1. Height in Humans
Height in pea plants is controlled by one of two alleles; the allele for a tall plant is the dominant allele, while the allele for a short plant is the recessive one. What about people? Are the factors that determine height more complicated in humans?

2. Bell Work
1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches.
2. What can you observe about the heights of the ten people?
3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer.

3. Exploring Mendelian Genetics

4. Key Concept
What inheritance patterns exist aside from simple dominance
Incomplete dominance and Codominance
The influence of the environment on genetic expression

5. A Summary of Mendel’s Principles:
Gregor
Mendel
experimented with
concluded that
Pea
plants
“Factors”
determine
traits
Some alleles
are dominant,
and some alleles
are recessive
Alleles are
separated during
gamete formation
which is called the
which is called the
Law of
Dominance
Segregation

6. A Summary of Mendel’s Principles:
1) The inheritance of biological characteristics is determined by individual units know as genes → genes are passed from parent to offspring
Example: Offspring tend to resemble their parents

7. A Summary of Mendel’s Principles:
2) In cases in which 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 → this is known as The Principle of Dominance
Example: Tongue Rolling → R = can roll tongue (dominant), r = can’t roll tongue (recessive)

8. A Summary of Mendel’s Principles:
3) In most sexually reproducing organisms, each adult has two copies of each gene (one from each parent) → These genes are segregated from each other when gametes are formed
→ known as The Principle of Segregation
Example: Rr → body cell
/ \
gametes→ R r

9. The Principle of Independent Assortment
The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes
This helps account for the many genetic variations observed in plants, animals, and other organisms

10. A Two Factor Cross

11. Beyond Dominant and Recessive
Despite the importance of Mendel’s work, there are important exceptions to most of his principles
Some alleles are neither dominant nor recessive
Many traits are controlled by multiple alleles or multiple genes

12. Incomplete Dominance
One allele is not completely dominant over another one
The heterozygous phenotype is somewhere in between the two homozygous phenotypes (both alleles contribute to the phenotype)

13. Example: The Dosage Effect
Flower color in 4 o’clock plants
R = red flowers
W = white flowers
P-generation: RR x WW

14. Codominance
Another situation in which both alleles contribute to phenotype
A heterozygous phenotype shows distinct characteristics from each allele

15. Example: Chickens
In chickens there is an allele for white feathers and black feathers
A heterozygous individual will have both color feathers

16. Multiple Alleles
Genes that have more than two alleles (individuals will still only have two copies)
Example: Fig Coat color in rabbits

17. Polygenic Traits
Traits that are produced by the interaction of two or more genes
Example: skin color in humans is influences by at least 4 different traits

18. Applying Mendel’s Principles
Mendel’s principles don’t just apply to plants
Example: Drosophila (the common fruit fly) is a popular organism for genetic research. Why?
Fruit flies are small, develop quickly, and produce many offspring

19. Genetics and the Environment
The characteristics of any organism are not solely determined by the genes it inherits → characteristics are determined by the interaction between genes and the environment
Example: Many genes affect a Sunflower’s height and color; however these traits are also influenced by climate, soil conditions, and the availability of water

20. Conclusion
Genes provide a plan for development, but how that plan unfolds also depends on the environment