Gregor Mendel conducted experiments with pea plants to study heredity. He found that pea plants have distinct traits that are inherited, such as flower color. Through controlled breeding experiments of pea plants with different traits, Mendel discovered that traits are passed from parents to offspring through discrete units (now called genes). His experiments showed that for many traits, one gene variant (allele) is dominant and will be expressed over another recessive allele. Mendel's work established the foundations of classical genetics and heredity through his laws of segregation and independent assortment.

1. Mendel & Heredity
Chapter 6

2. The Origins of Genetics
 Genetics – the study of biological
inheritance patterns and variation in
organisms
 Traits – distinguishing characteristics that
are inherited
 Heredity – the passing of characters (traits)
from parents to offspring

3. Mendel’s Breeding Experiment
 Gregor Johann Mendel
 Austrian monk
 Experiments with garden
pea plants led to our
modern understanding of
heredity

4. Why are Pea Plants Good Subjects?
 Several characters (traits) of the plant exist in two
clearly different forms
 Flower color
 Seed color
 Seed shape
 Pod color
 Pod shape
 Flower position
 Plant height
 Easy to grow
 Short life cycle

5. Mendel’s
Experiment
 Monohybrid cross –
involves one pair of
contrasting traits
 Step 1 – allowed each
variety of garden pea
to self-pollinate for
several generations to
ensure they are
genetically uniform,
or purebred.

6. Mendel’s Experiment
 Step 2 – cross-pollinated first two parent plants
(P generation) that had contrasting traits
 The offspring of these plants were called the first
filial generation (F1 generation)
 He examined each F1 plant and recorded the
number of plants expressing each trait

7. Mendel’s Experiment
 Step 3 – allowed the F1 generation to self-
pollinate
 The offspring are called the F2 generation
 Each F2 plant was examined and the traits
recorded

8. Mendel’s Results

9. Mendel’s Results
 In F1 generation, all plants were purple
 In F2 generation, the ratio of purple to white
plants was 3 to 1
 Ratio: 3:1 or 3 to 1 or 3/1

10. Mendel’s Hypotheses
1. There are alternative versions of genes
 Example: the gene for flower color in peas
can exist in a “purple” version or a “white”
version
Today the different versions of a gene are called its
alleles.

11. Mendel’s Hypotheses
2. When two different alleles occur together, one
of them may be completely expressed, while
the other may have no observable effect on the
organism’s appearance.
Expressed form of the character – dominant
Trait that is not expressed - recessive

12. Mendel’s First Law or
Law of Segregation
 Organisms carry two copies of each gene, one
from each parent
 When gametes are formed, the alleles for each
gene in an individual separate. Thus, gametes
carry only one allele for each inherited character.

13. The same gene can have many
versions
 Gene – a piece of DNA that holds the instructions
to make a certain protein
 Allele – any of the alternative forms of a gene
that are at a certain locus on a chromosome

14. Homozygous vs. Heterozygous
 If two alleles of a particular
gene present in an individual
are the same, the individual is
said to be homozygous for
that trait.
 If the alleles of a particular
gene present in an individual
are different, the individual is
said to be heterozygous for
that trait.

15. Genotype vs. Phenotype
 Genome – all of an organism’s genetic material
 Genotype – the set of alleles that an individual has
 Phenotype – the physical appearance of a character
Genotype Phenotype
PP Purple flower
Pp Purple flower
pp White flower

16. Punnett Squares
 A diagram that
predicts the outcome
of a genetic cross by
considering all
possible combinations
of gametes in the
cross.

18. Outcomes of Crosses
 Probability – the likelihood that a specific event
will occur
Probability = # of 1 kind of outcome
total # possible outcomes