. Exceptions to Mendel’s Principles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
P generation = Parent generation
F1 generation = First offspring generation
F2 generation = Second offspring generation

3. Incomplete Dominance
Incomplete Dominance – Cases when one allele is not completely dominant over another.
The heterozygous phenotype is between the two homozygous phenotypes
Ex: RR (Red flower) x rr (White lower) = Rr (Pink flower)

4. Codominance
Codominance – Both alleles contribute to the phenotype of the organism
Ex: Sickle Cell Anima – disorder in which the body makes sickle- shaped red blood cells. (“Sickle-shaped” = crescent shaped). Due to abnormal hemoglobin shape. Sickle cells are stiff and sticky and tend to block blood flow in blood vessels. Can cause pain, organ failure and raise the risk of infection

5. Sickle Cell Anemia
Sickle Cell Anemia – A patient’s Journey -
Evolution of Sickle Cell Malaria -

6. Multiple Alleles
Multiple Alleles – when genes have more than two alleles in the population
An individual can only have two alleles but a population can have more than two alleles

7. Sex-Linked Genes
Sex-linked Genes – genes located on the X and Y chromosomes
Ex: Colorblindness – an inability to distinguish colors
More common in males because males have just one X chromosome.
In males, all sex-linked alleles are expressed, even if they are recessive.
Females are normally carries for sex-linked alleles (meaning the female will not express the allele but can pass the allele on to an offspring). This is because females has two X chromosomes. Females will express the trait if the allele is present on both X chromosomes.

8. Sex-Linked Genes
A female would need for the allele to be located on both X chromosomes for a sex-linked trait to be expressed
In female cells, one X chromosome is turned off.
Ex: Calico Cat are always female because the gene for fur color is located on the X chromosome. Different X chromosomes are expressed on a single cat