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Explain the principles of Dominance, Segregation, and independent
assortment
Distinguish between dominant and recessive traits, homozygous
and heterozygous traits; and the genotype and the phenotype
Predict genotypes and phenotypes of parents and offspring using
the laws of inheritance, and
Apply Mendel’s principles in solving genetic problems on
monohybrid and dihybrid crossing
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Born in a German speaking family in the Silesian part of
the Austrian Empire and gained posthumous recognition
as the Founder of the Modern Science of Genetics
Austrian Monk at the Monastery of St Thomas in Czech
Republic
High School teacher of Physics and Natural History
Spent most of his time conducting Biological
Experiments
Famous on his experiments on Crosses of Garden Peas
(Pisum sativum) from 1856-1863
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Charles Robert Darwin (1809-1882): English naturalist Darwin
introduced the Theory of Natural Selection. He expressed his
findings and ideas in his famous research publication, "Origin of
Species by Means of Natural Selection" in the year 1859, after
observing the living communities of Galapagos Islands.
Charles Darwin thought that the traits from parents were
transmitted to their offspring by BLOOD, thus came the blood
theory of heredity. This theory gives rise to the expressions
“bloodline” “blue blood” and “blood relative”
Early concepts about transmission of traits are now considered
inaccurate as we now know that both parents contribute hereditary
traits to their offspring and that these traits are not contained in
blood but in sex cells.
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GENETICS – branch of biology that deals with
heredity and variation of organisms.
Chromosomes carry the hereditary information
(genes)
 Arrangement of nucleotides in DNA
 DNA  RNA  Proteins
Chromosomes (and genes) occur in pairs
Homologous Chromosomes
New combinations of genes occur in sexual
reproduction
◦ Fertilization from two parents
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1. Garden Peas have several varieties that have
observable, contrasting characteristics
2. Garden Peas reproduce by self-pollination (they
reproduce by themselves)
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Published these findings in its Scientific journal in 1866 after he
reported it to a local natural history society
His work was not appreciated until 3 scientists rediscovered his
work at around 1900
 Hugo de Vries, Karl Correns and Erich von Tschermak
* His achievement was only recognized after the turn of the century, many
years after his death
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Gametes: reproductive cells produced by sexually
reproducing organisms.
◦ Two types:
 male gametes = sperm
 In plants: contained in pollen
 Female gametes = eggs
 In plants: contained in ovules
 Ovules contained in carpels
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Fertilization: fusion of egg and sperm
◦ Self-fertilized: fusion of sperm and egg from
same plant
◦ Cross fertilized: fusion of egg and sperm from
two different plants
 Produced hybrids
F1: first generation
F2: second generation
Mendel’s Interpretations
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Modified to incorporate today’s vocabulary
◦ Genes: the hereditary information that determines a single trait
◦ Alleles: alternate forms of a gene
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When an organism inherits two identical alleles for a trait,
organism is said to be homozygous for the trait.
 ex. TT, rr, YY, RR, ss
When an organism inherits to different alleles for one trait,
the organism is called heterozygous for the trait.
 ex. Tt, Rr, Yy, Ss, Ww
Mendel stated that physical traits are inherited as “particles”
Mendel did not know that the “particles” were actually
Chromosomes & DNA
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The initial letter of the name of a recessive trait,
lowercased and italicized, denotes the recessive allele
The same letter in uppercase refers to the dominant
allele
Ex. Tall (S)
Short (s)
1.
2.
3.
4.
Unit Factors in Pairs
Dominance/Recessiveness
Segregation
Independent Assortment
1.
Unit Factors in Pairs (paired Unit Factors)
>Genetic characters are controlled by unit factors that exist in
pairs in individual organisms. Each individual carries a pair of
factors for each trait, and they separate from each other during
fertilization.
>Mendel observed that traits did not blend, he argued that some
invisible factor must determine each of the traits he investigated.
>He termed this factor as merkmal** which means “character”
**Today, the factor that determines trait is called GENES
2.
Law of Dominance/Recessiveness
If an organism inherits different alleles for the
same trait, one allele may be DOMINANT over the
other.
In Mendel’s Experiment,
Why were all the seeds in the F1 generation round?
Dominant vs. Recessive Allele
Dominant: an allele that is expressed whenever it is
present
Recessive: an allele that is masked whenever the
