Inheritance

 One of the most exciting
fields of biology is the study
of patterns of inheritance or
GENETICS

What is genetics?
• It is the study of the mechanism of
heredity and variations by which traits or
characteristics are passed from one
generation to another
• These mechanisms are dependent on the
behavior of chromosomes during the
formation
of sex cells (meiosis) and the way in which
this
• cells are brought together in fertilization

What is heredity?
Refers to the similarities between
parents and offspring
It is the transfer of genetically
controlled characteristics from one
generation to the next
Examples: hair color and color of the
eyes, height, intelligence(human)
flower color (plants)

What is variation?
Variation is difference, whether in
the expression of somatic characters
or in the elements of the germinal
substance, exhibited among groups of
organisms related by descent
(ancestors).

Example of variation:
At the time the chromosome theory of
inheritance was proposed, scientists began
studying the inheritance of traits in the fruit fly
(Drosophila melanogaster)
In a bottle containing normal (wild type)
red-eyed flies, a white- eyed fly was
discovered in a bottle. This variation was
produced by mutation- an inherited change
in the gene controlling the eye color

Chromosomal mutation affect the number
and structure of chromosomes.
Mutation whether genetic or chromosomal
are defined as any heritable change and
are the sources of all genetic variation
Mutation therefore is a random change in a
gene or chromosome resulting in a new
trait or characteristic that can be inherited.
 Mutation can be a source of beneficial
genetic variation, or it can be neutral or
harmful in effect.

History ofHistory of
GeneticsGenetics

9
Gregor MendelGregor Mendel
(1822-1884)(1822-1884)
ResponsibleResponsible
for the Lawsfor the Laws
governinggoverning
Inheritance ofInheritance of
TraitsTraits

10
Gregor Johann MendelGregor Johann Mendel
Austrian monkAustrian monk
Studied theStudied the
inheritanceinheritance of traitsof traits
inin pea plantspea plants
Developed theDeveloped the
laws of inheritancelaws of inheritance
Mendel's work wasMendel's work was
not recognized untilnot recognized until
the turn of thethe turn of the 20th20th
centurycentury

11
Gregor Johann MendelGregor Johann Mendel
BetweenBetween 1856 and1856 and
1863,1863, MendelMendel
cultivated andcultivated and
tested sometested some 28,00028,000
pea plantspea plants
He found that theHe found that the
plants' offspringplants' offspring
retainedretained traits of thetraits of the
parentsparents
Called theCalled the “Father“Father
of Genetics"of Genetics"

12
He bred gardenHe bred garden
peas in thepeas in the
monastery gardenmonastery garden
and analyzed theand analyzed the
offspring of theseoffspring of these
crossescrosses
Site of GregorSite of Gregor
Mendel’sMendel’s
experimentalexperimental
garden in thegarden in the
Czech RepublicCzech Republic

13
Reasons why Mendel choseReasons why Mendel chose
garden peas (garden peas (Pisum sativum)Pisum sativum)
1.1. Can be grown in aCan be grown in a
small areasmall area
.2 Plants Reproduce.2 Plants Reproduce
at a fast rateat a fast rate
. ,3 Thus a number. ,3 Thus a number
ofof
generations cangenerations can
bebe
produced withinproduced within

14
.4.4 They are hardyThey are hardy
plants which doplants which do
not need muchnot need much
caring andcaring and
cultivatingcultivating
.5 Plants Produce.5 Plants Produce
lots of offspringlots of offspring

15
66. Produce pure plants. Produce pure plants
when allowed towhen allowed to
-self-self
pollinate severalpollinate several
generationsgenerations
.7 Can be artificially.7 Can be artificially
-cross pollinated-cross pollinated
.8 Garden pea has.8 Garden pea has
several contrastingseveral contrasting
characterscharacters

16
Reproduction in Flowering PlantsReproduction in Flowering Plants
Pollen contains spermPollen contains sperm
Produced by theProduced by the
stamenstamen
Ovary contains eggsOvary contains eggs
Found inside theFound inside the
flowerflower
Pollen carries sperm to thePollen carries sperm to the
eggs for fertilizationeggs for fertilization
Self-fertilizationSelf-fertilization cancan
occur in the same floweroccur in the same flower
Cross-fertilizationCross-fertilization cancan
occur between flowersoccur between flowers

