Chapter 4
Mendel’s Work
Section 1
Gregor Mendel
 Mid
nineteenth century
 Priest named Gregor Mendel
 Worked in a garden at a monastery
 Heredity:
the passing of physical
characteristics from parents to offspring
Why do pea plants have
different characteristics?
 Traits:

physical characteristics
Examples: stem height (tall or short), seed
color (green or yellow)
 Pea
plant TRAITS were often similar to
those of their parents and sometimes they
were different
 His studies laid the foundation for
GENETICS– the scientific study of heredity
Mendel’s Experiments
 The
flower’s petals surround the pistils and
the stamens


Pistil: produces female sex cells, or eggs
Stamen: produce pollen, which contains
male sex cells, or sperm
 Fertilization:
new organisms can form
when the egg and the sperm join in this
process
Self-Pollinating


Pea plants are usually self-pollinating
Pollen from a flower lands on a pistil of the same
flower
Cross-Pollinating


“crossed”
Mendel removed pollen from
a flower on one plant,
then brushed the pollen
onto a flower on a second
plant
Crossing Pea Plants
 How
are you going to study the
inheritance of traits in pea plants?

Mendel decided to “cross” plants with
contrasting traits
 Example:
 He
tall plants with short plants
started with purebred plants
 Purebred: an organism is the offspring of
many generations that have the same
trait
The F1 and F2 Offspring
Mendel crossed purebred tall plants with
purebred short plants
---This is called the parental generation, or the P
generation
F1 offspring are all tall
---The offspring from this cross are the first filial
generation, or the F1 generation
He then allowed for the F1 offspring to selfpollinate
F2 offspring are tall and short
---The offspring from this cross are the second
filial generation, or the F2 generation
Results of a Cross
F1 generation
(both tall)
P generation
(one tall, one short)
F2 generation
(mix of tall and short)

What other traits did Mendel study?


Pod color, seed shape, flower position
What were the two forms of the seed shape?

Round and wrinkled
Genes and Alleles
 Genes:
factors that control a trait
 Alleles: different forms of a gene
 An
organism’s traits are controlled by
alleles it inherits from its parents. Some
alleles are dominant, while others are
recessive
Dominant Alleles
vs. Recessive Alleles
 Dominant
Allele: an allele whose trait
always shows up in the organism when
the allele is present
 Recessive Allele: an allele that is masked
when a dominant allele is present
Let’s do some math….
Examples for Stem Height
 One
recessive allele + one dominant
allele = dominant traits (tall stem)
 One
recessive allele + one recessive allele
= recessive traits (short stem)
Alleles in Mendel’s Crosses
 Combination



of Alleles:
Hybrid tall--- one allele for tall stems and
one allele for short stems (one dom, one
rec)
Purebred tall--- two alleles for tall stems
(two dom)
Purebred short--- two alleles for short stems
(two rec)
Symbols for Alleles
 Geneticists
use letters to represent alleles
 Dominant allele is represented by a
capital letter

Example: tall stems = T
 Recessive
allele is represented by a
lowercase letter

Example: short stems = t
2
dominant alleles for tall stems
= TT
2
recessive alleles for short stems
= tt
1
recessive allele for short stems and 1
dominant allele for tall stems
= Tt
Significance of
Mendel’s
Contribution
 The
importance of Mendel’s discovery
was NOT recognized during his lifetime
 Then, in 1900, three scientists discovered
his work
 They quickly realized the importance of
his work
 Mendel is often called the Father of
Genetics