1. Genetics The study of heredity - how traits are passed from parent to offspringor x = or

2. The study of heredity started with the work of Gregor Mendel and his
pea plant garden
Mendel was an Austrian Monk that lived
in the mid 1800’s

3. Why did Mendel work with pea plants?
Fast to grow
Easy to cross pollinate
Two distinct forms for traits
Tall or short
Yellow or green pods
Smooth or pinched pods
Yellow or green peas
Round or wrinkled peas
Colored or white seed coats
Side flowers or end flowers

4. Mendel’s cross between tall pea plants yielded all tall pea plants
Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between short pea plants yielded all short pea plants. X = X =
Mendel’s cross between tall pea plants and short pea plants yielded all tall pea plants. x =

5. Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being short.x =

6. Mendel’s Factors
Mendel repeated this experiment with many pea plants with many different traits
For each, he discovered that there seem to be “factors” that disappeared in one generation and then reappeared in the next
This went against the idea that an organism is a blend of their parents’ traits

7. Why was Mendel’s work so important?
Mendel worked at time when microscopes were not very good and the cell theory was only just being thought of
When scientists understood DNA and chromosomes, they understood how important his work with pea plants was

8. Dominant vs. Recessive Genes
There are TWO basic kinds of genes:
Dominant - A gene that is always expressed and hides others
Represented with a capital letter
Recessive - A gene that is only expressed when a dominant gene isn’t present
Represented with a lowercase letter

9. A Human Example A dominant gene will always mask a recessive gene.
Eddie Munster
A dominant gene will always mask a recessive gene.
A “widow’s peak” is dominant, not having a widow’s peak is recessive.
If one parent contributes a gene for a widow’s peak, and the other parent doesn’t, the off spring will have a widow’s peak.

10. Latin Roots
All organisms have two copies of each gene: one contributed by the father and the other contributed by the mother.
Homozygous - Two copies of the same gene
“homo” - same
Heterozygous - Two different genes
“hetero” - different

11. Genotypes and Phenotypes
For the widow’s peak:
Genotypes: WW, Ww, or ww
Phenotypes: widow’s peak or no widow’s peak
WW - has a widow’s peak Homozygous dominant
Ww - has a widow’s peak Heterozygous
ww - no widow’s peak Homozygous recessive

12. Eddie Munster’s Parents
Father = Herman - No Widow’s Peak
Mother = Lilly - Has a Widow’s Peak

13. What can you determine about a genotype from a phenotype?
Since Herman has no widow’s peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww”
Definitely ww Homozygous
recessive
Either Ww Heterozygous
or WW Homozygous dominant

14. Punnett Squares
A Punnett Square is a tool used to predict the possible gene combinations of the offspring. In this case, we can work backwards…
A Punnet Square begins with a box 2 x 2
One gene is called an “allele”
One parents pair is split into alleles on top, the other along the side
Each allele is crossed with the other allele to predict the traits of the offspring
Assume Lilly is heterozygous Ww W w
Assume Herman is homozygous recessive ww w Ww ww w Ww ww

15. How can you tell?
So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)? W w W W w Ww ww w Ww Ww w Ww ww w Ww Ww

16. More Information Needed
Recall that Herman and Lilly had another offspring, Marylin. She had no widow’s peak. Therefore, Lilly must be heterozygous.

17. Incomplete dominance
Two forms blend

18. Co-dominant Traits
both forms are shown

19. Multiple Alleles
more than two forms of a trait

20. Polygenic Traits
Controlled by many genes