dominant allele is present.
Dominant and recessive alleles influence an organism’s
phenotype
Mendel’s F1 Crosses
When Mendel crossed plants with contrasting traits, the hybrid offspring showed
traits of only one parent.
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Genotype: Genetic makeup of an individual. It is
determined by the alleles present for each trait.
ex. Hh, TT, yy, Bb, Rr
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Phenotype: Physical appearance of a trait. It is the
expression of the genotype.
ex. Tall, long, brown, hairy, wrinkled
3.
Law of Segregation
During the formation of gametes, the paired unit factors
separate or segregate randomly so that each gamete
receives one or the other with equal likelihood.
A parent contributes only one of its alleles for a trait to
each offspring.
If parent is heterozygous for a trait, the particular allele
donated to the offspring is random.
Parent Generation: RR (Round Seeds) xrr (Wrinkled Seeds)
Gametes:R (Female Gamete) r (Male Gamete)
R
Genotypic ratio: 4 Rr or 100% Rr (4:0 Rr)
r
Rr
Phenotypic ratio: 4 Round (4:0 Round: wrinkled)
(Round)
r
Rr
(Round)
First filial generation (F1) Rr (Round Seeds) Self-fertilized
Gametes: R and r
Male Gametes = Rr
Female Gametes = Rr
Rr x Rr
Second Filial Generation (F2)
Genotypic ratio: 1 RR : 2 Rr : 1 rr
Phenotypic ratio: 3 Round (RR + Rr) : 1 Wrinkled (rr)
R
Rr (Round)
Rr(Wrinkled)
4.
Independent Assortment
During gamete formation, segregating
pairs of unit factors assort
independently of each other.
Alleles of one gene are passed to
offspring independently of the alleles of
other genes.
Applies to the inheritance of two or
more genes simultaneously.
Punnett Squares- named after British geneticist
Reginald C. Punnett, the person who first
devised this method of analysis
-Uses a checkerboard or table format to show
the genotypes of possible gametes from each
parent. The vertical columns represent those of
the female parent, and the horizontal rows
represent those of the male parent.
- By filling out the Punnett square, we are
listing all possible random fertilization events.
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Testcross is used to determine the genotype of an individual
◦ Testcross: crossing an organism with unknown genotype with
one that is homozygous recessive for the trait.
◦ Example:
 Plant that has round (R) seeds crossed with one that is
homozygous recessive (rr) for wrinkled seeds
R?
x
rr
R?
x
rr
? = R or r
?=R
?=r
R
r
r
Rr
rr
r
Rr
rr
Genotypic ratio: 100% Rr or 4:0 (Rr)
Genotypic ratio: 2:2 (Rr : rr)
Phenotypic ratio: 100% Round (4:0 Round wrinkled) Phenotypic ratio: 2:2(round:wrinkled)
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Used to predict possible offspring genotypes
Place alleles for each parent on each side
T
Mother’s genotype
TT
x
T = being tall
t = short
Father’s genotype
Tt
T
T
t
Given parents’ genotypes,
you can predict offspring’s genotypes and phenotypes
T
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Can use the inheritance of traits in fruit flies to
illustrate the law of independent assortment.
Dihybrid cross
◦ A fertilization in which the parents differ in two
distinct traits or characteristics.
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Gene – a unit of heredity; a section of DNA sequence
encoding a single protein
Genome – the entire set of genes in an organism
Alleles – two genes that occupy the same position on
homologous chromosomes and that cover the same
trait (like ‘flavors’ of a trait).
Locus – a fixed location on a strand of DNA where a
gene or one of its alleles is located.
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Homozygous – having identical genes (one from each
parent) for a particular characteristic.
Heterozygous – having two different genes for a
particular characteristic.
Dominant – the allele of a gene that masks or
suppresses the expression of an alternate allele; the
trait appears in the heterozygous condition.
(represented by uppercase letters)
Recessive – an allele that is masked by a dominant
allele; does not appear in the heterozygous condition,
only in homozygous. (represented by lowercase letters)
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Genotype – the genetic makeup of an organisms
Phenotype – the physical appearance of an organism
(Genotype + environment)
Monohybrid cross: a genetic cross involving a single pair
of genes (one trait); parents differ by a single trait. e.g.
flower color
Dihybrid cross - cross involving two traits
e.g. flower color & plant height
P = Parental generation
F1 = First filial generation; offspring from a genetic cross.
F2 = Second filial generation of a genetic cross
Trait - any characteristic that can be passed from parent
to offspring
Heredity - passing of traits from parent to offspring
 Homozygous genotype - gene combination
involving 2 dominant or 2 recessive genes
(e.g. RR or rr); also called pure
 Heterozygous genotype - gene combination of
one dominant & one recessive allele
(e.g. Rr); also called hybrid
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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
The gametes are:
T

