MENDEL AND
GENETICS
What is genetics?
Genetics, a branch of biology, is
the scientific study of heredity
and variation of organisms.
• Gregor Mendel, an Austrian monk who studied
botany. He made important progress in
understanding heredity.
• Mendel studied the traits of ordinary garden peas.
• During Mendel’s time, the belief was that the
traits of the parents blended.
Gregor Mendel’s work with plants
• Most plants have female and male parts on the same
flower.
**Male part of the
flower produces pollen
(sperm).
**Female part of the
flower produces ova
(eggs)
Gregor Mendel’s work with plants
• Plants reproduce sexually.
• Pollen (male gamete) and ovum (female gamete) join making a
new diploid cell.
• Fertilization produces a new diploid cell.
• Two options for the same
flower:
• Self-pollinating: pollen and
ovum come from the same
flower.
• Cross-pollinating: ovum on
one flower and pollen from
flower on a different plant..
Mendel’s peas
• Mendel had pea plants that
were true breeding.
• Seed from purple flowered plants
had only purple flowered
offspring.
• True breeding plants are ones
that produce offspring like
themselves.
• Mendel experimented with
cross pollination between
different parents.
Gregor Mendel’s work with peas
• A trait is a specific observable characteristic that varies from
one individual to another.
• Seven traits of peas studied by Gregor Mendel: dominant
version of trait is on the left, recessive on the right.
Crossing different pea plants
• P = parental generation
• F1 = first filial generation
(offspring or children)
• F2 = second filial generation
(grandchildren)
Mendel’s Cross pollination
P two different parents, example:
trait = height; alleles = tall or short
F1 offspring are all heterozygous
plants. He used F1 as parents
Heterozygotes have different
alleles.
Homozygotes have the same
alleles.
F2 ‘grandchildren’ have ratios of 3
out of 4 dominant trait or 75% and
1 out of 4 recessive trait or 25%.
Mendel—Father of Genetics
• Mendel’s application of scientific method and
mathematics to heredity in peas led to his
discovery of ‘factors’.
• Mendel concluded that biological inheritance is
determined by factors passed from one
generation to the next.
Today, scientists call the factors that
determine traits genes.
Mendel—Father of Genetics
• Each trait that Mendel studied was controlled by
one gene that occurred in two contrasting forms.
• The different forms of a gene are called
alleles
• Example—trait is flower color
•
•
•
Gene controls flower color
Two forms are purple and white
Alleles are P-purple, p-white
Mendel and his work
Mendel predicted:
• The role of genes in heredity
• Alleles for a gene occur in pairs for an
organism
• One allele for a gene is in each gamete
Mendel had three key concepts:
Dominance
Segregation
Independent Assortment (Day 3)
Two of Mendel’s Principles
• 1. Law of Dominance:
One allele does the ‘talking’. One allele is
expressed. One allele was dominant over the other in the
F1 generation.
• 2. Principle of Segregation:
When gametes are formed, the pairs of hereditary
factors (alleles) become separated. Each sex cell
(egg/sperm) receives only one version of the gene.
• 3. Principle of Independent Assortment:
Day 3 (Hint: multiple chromosomes divide in
meiosis)
Gregor Mendel’s ideas
• Mendel observed that some traits appeared to dominate.
• Dominant allele of gene is always expressed in classic
•
•
•
•
Mendelian genetics.
Other traits skipped a generation. Recessive trait.
A trait is an observable characteristic that varies between
individuals.
Mendel counted the results of cross pollination. He
calculated the probability of outcomes from his
hypothesis.
Probability is how likely something will happen. Mendel
assumed the forms of a trait happened as random events
with equally likely probabilities.
• Consider flipping a coin. Two equally likely outcomes.
Using a Punnett Square
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a Punnett square
Parents are tall and short.
Parent genotypes:
TT and t t
Cross
TT  tt
Punnett square
4. "split" the letters of the genotype for each parent
& put them "outside" the p-square
5. determine the possible genotypes of the offspring by
filling in the p-square
6. summarize results (genotypes & phenotypes of
offspring)
T
TT  tt
t
t
Tt
Tt
T
Tt
Genotypes:
100% T t
Tt
Phenotypes:
100% Tall plants
Monohybrid cross: F2 generation
• If you let the F1 generation self-fertilize, the next
monohybrid cross would be:
Tt  Tt
(tall)
T
T
t
TT
Tt
t
Tt
tt
(tall)
Genotypes:
1 TT= Tall
2 Tt = Tall
1 tt = dwarf
Genotypic ratio= 1:2:1
Label your ratio TT:Tt:tt
Phenotype:
3 Tall
1 dwarf
Phenotypic ratio= 3:1
Label your ratios tall: short
Secret of the Punnett Square
• Key to the Punnett Square:
• Determine the gametes of each parent…
• How? By “splitting” the genotypes of each parent:
If this is your cross
T T

t t
The gametes are:
T
T
t
t
Once you have the gametes…
T
T

t
t
T
T
t
t
Tt
Tt
Tt
Tt
Another example: Flower color
For example, flower color:
P = purple (dominant)
p = white (recessive)
If you cross a homozygous Purple (PP) with a
homozygous white (pp):
PP

Pp
pp
ALL PURPLE (Pp)
Cross the F1 generation:

Pp
P
p
Pp
P
p
PP
Pp
Pp
pp
Genotypes:
1 PP
2 Pp
1 pp
Phenotypes:
3 Purple
1 White
Stations
• Each group will visit all stations.
• Station 1—Vocabulary A
• Station 2—Punnett Squares
• Station 3—Analysis of Punnett Square for Parents
• Station 4—Vocabulary B
• Station 5—Segregation
• Station 6—Phenotype/Genotype
• Use your handout to record your work at each station.
Resources
• Dr. Dennis O’Neill, 2013, Basic Patterns of Genetics,
Palomar College
http://anthro.palomar.edu/mendel/Default.htm
• Paul Anderson, 2013, Mendelian Genetics, Bozeman
Science,
http://www.youtube.com/watch?v=NWqgZUnJdAY
• CrashCourse, 2013, Heredity, Crash Course in Biology,
http://www.youtube.com/watch?v=CBezq1fFUEA
• Staff, 2013, Glossary of Terms, Palomar College,
http://anthro.palomar.edu/mendel/glossary.htm