1. Patterns of Inheritance
Gregor Mendel
Austrian monk
In 1860, developed laws of heredity
Performed crosses between garden pea plants
He concluded that plants transmitted distinct factors (genes) to offspring
Genes are found on chromosomes

2. Chromosomes come in pairs called homologous chromosomes
Same length
Same centromere location
Same gene location (locus)
May have different forms of the gene (allele)

3. Letters on homologous chromosomes stand for alleles
Ex. H = widow’s peak h = straight hairline
Capital letters represent dominant traits
Lowercase letters represent recessive traits
Based on his studies, Mendel came up with the law of segregation:
Each individual has two factors (genes) for each trait
The factors segregate (separate) during the formation of the gametes.
Each gamete contains only 1 factor (gene) from each pair of factors (genes).
Fertilization results in each new individual having 2 factors (genes) for each trait.

4. Phenotype: describes what the person actually looks like
Ex. Right handed or left handed
Purple petals or white petals
Genotype: refers to the alleles the chromosomes carry that are responsible for that trait
Ex. H = right hand, h = left hand
Dominant alleles will mask the expression of the recessive allele when they are together.

5. Homozygous: two alleles in the pair are the same
Ex. AA = homozygous dominant
Ex. aa = homozygous recessive
Heterozygous: two alleles in the pair are different
Ex. Aa
The gametes only have one allele for each trait.

6. One-trait Crosses
To see the probability of outcomes of crossing two parents, a Punnett square is used.
Monohybrid crosses: crossing of parent genotypes for 1 trait
Make a key to indicate parent genotypes
Combine all possible gametes using a Punnett square
Determine genotypes & phenotypes of all offspring
Write results as a ratio, fraction, or percent

7. The One-Trait Testcross
Testcross: the cross of a known homozygous recessive with an unknown genotype to see if the offspring have any dominant traits
Example: Both you and your brother are left-handed, but your parents are both right-handed. Right- handedness (H) is dominant over left-handedness (h). What are the genotypes of your parents?
Example: My husband and my two kids are left-handed, but I am right-handed. What is my genotype?

8. The Inheritance of Two Traits
Mendel’s law of independent assortment:
Each pair of factors (genes) separates independently (without regard to how the others separate).
All possible combinations of factors can occur in the gametes.
Dihybrid crosses: two different traits crossed together
A 16 square Punnett square is used
Example: Cross a father who is heterozygous for dimpled cheeks and a cleft chin with a mother who is the same.
D= dimples d= no dimples
R= cleft chin r= no cleft chin

9. Pedigrees
A pedigree is a chart of a family’s history with regard to a particular genetic trait.
Males = squares
Females = circles
A line between the two represents mating
A vertical lines branching off represents offspring
Affected individuals are shaded
Carriers are half shaded

10. Incomplete Dominance
Incomplete dominance: heterozygote is a mixture of the two alleles because neither allele dominates.
Example: Red four o’clock plants mixed with white four o’clock plants produce pink offspring. Cross a red four o’clock plant with a white four o’clock plant.
R1R1= red R2R2= white R1R2= pink

11. Codominance Both alleles show up in the heterozygote
Roan cattle are brown and white spotted cows
The allele for brown coat color is equally dominant to white coat color
Example:
R1R1= brown coat
R2R2= white coat
R1R2= brown & white spots (roan)
AB blood type

12. Multiple Alleles
There are several alleles for the same gene that control the inheritance of the trait.
A person can only have two of the possible alleles for the trait
ABO blood types are an example
IA= A antigen on red blood cells
IB= B antigen on red blood cells
i= neither A or B antigen on red blood cells
What are the possible genotypes for a person with A type,
B type, AB type, and O type blood?
Is it possible for a man who is type A blood and a woman who is type B blood to have a child who is type O blood?

13. Sex-Linked Inheritance
Traits controlled by genes on the sex chromosomes
Male sex chromosomes= XY
Female sex chromosomes= XX
X-linked genes are alleles found on the X chromosome
Most sex-linked genes are on X chromosome
The male Y chromosome is mostly blank for these diseases
Examples of sex-linked diseases: colorblindness, hemophilia, Duchenne’s Muscular Dystrophy

14. Polygenic Inheritance
Two or more sets of alleles determine the trait
Each dominant allele codes for a product
The more dominant alleles the “stronger” the phenotype
Results in a variety of phenotypes
Examples: skin color, height, weight, eye color
Aa and Bb alleles coding for skin color
AABB= very dark skin
AaBb= medium color skin
aabb= very light skin
Environmental factors, such as temperature and nutrition, can affect phenotypes.
Examples: height and nutrition, Himalayan rabbits & temperature

15. Inheritance of Linked Genes
All the alleles on one chromosome form a linkage group because they tend to be inherited together.
Example: red hair and freckles, blond hair and blue eyes
Genes that are farther apart on the chromosome are less likely to be linked because they separate more often in crossing over.