t t
T
t
t
T
t
T
T
t

T
t
Tt
Tt
Tt
Tt
t
• If either parent is HOMOZYGOUS
T
T
t

t
T
Tt
You only need one box!
t
Genotypes:
100% T t
Phenotypes:
100% Tall plants
T
T
t
t
Tt
Tt
Tt
Tt
t
=
Genotypes:
100% T t
Phenotypes:
100% Tall plants
T
Tt
A heterozygous with a homozygous
T
t
t

You can
still use the
shortcut!
t
T
t
Tt
t t
t
Genotypes:
50% T t
50 % t t
Phenotypes:
50% Tall plants
50% Dwarf plants
Pp
Pp
P
p

P
p
PP
Pp
Pp
pp
Genotypes:
1 PP
2 Pp
1 pp
Phenotypes:
3 Purple
1 White
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Parents differ by a single trait.
Crossing two pea plants that differ in stem size, one tall one
short
P = allele for Tall
p = allele for dwarf
PP = homozygous tall plant
pp= homozygous dwarf plant
PP  pp
Monohybrid cross for stem
length:
P = parentals
true breeding,
homozygous plants:
F1 generation
is heterozygous:
TT  tt
(tall)
(dwarf)
Tt
(all tall plants)
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Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2 alleles affect a
characteristic
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Possible combinations are:
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Genotypes
RR
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Phenotypes
RED
Rr
RED
rr
YELLOW
Cross 2 Pure
Plants
TT x tt
Results in
all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
Mendel’s Principles of Heredity apply
universally to all organisms.
Cystic Fibrosis: a lethal genetic disease
affecting Caucasians.
Caused by mutant recessive gene carried by 1
in 20 people of European descent (12M)
One in 400 Caucasian couples will be both
carriers of CF – 1 in 4 children will have it.
CF disease affects transport
in tissues – mucus is accumulated
in lungs, causing infections.
IF two parents carry the recessive gene of
Cystic Fibrosis (c), that is, they are
heterozygous (C c), one in four of their
children is expected to be homozygous
for cf and have the disease:
C C = normal
C c = carrier, no symptoms
c c = has cystic fibrosis
C
c
C
c
CC
Cc
Cc
cc
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Of course, the 1 in 4 probability of getting the
disease is just an expectation, and in reality, any
two carriers may have normal children.
However, the greatest probability is for 1 in 4
children to be affected.
Important factor when prospective parents are
concerned about their chances of having affected
children.
Now, 1 in 29 Americans is a symptom-less carrier
(Cf cf) of the gene.
Gaucher Disease is a rare, genetic disease. It causes lipidstorage disorder (lipids accumulate in spleen, liver, bone
marrow)
It is the most common genetic disease affecting Jewish
people of Eastern European ancestry
(1 in 500 incidence; rest of pop. 1 in 100,000)
Matings that involve parents that differ in two
genes (two independent traits)
For example, flower color:
P = purple (dominant)
p = white (recessive)
and stem length:
short
T = tall
t=
TT PP  tt pp
(tall, purple)
Possible Gametes for parents
TP
and t p
TP
(short, white)
tp
tp
tp
tp
TtPp
TP TtPp
TtPp
TtPp
TtPp
TtPp
TtPp
TtPp
TP
TtPp
TtPp
TtPp
TtPp
TP
TtPp
TtPp
TtPp
TtPp
F1 Generation: All tall, purple flowers (Tt Pp)
TT PP  tt pp
(tall, purple)
(short, white)
Possible Gametes for parents
T P
TP
tp
t p
Tt Pp
F1 Generation: All tall, purple flowers (Tt Pp)
If F1 generation is allowed to self pollinate, Mendel
observed 4 phenotypes:
Tt Pp X Tt Pp
(tall, purple) (tall, purple)
tP
tp
TTPP TTPp TtPP
Tp TTPp TTpp TtPp
TtPp
Ttpp
tP
TP
Possible gametes:
TP Tp tP tp
Tp
TP
TtPP
TtPp
ttPP
ttPp
tp TtPp
Ttpp
ttPp
ttpp
Four phenotypes observed
Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1)
9 Tall
purple
TP
Tp
tP
TP
3 Tall
white
3 Short
purple
1 Short
white
tp
TTPP TTPp TtPP
Tp TTPp TTpp TtPp
TtPp
Ttpp
tP
TtPP
TtPp
ttPP
ttPp
tp TtPp
Ttpp
ttPp
ttpp
Phenotype Ratio = 9:3:3:1
Dihybrid cross: 9 genotypes
Genotype ratios (9):
1
2
TTPP
TTPp
Four Phenotypes:
Tall, purple (9)
2
TtPP
4
TtPp
1
TTpp
2
Ttpp
1
2
ttPP
ttPp
Short, purple (3)
1
ttpp
Short, white (1)
Tall, white (3)
When you have an individual with an unknown
genotype, you do a test cross.
Test cross: Cross with a homozygous recessive
individual.
For example, a plant with purple flowers can either be
PP or Pp… therefore, you cross the plant with a pp
(white flowers, homozygous recessive)
P?

pp
Chromosomes carry hereditary info (genes)
Chromosomes (and genes) occur in pairs
New combinations of genes occur in sexual
reproduction
Monohybrid vs. Dihybrid crosses
Mendel’s Principles:
◦ Dominance: one allele masks another
◦ Segregation: genes become separated in gamete formation
◦ Independent Assortment: Members of one gene pair
segregate independently from other gene pairs during
gamete formation
Mendel was lucky!
Traits he chose in the
pea plant showed up
very clearly…
One allele was dominant over another, so
phenotypes were easy to recognize.
But sometimes phenotypes are not very
obvious…
Thank
You