Variations to Mendel's Laws

Study Notes

In Mendel's pea plant crosses, the F1 generation always resembled one of the two parental varieties, which is called complete dominance
Complete dominance occurs when the dominant allele has the same phenotypic effect whether present as one or two copies
Mendel's laws cannot explain all inheritance patterns
Variations exist in inheritance that Mendel's laws do not fully explain

Incomplete dominance results in intermediate phenotypes
The F1 generation hybrids fall between the two parental varieties in incomplete dominance
When red snapdragons are crossed with white snapdragons, the F1 hybrids have pink flowers
This third phenotype results from the heterozygous flowers having less red pigment than the red homozygotes
The blending hypothesis suggests that the red and white flowers would never reappear from matings between the pink flowers
Incomplete dominance occurs in humans
Hypercholesterolemia, a disease causing dangerously high cholesterol levels, is an example
Normal individuals are HH for hypercholesterolemia
Heterozygotes (HH') have cholesterol levels about 2x as high as normal and are prone to atherosclerosis and heart attacks around age 30
Homozygous recessive individuals (H'H') have about 5x the normal cholesterol level and can have heart attacks as early as age 2
Normal individuals have receptors on their cells that pick up cholesterol from the blood stream and promote its breakdown
Heterozygous individuals have less of these receptors
Homozygous recessive individuals have none of these receptors

Many genes have more than 2 alleles in the population
Most of the time, there are more than 2 alleles
An individual carries only two alleles at a time; however, there can be different combinations of two from others
Human blood type is an example; people have A, B, AB, or O blood types
Three alleles result in four different blood types: A, B, AB, or O
The letters refer to two carbohydrates: A and B
These are found on the surface of human red blood cells

If a person has carb A, they have type A blood
If they have carb B, they have type B blood
If they have both, they have AB blood
If they have neither, then they have the O blood type
If someone with type A blood receives a transfusion from someone with type B blood, the body makes antibodies against the B blood because it doesn't recognize the B carbohydrate
These antibodies will cause the blood to clump up and form clots which is potentially fatal
The blood-typing test uses clumping to determine blood type
Type O blood lacks a sugar, so the recipient's body will not attack it as foreign as the universal donor and will not clot
Type AB blood is the universal acceptor because they can accept any blood type without causing clumping.
There are three different alleles - IA is the A allele (I is the protein that adds the sugars onto the blood cells), IB is the B allele, and i is the O allele
IA and IB are dominant to i
There are 6 possible genotypes: I^i and IAIA (type A), IBi and IBIB (type B), IAIB (type AB), and ii (type O)
IA and IB are codominant, so individuals with type AB blood express both alleles