1. Complex Inheritance

2. Mendelian Genetics
Mendel dealt with simple inheritance…dominant vs. recessive
Dominant traits only require one dominant allele to be present in order for the trait to be expressed
Hitchhiker’s thumb, tongue rolling, free ear lobes, hair on fingers
Recessive traits require two recessive alleles to be present in order for the trait to be expressed.
Most genetic disorders are caused by recessive alleles: Cystic Fibrosis, Tay-Sachs, PKU

3. Incomplete Dominance
Phenotype of the heterozygote is an intermediate between the two homozygous parents
Neither allele is dominant…the alleles blend together if offspring is heterozygous

4. Incomplete Dominance
In snapdragons, neither red or white is completely dominant
Red is represented by R and white by R’ (R prime)
RR=red flowers
R’R’= white flowers
RR’=pink flowers
In humans, curly hair is incompletely dominant
HH = straight hair
H’H’ = curly hair
HH’ = wavy hair

5. Codominance
BOTH alleles are dominant AND BOTH alleles will be seen if genotype is heterozygous
Chickens with black feathers are homozygous for the B allele (BB)
Chickens with white feathers are homozygous for the W allele (WW)
A heterozygous chicken (BW) would have black and white feathers

6. Codominance in Humans Sickle Cell Anemia
Hemoglobin crystallizes, changing the shape of RBC
Sickle shaped cells slow blood flow and block small vessels
Healthy People with NO Sickle Cell are AA
Affected people with severe sickle cell are SS
Carriers who may show symptoms are AS

7. Multiple Alleles
The presence of 3 or more alleles for a genetic trait. ALL alleles are on the SAME gene!
3 or more genotypes are possible for one trait…so you get 3 or more phenotypes are possible for one trait
Ex: Pigeons
There are three alleles that code for feather color, but each individual can have only two.
BA=ash red > B=grey > b = brown
BABA, BAB, BB, Bb, bb

8. Multiple Alleles in Humans
In humans, there are 3 alleles that code for blood type
3 alleles (IA, IB, and i) determine 4 blood types (A, AB, B, and O).
IAIA or IAi result in type A blood
IBIB or IBi result in type B blood
IAIB results in type AB blood
ii results in type O blood
If a person is given the wrong blood during a transfusion, incompatible blood cells can clump together and cause death.

9. Sex-Linked Inheritance
Traits are controlled by genes located on sex chromosomes
Sex chromosomes determine sex
XX = female
XY = male
Y chromosome doesn’t have same genes as X
Disorders affect males more than females, because males only have ONE X chromosome

10. Sex-linked Inheritance
Ex: Fruit flies
XR=Red Eyes
Xr=White Eyes
Females
XRXR or XRXr = Red Eyes
XrXr = White Eyes
Males
XRY = Red Eyes
XrY = White Eyes

11. Sex-Linked Inheritance in Humans
Red-green color blindness
Person cannot tell the difference between red and green.
Recessive allele affects the red and green receptors in the eye
Inherited on the X chromosome

12. Sex-Linked Inheritance in Humans
Hemophilia
Missing clotting factor so cannot stop bleeding
Treatments include clotting factor injections and blood transfusions
Inherited on the X chromosome

13. Polygenic Inheritance in Humans
Involves 3-4 different genes
Offspring look like an intermediate or combination of parents’ skin color
Eye and Hair Color are also polygenic