This document provides an overview of Gregor Mendel's experiments with pea plants and the principles of heredity and genetics that he discovered. It discusses Mendel's work crossing pea plants with different traits, such as flower color, and recording the results in subsequent generations. His experiments showed that traits are inherited in discrete units (now known as genes) and follow predictable patterns, such as the 3:1 ratio he observed for dominant and recessive traits in the F2 generation of a monohybrid cross. The document also covers Mendel's principle of independent assortment observed in dihybrid crosses.

1. INTERACTIVE CLASS QUIZ
Lecture on DNA and the Central Dogma of
Molecular Biology

2. LECTURE OUTLINE
Mendelian Genetics and Heredity
DNA versus RNA and Why DNA is the
genetic material?
Crick’s Central Dogma of Molecular
Biology

3. Mendel and the Genes
PART 1

4. HEREDITY AND FAMILY
RESEMBLANCE

5. HEREDITY BEFORE MENDEL
UNKNOWN: genetic principles that account for
the transmission of traits from parents to
offspring
“blending” hypothesis: genetic material
contributed by two parents mixes
“particulate” hypothesis: parents pass on
discrete heritable units called genes

6. HEREDITY BEFORE MENDEL
ASSUMPTION 1: CONSTANCY OF SPECIES
heredity occurs within species
e.g. camel + leopard = giraffe (by
breeding)
Species were thought to have been
maintained without significant
change from the time of their
creation

7. HEREDITY BEFORE MENDEL
ASSUMPTION 2: DIRECT TRANSMISSION OF TRAITS
traits are transmitted directly
Information from each part of the body was
supposedly passed along independently of the
information from the other parts
The child was formed after the hereditary
material from all parts of the parents’ bodies
had come together
example: red hair parents = red hair children

8. and then there was MENDEL…

9. WHO IS GREGOR
MENDEL?
carried out the first quantitative studies of inheritance
an Austrian monk
educated in a monastery and went on to study science
and mathematics at the University of Vienna
BUT…he failed his examinations for a teaching
certificate
initiated a series of experiments on plant hybridization
(using garden peas)

10. GUIDING PRINCIPLE FOR
MENDEL’S WORK
Variation is widespread in nature
Observable variation is essential for following
genes
Variation is inherited according to genetic laws
and not solely by chance
Mendel’s laws apply to all sexually reproducing
organisms

11. QUESTION #1:
Who was Gregor Mendel?
A. an English scientist who carried out
research with Charles Darwin
B. a little known Central European monk
C. an early 20th century Dutch biologist
who carried out genetics research
B

12. QUESTION #2:
Which of the following statements is true about
Mendel?
A. His discoveries concerning genetic inheritance
were generally accepted by the scientific community
when he published them during the mid 19th
century
B. He believed that genetic traits of parents will
usually blend in their children
C. His ideas about genetics apply equally to plants
and animals
C

13. WHY THE GARDEN PEA
(Pisum sativum)
Earlier investigators had produced hybrid peas by crossing different
varieties
can expect to observe segregation of traits among the offspring
Large number of true-breeding varieties of peas were available
Small and easy to grow, and they have a relatively short generation time
can conduct experiments involving numerous plants, grow several
generations in a single year, and obtain results relatively quickly
Sexual organs of the pea are enclosed within the flower
fertilization takes place automatically within an individual flower if it
is not disturbed, resulting in offspring that are the progeny from a
single individual

14. CROSSING
THE
PEA PLANTS
1
5
Removed stamens
from purple flower
4
3
2
Transferred sperm-bearing
pollen from
stamens of white
flower to egg-bearing
carpel of
purple flower
Parental
generation
(P)
Pollinated carpel
matured into pod
Carpel
(female)
Stamens
(male)
Planted seeds
from pod
Examined
offspring:
all purple
flowers
First
generation
offspring
(F1)

15. STUDIED DISCRETE
TRAITS

16. QUESTION #3:
Mendel believed that the characteristics of
pea plants are determined by the:
A. inheritance of units or factors from
both parents
B. inheritance of units or factors from one
parent
C. relative health of the parent plants at
the time of pollination
A

17. SOME IMPORTANT
TERMS
CHARACTER: a heritable feature
e.g. flower color
TRAIT: a variant of a given character
e.g. purple, white, yellow
ALLELES: alternative form of traits
P (purple); W (white); Y (yellow)

18. QUESTION #4:
An allele is:
A. another word for a gene
B. a homozygous genotype
C. a heterozygous genotype
D. one of several possible forms of a gene
D

19. SOME IMPORTANT
TERMS
Phenotype – observable characteristic of
an organism
Genotype – pair of alleles present in and
individual

20. QUESTION #5:
Phenotype refers to the
____________________ of an individual
A. genetic makeup
B. actual physical appearance
C. recessive alleles
B