17
Eight Pea Plant TraitsEight Pea Plant Traits
Seed shapeSeed shape ------ Round (R) or Wrinkled (r)Round (R) or Wrinkled (r)
Seed ColorSeed Color -------- Yellow (Y) or Green (Yellow (Y) or Green (yy))
Pod ShapePod Shape ------ Smooth (S) or wrinkled (Smooth (S) or wrinkled (ss))
Pod ColorPod Color ------ Green (G) or Yellow (g)Green (G) or Yellow (g)
Seed Coat ColorSeed Coat Color ------Gray (G) or White (g)Gray (G) or White (g)
Flower positionFlower position------Axial (A) or Terminal (a)Axial (A) or Terminal (a)
Plant HeightPlant Height ------ Tall (T) or Short (t)Tall (T) or Short (t)
Flower color ---Flower color --- Purple (P) or white (Purple (P) or white (pp))
TRAITS
CORRESPONDING
CHARACTERS

19
How Mendel Began his expt.How Mendel Began his expt.
Mendel produced pure strains by allowing the plantsMendel produced pure strains by allowing the plants
to self-pollinate for several generationsto self-pollinate for several generations
This way he was
sure that the
character (ex.
round seeds) was
“pure”.
All the pea plants
which he
cultivated for this
purpose produced
only “round
seeds”

20
How Mendel Began his expt.How Mendel Began his expt.
•He describe such
plants which produce
the same character
from generation to
generation as pure-
breeding or “breed
true”.
Mendel did the same
with the plants
bearing wrinkled
seeds

Second step: he cross pollinate the
pure breed of round seeded plants with
the wrinkled seeded plants
Round seed x Wrinkled seedRound seed x Wrinkled seed
F1: All round seedF1: All round seed

TRAIT P1 GENERATION
(Cross Pollinated)
F1 GENERATION
Shape of Seed Round X Wrinkled All round
Color of Seed Yellow X Green All yellow
Color of Seed Coats Colored X White All colored
Shape of Pod Inflated X Constricted All inflated
Color of Pod Green X Yellow All green
Position of Flowers Axial X Terminal All axial
Length of Stem Long X Short All long
Color of the Flowers Purple X White All purple
FIND OUT FROM THE TABLE BELOW
WHICH OF THE CHARACTERS ARE
DOMINANT AND WHICH ARE
RECESSIVE
Mendel’s P1 Crosses (Cross-Pollination)

MENDELLIAN LAWS OF
GENETICS
From the results of his experiments,
he formulated the three laws:
1.Law of Dominance
2.Law Segregation
3.Law of Independent
Assortment

Law of Complete DominanceLaw of Complete Dominance
States that a cross between homozygous
dominant genes and homozygous recessive genes
will result in a progeny of heterozygous genes
determining all dominant traits (all the offspring
will show only one of the characters)
The trait that is observed in the
offspring is the dominant trait
The trait that disappears in the
offspring is the recessive trait

Parental Phenotypes:Parental Phenotypes:
Curly Hair x Straight HairCurly Hair x Straight Hair
Genotypes:Genotypes: CCCC cccc
Curly HairsCurly Hairs
CcCc
EXAMPLE: FOUND IN YOUR BOOK
Phenotype of offspring:
Genotype of offspring:
100% curly
100% Cc

Third Step Of His ExperimentThird Step Of His Experiment
F2: 5474 round: 1850 wrinkledF2: 5474 round: 1850 wrinkled
(3/4 round to 1/4 wrinkled)(3/4 round to 1/4 wrinkled)
F1: All round seedF1: All round seed
F1 round plantsF1 round plants xx F1 round plantsF1 round plants
He allowed the F1 plants to self
pollinate

