Gregor Mendel conducted experiments with pea plants in the 1860s to study inheritance patterns in offspring. He found that traits are determined by discrete factors (now known as genes) that are transmitted from parents to offspring. Mendel discovered that for each trait, there are two alleles - one dominant and one recessive. In the F1 generation, the dominant allele is expressed. In the F2 generation, the recessive allele reappears in approximately 25% of offspring due to independent assortment of alleles. Mendel's laws of inheritance formed the foundation of classical genetics.

1. GENETICS 1:
Mendel
Robin D. Seamon

2. 1. REVIEW: Amoeba Sisters: Intro to Heredity
(8 min)
2. Amoeba Sisters: Alleles & Genes (8 min)

3. INHERITANCE
something we receive from each of our parents
Heredity- the delivery of characteristics from
parent to offspring
Genetics- science of the study of heredity

4. Gregor Mendel- 1822 Austrian monk; studied
science and math at University of Vienna after
becoming a priest
• Worked at the monastery & teaching high
school
• In charge of the monastery garden
• Scientifically inclined, he studied garden peas:
• Small, easy to grow & study hundreds of
offspring in a short period of time
• Short generations made easier
observance of patterns

5. Gregor Mendel
1822- 1884

6. Pollination- Mendel understood sexual
reproduction in angiosperms resulted from a
female part of a flower (ovary) and the male part
of the flower (pollen) & that peas are self-
fertilizing by wind & insects
• Fertilization- Joining the male gamete with the
female gamete to form a zygote (seed)

7. True-breeding- organisms that produce offspring
that are identical to the one parent; same traits
Trait- specific characteristic (seed color, plant
height, seed shape) of an individual
SEVEN Observable Traits of Pea Plants:
plant Long or short stems
Flower position on stem Axial, terminal
Pod color Green or yellow
Pod appearance Inflated or constricted
Seed texture Smooth or wrinkled
Flower color Purple or white
Seed color Yellow or green

9. To learn how the traits were determined, Mendel
crossed true-breeding stocks & observed results
• He needed to control pollination:
• Cut off stamens (pollen part of flower)
• Used a brush to ‘paint’ desired pollen
onto the female stigma
hybrid- crosses between parents
of two different traits

10. Parental generation: P
Offspring generation: F1 BREAK IT DOWN:
F = filial
= (son or daughter)
Let’s consider 1 of the traits:
Flower color…

11. Parental generation: P
Offspring generation: F1
SURPRISING RESULTS:
- F1 generation had characteristics of only one
parent
They’re ALL
purple.

12. MENDEL’S CONCLUSIONS:
Heredity = genes: An individual’s characteristics are
determined by factors that are passed from one
parental generation to the next (genes)
• Each trait was determined by a single gene that
occurred in one of two ways (alleles)
• Purple flower or white flower
• Green or yellow
• Wrinkled or smooth
• Tall or short
• etc.

13. Principle of dominance – some alleles are
dominant over other (recessive) ones
• Organisms with at least one dominant allele will
express that gene
• Organisms with the recessive allele will only
express it if a dominant allele is NOT present
Dominant: trait that appears in the F1 generation
Recessive: trait that did not appear in the F1
generation

14. PEA PLANT ALLELES
DOMINANT ALLELES
White
X
purple
purple
Flower
color

15. He looked at the offspring of the F1 generations
to find out:
• He allowed 7 kinds of hybrids
to self-pollinate & observed
the offspring
• F2 generation: offspring of
the F1 generation
Had the recessive alleles just disappeared or were
they still present in the new plants but masked?