21. QUESTION #6:
When the genotype consists of a dominant
and a recessive allele, the phenotype will be
like _________________ allele.
A. the dominant
B. the recessive
C. neither
A

22. SOME IMPORTANT
TERMS
Homozygous – two alleles of trait are the
same (YY or yy)
Heterozygous – two alleles of trait are
different (Yy)

23. Figure 14.6
3
1 1
2
1
Phenotype
Purple
Purple
Purple
White
Genotype
PP
(homozygous)
Pp
(heterozygous)
Pp
(heterozygous)
pp
(homozygous)
Ratio 3:1 Ratio 1:2:1

24. SOME IMPORTANT
TERMS
Capitalized traits = dominant
phenotypes
Lowercase traits= recessive phenotypes

25. QUESTION #7:
Assuming that both parent plants in the
diagram below are homozygous, why
would all of the f1 generation have yellow
phenotypes?
A. because the f1 genotypes are
homozygous
B. because yellow is dominant over
green
C. because both parents passed on
yellow alleles
B

26. QUESTION #8:
C

27. QUESTION #9:
A

28. QUESTION #10:
B

30. DOMINANT & RECESSIVE
TRAITS
D
R

31. QUESTION #11:
3) TT

32. QUESTION #12:
2) SHORT

33. QUESTION #13:
2) ALL tt

34. SOME IMPORTANT
TERMS
Generations:
P = parental generation
F1 = 1st filial generation, progeny of
the P generation
F2 = 2nd filial generation, progeny of
the F1 generation (F3 and so on)

35. SOME IMPORTANT
TERMS
Crosses:
Monohybrid cross = cross of two different
true-breeding strains (homozygotes) that
differ in a single trait.
Dihybrid cross = cross of two different
true-breeding strains (homozygotes) that
differ in two traits.

36. THE OBSERVATIONS
P Generation
(true-breeding
parents) Purple
flowers
White
flowers
×
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation
EXPERIMENT True-breeding purple-flowered pea plants and
white-flowered pea plants were crossed (symbolized by ×). The
resulting F1 hybrids were allowed to self-pollinate or were cross-pollinated
with other F1 hybrids. Flower color was then
observed
in the F2 generation.
RESULTS Both purple-flowered plants and white-flowered
plants appeared in the F2 generation. In Mendels
experiment, 705 plants had purple flowers, and 224 had white
flowers, a ratio of about 3 purple : 1 white.

37. BASED ON THE RESULTS…
In the F1 plants, only the purple trait was
affecting flower color in these hybrids
Purple flower color was dominant, and
white flower color was recessive

38. BASED ON THE RESULTS…
In the F2 plants, a 3:1 inheritance pattern was
observed
Possible Heredity concepts:
alleles account for the variation
inherits two alleles, one from each parent
if the two alleles at a locus differ, the
dominant allele determines the organism’s
appearance

39. ALLELES
Allele for purple flowers
Locus for flower-color gene Homologous
pair of
chromosomes
Allele for white flowers
The law of segregation - the two alleles for a
heritable character separate (segregate)
during gamete formation and end up in
different gametes

40. MECHANISM OF GENE
TRANSMISSION
GAMETOGENESIS
alleles segregate
FERTILIZATION
alleles unite

41. THE PUNNET SQUARE
Each true-breeding plant of the
P Generation
× Gametes:
F1 Generation
F2 Generation
P p
P p
P p
P
p
PP Pp
Pp pp
Appearance:
Genetic makeup:
Purple flowers
PP
White flowers
pp
Purple flowers
Pp
Appearance:
Genetic makeup:
Gametes:
F1 sperm
F1 eggs
1/2 1/2
parental generation has identical
alleles, PP or pp.
Gametes (circles) each contain only
one allele for the flower-color gene.
In this case, every gamete produced
by one parent has the same allele.
Union of the parental gametes
produces F1 hybrids having a Pp
combination. Because the purple-flower
allele is dominant, all
these hybrids have purple flowers.
When the hybrid plants produce
gametes, the two alleles segregate,
half the gametes receiving the P
allele and the other half the p allele.
3 : 1
This box, a Punnett square, shows
all possible combinations of alleles
in offspring that result from an
F1 × F1 (Pp × Pp) cross. Each square
represents an equally probable product
of fertilization. For example, the bottom
left box shows the genetic combination
resulting from a p egg fertilized by
a P sperm.
Random combination of the gametes
results in the 3:1 ratio that Mendel
observed in the F2 generation.

