1. Complex Inheritance

2. Simple Recessive Heredity
Most genetic disorders are caused by recessive alleles; the person must be homozygous recessive in order to express the disorder
Cystic Fibrosis
Tay-Sachs
PKU

3. Simple Dominant Heredity
Only one dominate allele must be present for the person to express the phenotype
Some examples of traits that follow this pattern are:
Hitchhiker’s Thumb
Tongue rolling
Free earlobes
Mid-digital hair

4. 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

5. 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

6. Incomplete Dominance in Humans
Curly hair is incompletely dominant
HH = straight hair
H’H’ = curly hair
HH’ = wavy hair

7. Codominance
Phenotypes of both homozygous parents are expressed in the heterozygous offspring in equal amounts
BOTH alleles are dominant AND BOTH alleles will be seen if genotype is heterozygous

8. Codominance
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

9. Codominance in Humans Sickle Cell Anemia
Common in African-American populations
Hemoglobin crystallizes, changing the shape of RBC
Sickle shaped cells slow blood flow and block small vessels
Sickle cells have short life span causing low blood cell count and anemia

10. Multiple Alleles The presence of 3 or more alleles for a genetic trait
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

11. Multiple Alleles BABA or BAB = Ash red feathers
BB or Bb = Grey feathers
bb = Brown feathers

12. 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.

13. 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

14. 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

15. 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

16. 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

17. Polygenic Inheritance
Inheritance pattern of a trait controlled by two or more genes…genes may be on the same chromosome or different chromosomes
2 or more genes control the trait
Genes can be on different chromosomes

18. 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

19. Environmental Influences
Temperature
Siamese Cats: body temp determines fur color
Warm areas—white fur
Cooler areas—black/brown fur
Light
Sun exposure increases melanin production, resulting in tan skin
Chemicals
Pesticide use can result in mutations and resistance

20. Environmental Influences
Hormones
Feathers of peahens versus peacocks
Females are dull and males have bright, attractive plumage
Sex changing fish
Age
Cells cannot divide indefinitely, eventually cannot repair themselves