1. Modes of Inheritance

2. A Summary of Mendel's Principles
Genes are passed from parents to their offspring.
If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

3. In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated (separated) from each other when gametes are formed.
The alleles for different genes usually segregate independently of one another.

4. Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

5. Incomplete Dominance 
When one allele is not completely dominant over another it is called incomplete dominance.
In incomplete dominance, the heterozygous phenotype is a blend between the two homozygous phenotypes.

6. RR
A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW
Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring.

7. Codominance 
In codominance, both alleles contribute to the phenotype.
In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.
Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.

9. Heterochromatic eyes are codominant

12. Rh Factors and blood typing reference

13. Multiple Alleles 
Genes that are controlled by more than two alleles are said to have multiple alleles.
An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.
Blood type is determined by a single gene that has at least three different alleles
A rabbit's coat color is determined by a single gene that has at least four different alleles.

14. Different combinations of alleles result in the colors shown here.
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles
ch = Himalayan; color in certain parts of the
body; dominant to
c allele
c = albino; no color;
recessive to all other
alleles
Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc.
AIbino: cc
Himalayan: chc, or chch
Full color: CC, Ccch, Cch, or Cc
Chinchilla: cchch, cchcch, or cchc

15. What is Sex Linkage?
In addition to their role in determining sex, the sex chromosomes have genes for many characters.
Genes located on a sex chromosome are called sex linked genes.
In humans the term usually refers to X-linked characters: genes located only on X chromosomes.
Fathers can pass X-linked alleles to their daughters, but not sons.
Mothers can pass sex-linked alleles to both sons and daughters.

16. Red-green color blindness
X chromosome has a locus for color vision with two alleles:
XN = Normal color vision
Xn = Red-green color blindness
Y chromosome does not have a color vision locus.
If a male receives the Xn allele he will have impaired color vision (Xn Y), whereas a female with XNXn will not.

17. Red-green color blindness
Parental
Phenotypes Carrier Female x Normal Male
Genotypes XNXn XNY
Gametes XN Xn XN Y
Female Gametes XN Xn
Male Gametes
XN XN
XN Xn Y
XN Y
Xn Y
Phenotypes
Normal Female : Carrier Female : Normal Male : Color blind Male
: : :

19. COLORBLINDNESS

20. HEMOPHILIA

21. HEMOPHILIA

22. HEMOPHILIA

23. MUSCULAR DYSTROPHY

24. MUSCULAR DYSTROPHY

25. HUMAN BALDNESS

26. HUMAN BALDNESS

27. Beyond Dominant and Recessive Alleles
Polygenic Traits  
Traits controlled by two or more genes are said to be polygenic traits.
Skin color in humans is a polygenic trait controlled by more than four different genes.