Beyond Mendel - Genetics
Incomplete Dominance
- In some cases, an intermediate phenotype is shown where neither allele is dominant
- In snapdragons, flower color can be red, pink, or white. The heterozygous condition results in pink flowers (or an intermediate trait)
- A white snapdragon crossed with a red snapdragon produces all pink offspring
- Two pinks crossed together produce 1/4 white, 2/4 pink, and 1/4 red
- When dealing with incomplete dominance and codominance it does not matter what letter you use, as long as the heterozygous condition always denotes the intermediate trait. In the diagram R is used, but you could also use W or even P. Ww = pink, Pp = pink if these letters are used.
Other Examples of Incomplete Dominance
Sickle cell disease in humans.
AA x aa = Aa (sickle cell trait), where some blood cells will have abnormal shapes
Carriers (Aa) can have a child with sickle cell disease (1/4)
Codominance
- Both alleles can be expressed or visible
- For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.
- Rr x Rr = 1/4 red, 1/2 roan, 1/4 white
Blood Types - Multiple Alleles and Codominance
- In humans, there are four blood types (phenotypes): A, B, AB, and O
- Blood type is controlled by three alleles. A, B, O
- O is recessive, two O alleles must be present for the person to have type O blood
- A and B are codominant. If a person receives an A allele and a B allele, their blood type is type AB
- Crosses involving blood type often use an I to denote the alleles - see chart.
When doing blood type crosses, you will need to know whether at type A or B person is heterozygous or homozygous. Type O's are automatically OO and type AB is automatically AB. Crosses are performed the same as any other.
Multiple Allele Traits and Epistasis
In labrador retrievers, coat color is determined by one gene with four different alleles.
Black (B) is dominant to brown (b)
The yellow coat color (ee) is epistatic. When it both (ee) are present the coat will be yellow
Polygenic Traits
- Polygenes mediate quantitative inheritance `````````````
- Individual heritable characters are often found to be controlled by groups of several genes, called polygenes.
- Each allele intensifies or diminishes the phenotype.
- Variation is continuous or quantitative (adding up) - also called quantitative inheritance
- Seed Color in wheat - aabbcc, Aabbcc, AaBbcc, AaBbCc, AABbCc, AABBCC (light, intermediate colors, dark)
- In humans - hair color, height, skin color
Environment and Phenotype
- Temperature, water, food sources can have an affect on how a gene is expressed
- Siamese cats have a gene that codes for darker pigments - this gene is more active at low temperatures. Parts of the body that are colder will develop the darker pigmentation - ears, feet tail of the siamese cats
