Chapter 11.5 - Beyond Mendelian Genetics

Incomplete Dominance

• In some cases, an intermediate phenotype is shown
• 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.

• Sickle cell disease is incompletely dominant in humans. AA x aa = Aa (sickle cell trait), where some blood cells will have abnormal shapes

Codominance

• Both alleles can be expressed
• For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.
• This phenotype might seem to support the blending theory. (The blending theory predicts pink F1 progeny.)
• The F2 progeny, however, demonstrate Mendelian genetics. When the F1 roan individuals self-fertilize, the F2 progeny have a phenotypic ratio of 1 red:2 roan:1 white.
• This mode of inheritance is called incomplete dominance.
• The phenotypic outcomes for cow color and incomplete dominance in general can be explained biochemically.
• One allele of the gene codes for an enzyme that functions in the production of the red color. The other allele codes for the gene to make white color. If both alleles are present, both are expressed, resulting in a cow that has some red and some white.
• Mendel's laws are not compromised here, he just happened to find in peas examples of complete dominance only.

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.

• The blood type determines what antibodies are located within the blood. Type A blood has type B antibodies. If type B blood is put into their bodies, their immune system reacts as if it were a foreign invader, the antibodies clump the blood - can cause death.
• Type AB blood has no antibodies, any blood can be donated to them - they are called the "universal acceptors"
• Type O blood has no surface markers on it, antibodies in the blood do not react to type O blood, they are called the "universal donors"

Many Genes Have Multiple Alleles

• A population might have more than two alleles for a given gene.
• In labrador retriever, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles.
• Even if more than two alleles exist in a population, any given individual can have no more than two of them: one from the mother and one from the father.

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