1. Mendel and the Gene Idea

2. Gregor Mendel
Studied pea plants for specific traits and how those traits passed from one generation to the next
He noticed a 3:1 inheritance pattern which led to the Law of Segregation
Alternative versions of genes account for variations in inherited characters
For each character, an organism inherits 2 alleles, one from each parent
If the 2 alleles at a locus differ, then one, the dominant allele, determines the organisms appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
The two alleles for a heritable character segregate during gamete formation and end up in different gametes

3. Law of Segregation

4. Law of Independent Assortment
Each pair of alleles segregates independently of each other pair of alleles during gamete formation

5. Rules of Probability
The outcome of any event is unaffected by what has happened previously
The multiplication rule
To determine the probability of 2 events that do not depend on each other, multiply the two probabilities
The addition rule
The probability that any one of 2 or more mutually exclusive events will occur is calculated by adding their individual probabilities

6. Beyond Mendel
Mendel looked at traits that had 2 possible characteristics and 2 possible alleles, but some traits are more complex
In single genes this can be seen in:
Codominance
Multiple Alleles
Pleiotropy

7. Incomplete Dominance
For some genes, neither allele is completely dominant over the other
Example: When red snapdragons are crossed with white snapdragons, the offspring are pink
Should not be confused with codominance where both traits are seen, but it is not a mix or intermediate (ie AB blood)

8. Multiple Alleles
Many genes exist with more than two possible allelic forms
Example: ABO blood groups can take the form of type A, type B, Type AB or type O, where A, B and AB are all dominant forms and O is recessive

9. Pleiotropy
A single gene can have multiple affects on a number of characteristics in an organism
Example: In pea plants, the gene that controls flower color also affects the seed coat color

10. Beyond Mendel
At times, multiple genes are involved in determining a particular phenotype
Epistasis
Polygenic Inheritance

11. Epistasis
A gene at one locus alters the phenotypic expression of a gene at a second locus
Example: In mice, black coat color is dominant to brown. A second gene determines if pigment is deposited in the fur. If the second gene is homozygous recessive it causes white fur regardless of the first genotype

12. Polygenic Inheritance
When multiple genes work together, the number of alleles involved cause a variation like that on a continuum; quantitative characters
Example: Skin pigmentation in humans is thought to be controlled by at least 3 separate genes that together give us the variation in skin colors. The greater number of dominant alleles, the darker the skin

13. Nature vs. Nurture
Genotype will influence what the phenotype will be, but the environment and outside influences also play roles
Available nutrients
Weather

14. Pedigrees
Show traits in a family tree format

15. Recessive Disorders These are disorders caused by recessive alleles
Those individuals that are heterozygous or carry one allele for a recessive disorder are considered carriers
They can pass on the disease, but they do not have the disease
Examples: Albinism, cystic fibrosis, sickle cell disease

16. Dominant Disorders
A disorder or disease that is seen with the presence of one dominant allele
Dominant alleles that cause lethal diseases are much less common than recessive
All LETHAL alleles arise by mutations and dominant alleles are more lethal. You can not ‘carry’ and pass on the disorder
Examples: Achondroplastic dwarfism, Huntington’s Disease

17. Genetic Testing and Counseling
Today couples can go through genetic testing to determine risk of genetic disorders prior to having a family, during pregnancy or after birth
Tests for identifying carriers
Often done with Tay-Sachs, cystic fibrosis and sickle cell
Fetal testing can be accomplished using amniocentesis or chorionic villus sampling
Newborns are also screened for many diseases at birth