Law of DominanceLaw of Dominance
29

Generation “Gap”Generation “Gap”
Parental PParental P11 GenerationGeneration == the parentalthe parental
generation in a breeding experimentgeneration in a breeding experiment..
FF11 generationgeneration == the first-generationthe first-generation
offspring in a breeding experiment.offspring in a breeding experiment.
(1st filial generation)(1st filial generation)
From breeding individuals from the PFrom breeding individuals from the P11
generationgeneration
FF22 generationgeneration == the second-generationthe second-generation
offspring in a breeding experiment.offspring in a breeding experiment.
(2nd filial generation)(2nd filial generation)
From breeding individuals from theFrom breeding individuals from the30

Following the GenerationsFollowing the Generations
31
Cross 2Cross 2
PurePure
PlantsPlants
TT x ttTT x tt
Results inResults in
allall
HybridsHybrids
TtTt
Cross 2 HybridsCross 2 Hybrids
getget
3 Tall & 1 Short3 Tall & 1 Short
TT, Tt, ttTT, Tt, tt

TRAIT P1 GENERATION
(Cross Pollinated
F1 GENERATION Actual
Ratio
Shape of Seed Round X Round 5, 474 round
1,850 wrinkled
7,324 total
2.93:1
Color of Seed Yellow X Yellow 6,022 yellow
2001 white
8023 total
3.01:1
Color of Seed
Coats
Colored X Colored 705 colored
224 white
929 total
3.15:1
Shape of Pod Inflated X Inflated 882 imnflated
229 constricted
1181 total
2.95:1
Color of Pod Green X Green 428 green
152 yellow
580 total
2.82:1
Position of Flowers Axial X Axial 651 axial
207 terminal
858 total
3.4:1
Length of Stem Long X Long 787 long
288 short
1064 total
2.84:1
Mendel’s P2 Crosses (Self pollination)

Mendel analyzed the data closely.
To his surprise there was a pattern in
the results. He also noticed that the
greater number of offspring
produced, the clearer was the pattern
It shows in the 4th
column that in the
F2 generation, the ratio of the plants
with the dominant character to the
plants with the recessive character
was almost 3:1

From the results of his P2 crosses, he
formulated his second law –The law of
Segregation- states that individuals carry
two hereditary alleles affecting any given
character
According to this law also that, the determiner
of a recessive character, which does not
appear in the F1 (Rr) generation, separates
from the chance for the determiners of the
recessive character to combine during
reproduction, and appear in the F2
Law of SegregationLaw of Segregation

Mendel reasoned that each of the F1
plants must have contained two
determiners, one for round seed and
one for wrinkled seed. However , the
determiner for round seed masked the
determiner fro wrinkled seed . Mendel
used the term “unit determiner” for
what we called as “GENES” now

The same gene can have many versions
• A gene is a piece of DNA that directs a cell to
make a certain protein.
• Each gene has a locus, a specific position on
a pair of homologous chromosomes.

• An allele is any alternative form of a
gene occurring at a specific locus on a
chromosome.
– Each parent donates one
allele for every gene.
– Homozygous describes
two alleles that are the
same at a specific locus.
– Heterozygous describes
two alleles that are
different at a specific
locus.
– The 2 alleles are known
to be found on copies of
chromosomes – one from
each parent–

Alleles can be represented using letters
– A dominant allele is
expressed as a phenotype
when at least one allele is
dominant.
– A recessive allele is
expressed as a phenotype
only when two copies are
present.
– Dominant alleles are
represented by uppercase
letters; recessive alleles by
lowercase letters.

To make easier for him to analyze the
results of his experiments, he designated
symbols for the characters
PLANT (GARDEN
PEA)
SYMBOLS
Pure breeding P1
plant bearing round
seeds
RR
Pure breeding P1
plant bearing
wrinkled seeds
rr
F1 Plants bearing
round seeds
Rr

Mendel said that during formation of gametes, the
determiners separate. Hence only one member of a
pair of determiners goes to a gamete
HomozygousHomozygous
DominantDominant
HomozygousHomozygous
RecessiveRecessive
HeterozygousHeterozygous
RR rrRR rr
F1: All round seed coatsF1: All round seed coats
RrRr

R R R R
Homozygous parents can only pass oneHomozygous parents can only pass one
form of an allele to their offspring.form of an allele to their offspring.

