Chapter 12
Mendel and Heredity
BIOLOGY I
MRS. DOERFLER
8.1 Bellringer
 List 5 characteristics that are passed on in families.
 Name two characteristics that are inherited but that
may also be influenced by behavior or environment.
Section 1 – Origins of Hereditary Science
 Mendel’s Studies of Characters
 Many resemble your parents
 The passing of characters from parents to offspring is
called heredity

Was one of the greatest mysteries of science before DNA and
chromosomes were discovered
Mendel’s Breeding Experiment
 Study of heredity began more 100 years ago by the
work of an Australian monk named Gregor Johann
Mendel


Performed experiments with the garden pea
First to develop rules that accurately predicted patterns of
heredity




Patterns that Mendel discovered form the basis of genetics, the
branch of biology that focuses on heredity
His parents were peasants, so he learned a lot about
agriculture
As a young man he studied theology and was ordained as a
priest
Studied science and mathematics at the University of Vienna
Mendel, cont.
 Repeated the experiments of T.A. Knight
 Experimented with garden pea heredity by cross-pollinating
plants with different characteristics
Useful features in Peas
 Several traits of the pea exist in two clearly different
forms
 The male and female reproductive parts are enclosed
in the flower

This allows you to easily control mating

Self fertilization or cross-pollination
 In cross-pollination you would remove the stamens (male) from
one flower and dust the pistil (female) of a different flower
 Is small, grows easily, matures quickly, and produces
many offspring
Traits expressed as Simple Ratios
 Mendel’s initial experiments were monohybrid
crosses

Cross that involves one pair of contrasting traits

Ex – crossing a plant with white flowers with a plant with purple
flowers
3 step experiments – Step 1
 Mendel allowed each variety of garden pea to self-
pollinate for several generations

Insured each variety was true-breeding


True-breeding – all of the offspring would display only one form of
the character
 Ex – true-breeding purple-flowering plant would only produce
plants with purple flowers
These plants served as the parental, or P generation

P generation are the first two individuals that are crossed in a
breeding experiment
Step 2
 Mendel then cross-
pollinated two P generation
plants that had contrasting
traits such as purple and
white flowers

Called the offspring of the P
generation the first filial, or F1
generation

He then examined and recorded
the number of F1 plants
expressing each trait
Step 3
 Mendel allowed the F1 generation to self-pollinate
 Called the offspring of the F1 generation, the second filial, of
F2 generation

Again, each was characterized and counted
Three Steps of Mendel’s Experiments
Mendel’s Results
 Each of Mendel’s F1 generation showed only one
form of the trait

The contrasting form had disappeared!

When the F1 generation was allowed to self-pollinate, the missing
trait reappeared in some of the plants of the F2 generation
 For each of the seven traits Mendel studied, he found
the same 3:1 ratio of plants expressing traits in the
F2 generation
Mendel’s Crosses & Results
8.1 DO NOW!
 Replicate(DRAW) the three steps in Mendel’s
experiments as illustrated in Figure 3, substituting
another trait from Table 1.
 Why did Mendel allow the pea plants to selfpollinate before beginning with his crosses?
 Define heredity