42. THE MONOHYBRID CROSS
White
(pp) Purple
Gametes Gametes
Purple
(Pp)
Purple
(PP)
p p P p
P
P
P
p
F1 generation
All purple
(Pp)
Gametes Pp PP Pp
F2 generation
¾ purple, ¼ white
Pp Gametes
Pp Pp Pp pp

43. THE MONOHYBRID CROSS

44. MENDEL OBSERVED THE SAME
PATTERN IN CHARACTERISTICS

45. QUESTION #14:
4) none

46. MENDEL’S LAW OF
INDEPENDENT ASSORTMENT
two characters at the same time (DIHYBRID CROSS)
whether alleles at 2 different gene loci segregate
dependently or independently
Crossing two, true-breeding parents differing in two
characters produces dihybrids in the F1 generation,
heterozygous for both characters
When gametes form, each pair of hereditary factors
(alleles) segregates independently of the other pairs

47. QUESTION #15:
The idea that different pairs of alleles are passed
to offspring independently is Mendel's principle
of:
A. unit inheritance
B. segregation
C. independent assortment
C

48. QUESTION
#16:
In the diagram, what accounts for the green pea seed
in the f2 generation?
A. On average, 1 out of 4 offspring of
heterozygous parents will be homozygous recessive
B. The yellow allele is dominant over the green one
C. The f1 generation parents are homozygous
yellow
A

49. QUESTION #17:
The idea that for any particular trait, the pair of
alleles of each parent separate and only one allele
from each parent passes to an offspring is
Mendel's principle of:
A. independent assortment
B. hybridization
C. segregation
C

50. DIHYBRID CROSS
P Generation YYRR
Gametes YR × yr
yyrr
YyRr
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
F1 Generation
F2 Generation
(predicted
offspring)
1⁄2 YR
YR
yr
1 ⁄2
1 ⁄2
Sperm
1⁄2 yr
YYRR YyRr
YyRr yyrr
3 ⁄4 1 ⁄4
Eggs
Phenotypic ratio 3:1
Eggs
1 ⁄4Y R
1 ⁄4 Yr
1 ⁄4 yR
1 ⁄4 yr
Sperm
1 ⁄4 YR 1 ⁄4 Yr 1 ⁄4 yR 1 ⁄4 yr
YYRR YYRr YyRR YyRr
YYrr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
9 ⁄16 3 ⁄16 3 ⁄16 1 ⁄16
Phenotypic ratio 9:3:3:1
315 108 101 32 Phenotypic ratio approximately 9:3:3:1
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with
green-wrinkled seeds—were crossed, producing
dihybrid F1 plants. Self-pollination of the F1 dihybrids,
which are heterozygous for both characters,
produced the F2 generation. The two hypotheses
predict different phenotypic ratios. Note that yellow
color (Y) and round shape (R) are dominant.
CONCLUSION The results support the hypothesis
ofindependent assortment. The alleles for seed color
and seed shape sort into gametes independently of
each other.

51. DIHYBRID
CROSS

52. LAWS OF PROBABILITY
Rr ×
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
×
R r
r
R
R
R
1⁄2 R
1⁄2 1⁄2
1⁄ 1⁄4 4
1⁄4 1⁄4
1⁄2 r
r R r
r
Sperm
Eggs

53. SUMMARY OF MENDEL’S
EXPERIMENTS
Genes are distinct entities that remain
unchanged during crosses
Each plant has two alleles of a gene
Alleles segregated into gametes in equal
proportions, each gamete got only one
allele
During gamete fusion, the number of alleles
was restored to two (FERTILIZATION)

54. MENDEL AND HUMAN
TRAITS

55. EXCEPTIONS TO MENDEL’S LAWS

56. INCOMPLETE
DOMINANCE
P Generation
Red
CRCR
F1 Generation
F2 Generation
×
Gametes CR CW
White
CWCW
Pink
CRCW
Sperm
Gametes CR
CR
CR
Eggs 1⁄2
CR
Cw
CR
1⁄2 1⁄2
1⁄2
1⁄2
1⁄2
CR CR CR CW
CR CW CW CW
Neither allele is dominant and
heterozygous individuals have an
intermediate phenotype

57. QUESTION #18:
What is the ratio of F2 heterozygotes during
incomplete dominance?
A. 1
B. 2
C. 3
B

58. CODOMINANCE
Neither allele is dominant and both alleles are expressed in
heterozygous individuals

59. QUESTION #19:
If your blood type is AB what is the dominant
allele?
A. A
B. B
C. both
C

60. POLYGENIC TRAITS
traits that are not controlled by a single gene
locus, but by the combined interaction of many
gene loci
Polygenic traits often show continuous
variation, rather then a few discrete forms
example: eye color, hair color, skin color

61. POLYGENIC TRAITS

62. EPISTASIS
alleles at one gene locus can hide or prevent the
expression of alleles at a second gene locus

63. PLEIOTROPY
This is when a single gene locus affects more
than one trait

64. QUESTION #20:
Give an example of a polygenic trait…
hair, skin or eye color