16. RESULTS- Mendel’s Discovery:
Traits controlled by the recessive alleles
REAPPEARED in the second generation.
• about ¼ of F2 offspring showed the recessive
trait

17. Since: ¼ of the F2 showed the recessive trait,
what had happened?
Mendel’s first assumption that the dominant gene
had masked the recessive one wasn’t true in the
F2 cross… so he studied the matter.
Law of Segregation- he hypothesized that the
alleles for purple and white flowers must have
separated (segregated) during the formation of
sex cells (egg &pollen)

18. He was correct.
During gamete formation, each gamete carries
only ONE allele (not both) for each gene.
SO… the recessive
trait reappears in the
F2 generation.
ONE
allele
Gametes: You’re only
getting two of these…
one from mom, one
from dad

19. Genotype- what is in the genes

20. 3:1
ratio
4:4
ratio
Phenotype- what we physically see
VIDEO: How
Mendel’s Pea
Plants helped
us understand
genetics (3
min)

21. HUMAN phenotypes:
Make us look different

22. Predicting
Probability

23. 1. Amoeba Sisters: Punnett Square Guinea
Pigs 6:30
OPTIONAL
1. Amoeba Sisters: Dihybrid Crosses (7 min)

24. A Good Scientist:
Mendel was a good scientist since he made careful
observations, collected accurate data & was able to
draw correct conclusions.
Probability- likelihood a particular event will occur
• The way that alleles segregate during meiosis is
random… how can we predict the likelihood of
gene expression?
• We can use a Punnett Square to determine
probability

25. Homozygous- organisms that have two identical
alleles for a gene
(two capital/dominant)
(two lower case/recessive)
Heterozygous- organisms that have two different
alleles for a gene
(one capital/dominant & one lower case/recessive)
PP
pp
Pp
Homozygous recessive
Homozygous dominant

26. F1

27. F1
1. Establish parent
cross.
2. Assign letters: (use
letter of the
dominant trait)
3. Make an 8 Punnett
square for one trait.
4. Fill in chart.
Bb, BB purple
bb white

28. F2
How do alleles segregate when more than one
allele is involved?
Mendel wanted to be able to predict the
offspring in the F2 generation.
law of independent assortment- factors (alleles)
for one characteristic (gene) do not affect the
inheritance of factors for another trait.
EXAMPLE: green pod color doesn’t always go
with green pea color

29. Rr, RR smooth
rr wrinkled
YY, Yy yellow
Yy green
1. Establish parent
cross.
2. Assign letters: (use
letter of the
3. Make a 16 Punnett
square for two traits.
4. dominant traits)
5. Fill in chart.
F2

30. NON-MENDELIAN TRAITS
VIDEO: Incomplete dominance, Codominance,
Polygenic Traits (7 min)

31. OTHER PATTERNS OF INHERITANCE
Incomplete dominance- blending of traits; alleles
are neither dominant nor recessive
In this example,
plants with red
flowers bred with
white flowers
produce PINK
flowers!
• NEITHER is
DOMINANT

32. Codominance- phenotypes of BOTH alleles are
expressed in the heterozygous offspring
In this example,
white chickens
crosses with black
chickens create
speckled chickens
• NEITHER is
DOMINANT BOTH COLORS
expressed

33. Polygenic traits- traits controlled by 2 or more
genes
EXAMPLES:
(traits that vary a
lot)
Height
Skin color
Hair color

34. MULTIPLE ALLELES
VIDEO: Amoeba Sisters: Multiple Alleles (Blood
Type)

35. Multiple alleles- gene with three or more alleles,
but show only 2 at a time (since you only get two)
Example:
Rabbit fur color
(4 alleles)
ABO Blood types
(3 alleles)

36. Multiple alleles-
Example:
ABO Blood
types (3 alleles)

37. Multiple alleles-
Example:
ABO Blood types (3 alleles)

38. B allele (IBIB, IBi, IAIB)
(has B antigen)

39. A allele (IAIA, IAi, IAIB)
(has B antigen)

40. O allele (IAi, IBi, ii)
(has neither antigen)

43. Rh Factor
Rh factor is another protein on the surface of red
blood cells
Present (+)
Absent (-)
• Antibodies are made when Rh- blood is exposed
to Rh+ blood (not automatically like ABO

44. Only a problem when mother
is Rh- and fetus is Rh+