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
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.
17.
18. Outcomes of Crosses
Probability – the likelihood that a specific event
will occur
Probability = # of 1 kind of outcome
total # possible outcomes
