2. Review of Mendel’s Laws
Law of Dominance - some alleles are dominant others are recessive.
Law of Segregation - Organisms inherit 2 copies of each gene, 1 from each parent. Organisms donate only 1 copy of each gene in their gametes (egg or sperm).
Law of Independent Assortment – genes for different traits can segregate independently during the formation of gametes (egg and sperm).

3. Chromosome Theory of Inheritance
Genes are located on chromosomes and the behavior of chromosomes during meiosis accounts for inheritance patterns.

4. Exceptions to Mendel’s Laws
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
Exceptions to Mendel’s Laws include:
Incomplete dominance - Codominance
Multiple Alleles - Polygenic traits

5. Ex. Red and white flowers make pink flowers
Incomplete Dominance
The heterozygous phenotype is somewhere between the two homozygous phenotypes.
Neither allele is completely dominant or recessive.
Ex. Red and white flowers make pink flowers

6. Incomplete Dominance Example #1
In humans, curly hair (HH) is incompletely dominant to straight hair (H’H’).  The heterozygous individual has wavy hair (HH’).  Cross two people with wavy hair.
Genotype Ratio: Phenotype Ratio:

7. Incomplete Dominance Example #2
Fitz loves growing flowers for his friend Olivia. Her favorite flowers, roses, are found in red (RR), blue (R’R’), and purple (RR’). What would happen if Fitz crossed a blue rose with a purple rose?
Genotype Ratio: Phenotype Ratio:

8. Codominance Both traits are fully and separately expressed.
Ex. Red and white flowers make red and white speckled flowers.
Ex. Blood Type (AB blood type is fully A and fully B)
Blood type is also an example of multiple alleles

9. Codominance Example #1
In horses, gray horses (GG) are codominant to white horses (WW).  The heterozygous horses(GW) is an appaloosa horse (a white horse with gray spots on the rump and loins). Cross a white horse with an appaloosa horse.
________      X      ________
Genotypic Ratios:
Phenotypic Ratios:

10. Blood Type An example of Codominance AND Multiple Alleles
Blood Type AB is fully A and fully B
Multiple Alleles
Alleles = A, B, and i

11. Blood Type An example of Codominance AND Multiple Alleles
Ex. If a mother has Type AB blood and a father has Type B blood, what are the possible blood types of their children?

12. Blood Type Who’s Your Daddy? Genetics
Oh no! The hospital has made a big mistake and lost track of which baby belongs to each parents. Can you help them fix this before they get sued?
Babies:
Baby 1: O
Baby 2: AB
Baby 3: A
Parents:
Mr. and Mrs. Allele: AB and AB
Mr. and Mrs. DNA: O and AB
Mr. And Mrs. Gene: B and O

13. Multiple Alleles Having more than two alleles for one gene
Ex. Alleles for blood type = A, B, i
Ex. Fur color in rabbits

14. Multiple Alleles Example
Example: What are the possible fur colors of the offspring if you cross a dark grey rabbit (Ccch) and a Himalayan rabbit (chc)?
Phenotype Key:
Full color = CC, Ccch, Cch, Cc
Chinchilla = cchch, cchcch, cchc
Himalayan = chc, chch
Albino = cc

15. Polygenic Inheritance
A trait produced by two or more genes
Usually shows a RANGE in phenotype
Ex. skin color, eye color, height, personality
Occasionally, epistasis can occur
When one gene overshadows all of the others
Ex. Labrador retriever fur colors, eye color, Albinism
Eye Color (in general)
TWO Genes = B gene and G gene
BB or Bb = Brown
bb = not brown
GG or Gg + bb = green
gg + bb = blue
Lab Fur Color:
TWO Genes = B gene and E gene
BB or Bb = Black
bb = Brown
EE or Ee + bb = Brown
ee = masks ALL b genotypes (epistasis) and makes YELLOW

16. Linked Genes
Linked genes: genes that are physically located on the same chromosome will be inherited together.
Ex. Blonde hair and blue eyes, red hair and freckles, large ears and broad nose
Linked genes can only be separated or broken apart during Crossing Over

17. Sex-Linked Traits
Males and females have exactly the same chromosomes for pairs These are autosomes.
The last pair of chromosomes, called sex chromosomes, determine the gender of the individual.
Females have XX
Males have XY

18. X vs. Y Chromosome Y chromosome carries FEW genes.
X chromosome contains MANY genes that affect MANY traits.
Sex-Linked Genes = genes on sex chromosomes (X or Y)
Image © NIH Image Gallery 2014:

19. Genes on the X chromosome are X-Linked.
X-Linked Genes
Genes on the X chromosome are X-Linked.
Females inherit gene as normal & principle of dominance applies.
Males inherit the gene on the X, but not the Y.
Because males have only one X, they express that trait whether it is dominant or recessive!
Carrier Daughter
Normal Son
Affected Son
Normal Daughter

20. Sex-Linked Example Colorblindness - located on X chromosome
Normal vision = XB
Colorblindness = Xb
Y chromosome does NOT have the gene
Ex. Cross a woman who is a carrier for color blindness with a man who is normal.
Image: © Christian Heilmann