Mendel
* Gregor Mendel experimented with pea plants to
determine how parents pass traits to their offspring.
I. Mendel’s Experiments
A. Crossing Pea Plants
1. Mendel decided to cross pea plant with contrasting traits
ex: crossing tall plants with short plants
* Purebred – an organism which is the offspring of many generations that have the same
trait
II. Dominant and Recessive Alleles
* Mendel determined that each trait exists in pairs (one coming from each parent).
A. Genes and Alleles
An organism’s traits are controlled by the alleles it inherits from its
parents. Some alleles are dominant and some are recessive.
III. Mendel’s Results
A. Fertilization
1. First Cross
- The two purebred “parents” were
crossed and Mendel noticed all of the
offspring were tall.
2. Second Cross
- The offspring of the first cross were
crossed with each other and Mendel
noticed that about ¾ were tall and ¼
were short.
B. Alleles in Mendel’s Crosses
1. Purebred tall plants had two alleles for tall stems.
2. Purebred short plants had two alleles for short stems.
3. A hybrid (heterozygous) organism has two different alleles for a trait
C. Symbols for Alleles
1. A dominant allele is represented by a capital letter.
Ex: ‘T’ – represents the allele for tall plants
2. A recessive allele is represented by a lowercase letter.
Ex: ‘t’ – represents the allele for short plants
3. Purebreds and Hybrids are written as follows:
TT = Purebred Tall Plant
Tt = Hybrid Tall Plant
tt = Purebred Short Plant
D. Significance of Mendel’s Contribution
1. Before Mendel, traits were thought to be a blend of the parents.
2. Mendel’s experiments showed that traits don’t always blend.