1. Introduction to Genetics
How are traits passed on?

2. Gregor Mendel Austrian monk who studied pea plants
He chose these plants to study because they were:
Easy to grow
Mature quickly
Have sharply contrasting traits

3. Traits of Pea Plants

4. Pea Plants Naturally, pea plants self-pollinate.
Mendel observed that some plants, when they were allowed to self-pollinate, always “bred” true.
This means that these plants were “true-breeding” (Ex. Tall plants always produced tall plants.)
Mendel cross-pollinated some plants to observe the results.

5. When he crossed true-breeding tall plants and true breeding short plants, he found that he always got tall plants
Then, he crossed the resulting tall plants, and found that the short plant reappeared.
P = Parental Generation
F1 = first filial generation
F2 = Second filial generation

6. Why did Mendel get these results?
All living things have genes, or sections of chromosomes that have instructions for making different characteristics
There are always two versions of these genes, one from each parent
Called Alleles
Represented by letters

7. Alleles Alleles, or the two forms of each gene, can be Examples of
Dominant –
this trait will always show up if the dominant allele is present
Represented by capital letters
Recessive –
this trait will only show up in the absence of the dominant allele
Represented by lowercase letters
Examples of
Dominant traits – tongue rollers, brown eyes, dark hair, left thumb hand clasping
Recessive traits – non-tongue rollers, light eyes, light hair, right thumb hand clasping

8. Allele Combinations
Depending on the two alleles present, an organism can be considered homozygous or heterozygous for a trait
Homozygous – two alleles are the same, either both dominant or both recessive (TT or tt)
Heterozygous/Hybrid – two alleles are different, one dominant and one recessive (Tt)
The different arrangements of alleles is called an organism’s genotype

9. Phenotype
An organism’s genotype will determine its phenotype, or physical characteristic
For example:
A plant with the alleles TT is Homozygous Dominant & one with the alleles Tt is heterozygous, and tt is read Homozygous Recessive
Both plants have the dominant gene for tallness (T) and will therefore have a tall phenotype

10. Back to Mendel’s Plants
Why did the trait for short plants disappear in the F1 generation?
Why did it reappear in the F2 generation?
Punnett squares can be used to demonstrate the different possible combinations of alleles…..

11. Punnett Squares T T t T t T t F1 results crossed  t T t T t T t

12. Mendel’s Laws

13. Law of Dominance
When there are two forms of a gene for one trait, one form may be dominant over the other

14. Law of Segregation
Two forms of each gene are segregated during the formation of reproductive cells

15. Law of Independent Assortment
Genes for different traits may assort independently of one another