Gregor Mendel conducted experiments with pea plants to study inheritance and genetics. He found that traits are passed from parents to offspring through discrete units (now known as genes). Mendel identified two laws of inheritance: the Law of Dominance and the Law of Segregation. The Law of Dominance states that some traits (called dominant traits) are expressed even if only one gene is present, while other traits (called recessive traits) are only expressed if two copies of the gene are present. The Law of Segregation states that organisms pass only one of their two genes for each trait randomly to their offspring. Mendel's experiments were the foundation of classical genetics and laid the basis for understanding inheritance.

1. Classical Mendelian Genetics
Presented By: Mahrukh Shehzadi

2. Definition of Genetics
Branch of Biology that studies the ways
in which hereditary information is
passed on from parents to offspring.
Genetics as a science was first practiced
by Gregor Mendel
Mendel was an Austrian monk who
experimented for many years with pea
plants

3. Mendel (cont.)
Before his
experiments, people
believed that
“factors” were
inherited as a blend
of Mom and Dad’s
“factors”

4. Mendel (cont.)
Why Peas?
Mendel used peas to study inheritance
because:
True breeding strains were available
Peas are easy to grow and maintain
They have a short generation time

5. Mendel (cont.)
Peas have many easy to observe traits including:
 Seed color - Green or yellow
 Seed shape - Round or wrinkled
 Pod color - Green or yellow
 Pod shape - Smooth or constricted
 Flower color - White or purple
 Flower position - Axial or terminal
 Plant size - Tall or dwarf

6. Mendel (cont.)
Pea flowers are constructed
in such a way that they
typically self fertilize
Because of this, it is
relatively easy to control
crosses in peas
By removing the anthers of
one flower and artificially
pollinating using a brush,
crosses can be easily
controlled in peas.

7. The seven pea characteristics
studied by Mendel
Flower Color
Flower
Position
Seed Color
Seed Shape
Pod Shape
Pod Color
Plant Height
Dominant Recessive

8. Mendel’s First Experiment
•P generation –
(parental) true-
breeding
•F1- (first filial)
offspring of P
generation
•F2 – (second
filial) offspring
from F1 cross

9. Mendel’s First Experiment
In this breeding
experiment, Mendel
cross-pollinated
(hybridized) two
contrasting, true-
breeding pea varieties.
Mendel would then
allow the F1 hybrids to
self-pollinate to produce
an F2 generation.

10. Mendel’s First Experiment
When Mendel allowed
the F1 plants to self-
fertilize, the F2
generation included
both purple-flowered
and white-flowered
plants.
 The white trait, absent
in the F1, reappeared
in the F2.

11. Mendel’s Results
When crossing purple flowered peas
with white flowered peas, Mendel got
the following results:
In the first filial (F1) generation all
offspring produced purple flowers
In the second generation (second filial
or F2) approximately a 3:1 ratio of
purple to white flowers was seen

12. Mendel’s Results
Because the F1 generation did not produce
light purple flowers and because white
flowers showed up in the F2 generation,
Mendel disproved blended inheritance.
Mendel said that the parents had two sets of
factors (genes), thus two copies of the flower
color gene
Each gene has two varieties-called alleles
In the case of the flower color gene, the two
alleles are white and purple

13. Mendel’s Results
In the F1 generation, the white allele
was hidden by the purple “dominant”
allele
In the F2 generation, 1/4 of the
offspring wound up with two copies of
the white allele, thus they were white
Mendel termed this discovery “The Law
of Dominance”

14. Mendel’s First Law: The Law of
Dominance
If only one of the genes in a pair is
expressed, it is called the dominant allele
The gene which is present but not expressed
is called the recessive allele
Recessive alleles may be hidden and not
shown for many generations
When two copies of a recessive allele are
present then the recessive trait will show

15. Mendel’s 2nd Law-The Law of
Segregation
The two alleles
for each
character
segregate
(separate)
during gamete
production

16. Mendel’s 2nd Law-The Law of
Segregation

17. Mendel’s 3rd Law-The Law of
Independent Assortment
When Mendel crossed peas and looked at two
different traits, he discovered that the traits assorted
independently
In other words, if he was looking at the height of the
plants and the color of the flowers, all four possible
combinations of height and flower color were
produced:
Tall Purple
Tall white
Dwarf Purple
Dwarf white

18. Mendel’s 3rd Law-The Law of
Independent Assortment
• As long as genes are on different
chromosomes, they will assort
independently
• This meant that a seed’s color did not
depend on its shape, nor did height of
the plant affect the flower color

19. Mendelian Genetics Vocabulary
Character –heritable feature
Trait – each variant for a character
Dominant Allele: a gene that exerts
it’s full effect no matter what effect it’s
allelic partner may have.
The dominant allele is represented by a
capitalized letter (ex. T=tall)

20. Mendelian Genetics Vocabulary
Recessive Allele: a gene that is
masked by the dominant allele
The recessive allele is represented by
the same letter but lower case (ex.
t=short)
Remember: Diploid organisms have 2
copies of every gene, therefore they
have 2 alleles for a specific trait

21. Mendelian Genetics Vocabulary
Homozygous: an
organism that
contains identical
alleles (ex. TT or tt)
Also called pure-bred
Homozygous parents
can only pass one
form of an allele to
their offspring.
R R R R

22. Mendelian Genetics Vocabulary
Heterozygous: an
organism that
contains two
different alleles (ex.
Tt) Also called
hybrid
Heterozygous
parents can pass
either of two forms
of an allele to their
offspring.

23. Mendelian Genetics Vocabulary
Genotype: genetic make-up of an organism
or the specific alleles
 represented by letters
 ex. tt, TT, Tt
Phenotype: the observable appearance of
an individual resulting from it’s genotype
 ex. Tall, short, yellow

24. Solving Genetic Problems Involving
Different Types of Inheritance
The Key Steps
 1.) decide the type of
inheritance involved
 2.) choose a capital letter for
the dominant trait – it’s lower
case for the recessive
counterpart
 3.) decide your mating (cross)
 4.) construct a “Punnett”
Square to describe results of
mating
 5.) be sure to answer the
question(s) posed in the
problem

25. Making A Punnett Square
White
Flower
Parent
(pp)
Purple
Flower
Parent
(PP)
Gametes
p p
P
P

26. Basic Types of Inheritance +
Genetic Crosses
A.) Simple Dominance
– One trait dominates
over a second
 1.) Test Cross – Used
to determine
whether an organism
is homozygous or
heterozygous. Test
organism is crossed
to a pure recessive.

27. Basic Types of Inheritance +
Genetic Crosses
Double cross-Looking at the inheritance
of two separate traits.
If both organisms are hybrid for both
traits this becomes a “dihybrid-cross”
Mendel noticed that whenever two
dihybrids are crossed, the ratio of the
genotypes is 9:3:3:1

28. Incomplete Dominance
When traits are
crossed and a
blending of the
dominant and
recessive
phenotypes takes
place.

29. Basic Types of Inheritance +
Genetic Crosses
C.) Co-Dominance
(Multiple Alleles) –
Two or more alleles
both show over a
third (human blood
types)
D.) Sex-Linkage –
Traits are carried
only on the sex
chromosomes
Phenotype (Blood
Type)
Genotype
A IAIA or IA i
B IBIB or IB i
AB IAIB
O i i

30. Genetics and People
Genetic Diseases
1. sickle cell anemia
Production of abnormal, crescent shaped red
blood cells. It is autosomal (not in the sex
pair) and recessive (you can be normal, a
carrier, or have the disease)
caused by malformed hemoglobin
blood loses the ability to carry oxygen
red blood cells can stick to sides of blood
vessels and cause a clot

32. Genetics and People
2. Tay-Sachs disease:
It destroys the nervous system of
children. They usually do not live
beyond the age of four.
It is autosomal and recessive (you need
2 copies to have the disease)
common in Jews of eastern European
descent

33. Genetics and People
3. PKU (phenylketonuria):
It is an inherited condition of severe mental
retardation due to a failure to breakdown
phenylalanine, an amino acid.
It is autosomal and recessive.
Brain fails to develop during infancy
Can be prevented by eating a diet free of
phenylalanine
All newborns are screened for this disease

34. Genetics and People
4. Cystic fibrosis
A respiratory disorder caused by
excessive production of thick mucus in
the lungs and pancreas.
Autosomal and recessive.

35. Genetics and People
5. Thalassemia:
A severe form of anemia (loss of red
blood cells).
It is autosomal and recessive.
6. Huntington’s Disease:
An incurable form of insanity that starts
at age 40.
It is autosomal and dominant.

36. Genetics and People
7. Hemophilia- a sex-linked recessive
disease
Blood is missing a protein which allows
it to clot properly
Genetic engineering has led to
manufacture of this protein and better
treatment

37. Genetics and People
B. genetic screening
1. Amniocentesis: a
sample of amniotic
fluid is genetically
analyzed.

38. Genetics and People
2. Karyotype: an
enlarged
photograph of
arranged human
chromosomes.

39. Genetics and People
3. Genetic Counseling:
when high risk
groups are tested
for and advised on
possible genetic
disorders of their
offspring.
Examples:
genealogies or
family pedigrees

40. Genetic Applications
A. Plant and animal breeding: breeders
apply the principles of genetics to
improve plants and animals.
Breeding Methods:
1. Artificial Selection: plants or animals
having desired qualities are selected to
reproduce their kind.
Examples: horse and dog breeders

41. Genetic Applications
2. Hybridization
Crosses are made between organisms having
desirable qualities in the attempt to produce
some offspring that incorporate these
qualities.
Examples: American cattle and Brahman
cattle

42. Genetic Applications
3. Inbreeding
Is the mating of organisms of the same
strain or type
Example: German shepherds will only
be bred with German shepherds
Problem: offspring are more susceptible
to diseases and have shorter life spans.

43. Genetic Applications
4. Vegetative Propagation
Desirable traits in plants can be easily
produced by asexual means.
Example: seedless fruits

44. Heredity and Environment
Both the genetics and the environment
affect the phenotypes and development
of organisms.

45. Heredity and Environment
1. Effect of Temperature on hair color in the Himalayan rabbit
Himalayan rabbits are white
except for its black nose, ears,
tail, and feet.
In experiments, fur was removed from the
rabbits back, an ice pack was
applied while the fur grew back.

46. Heredity and Environment
This low temperature
produced black fur.
Conclusion: low
temperature prevents
the action of the
white fur gene and
enzyme.