R r R r
Heterozygous parents can pass eitherHeterozygous parents can pass either
of two forms of an allele to their offspring.of two forms of an allele to their offspring.

TERMINOLOGIES
• PHENOTYPE – the way an organism looks and
behaves- its physical characteristics (Ex. Tall, short,
green, white, brown hair, blue eyes, etc. )
• GENOTYPE – the gene combination (allelic
combination) of an organism – Ex. TT, Tt, tt, etc.)
PhenotypePhenotype:: Round seedRound seed
Genotype :Genotype : RR (homozygous dominant)RR (homozygous dominant)
Rr (heterozygous dominantRr (heterozygous dominant))
Phenotype:Phenotype: Wrinkled seedWrinkled seed
Genotype:Genotype: rr (homozygous recessive)rr (homozygous recessive)
Principles of Heredity

– HOMOZYGOUS – 2 ALLELES ARE THE SAME
when an organism possesses two identical
alleles.
ex. YY or yy
– HETEROZYGOUS – 2 ALLELES DIFFERENT
when an organism possesses different alleles.
ex. Yy

Law of Segregation
In the formation of gametes, the members of a
pair of alleles separate (or segregate) cleanly
from each other so that only one member is
included in each gamete.
Each gamete has an equal probability of
containing either member of the allele pair.
When gametes (sperm, egg) areWhen gametes (sperm, egg) are
formed, each gamete will receiveformed, each gamete will receive
one allele or the other.one allele or the other.

Law of Segregation
• 1. A pea plant contains two discrete
hereditary factors, one from each parent
• 2. The two factors may be identical or
different
• 3. When the two factors of a single trait are
• different
– One is dominant and its effect can be seen
– The other is recessive and is masked
• 4. During gametogenesis (meiosis), the
paired factors segregate randomly so that
half of the gametes received one factor and
half of the gametes received the other
– This is Mendel’s Law of Segregation 2-25

Monohybrid cross
(cross with only 1 trait)
• Problem:
• Using this is a several step process,
look at the following example
– Tallness (T) is dominant over shortness
(t) in pea plants. A Homozygous tall
plant (TT) is crossed with a short plant
(tt). What is the genotypic makeup of
the offspring? The phenotypic
makeup ?

Punnett Squares
• Genetic problems can be easily solved
using a tool called a Punnett Square.
– Tool for calculating genetic probabilities
A punnett squareA punnett square

Punnet Square Process
1. Determine alleles
of each parent,
these are given as
TT, and tt
respectively.
2. Take each possible
allele of each
parent, separate
them, and place
each allele either
along the top, or
along the side of
the punnett square.

Punnett process continued
• Lastly, write the letter
for each allele across
each column or down
each row. The
resultant mix is the
genotype for the
offspring. In this
case, each offspring
has a Tt
(heterozygous tall)
genotype, and simply
a "Tall" phenotype.

Punnett process continued
• Here we have some more
interesting results: First
we now have 3
genotypes (TT, Tt, & tt) in
a 1:2:1 genotypic ratio.
We now have 2 different
phenotypes (Tall & short)
in a 3:1 Phenotypic
ratio. This is the
common outcome from
such crosses.

F1 GENERATIONF1 GENERATION
FATHERFATHER MOTHERMOTHER
T tT t T tT t
TT TT TT tt tt tt
F2 GENERATIONF2 GENERATION
- the- the law of dominancelaw of dominance explained the heredity ofexplained the heredity of
the offspring ofthe offspring of the f1 generationthe f1 generation
- the- the law of segregationlaw of segregation explained the heredity ofexplained the heredity of
thethe f2 generationf2 generation

Another Example of Monohybrid Cross
Smooth and wrinkled parental seed strains crossed.
F1 genotypes F1 phenotypes
4/4 Ss 4/4 smooth

With the previous example ,
apply the law of segregation and
find the phenotypic ratio and
genotypic ratio of the F2.
Seat work:

ANSWER
Parentals:Parentals:
SS x ssSS x ss
S S s sS S s s
SS
SS
s ss s
F1 x F1:F1 x F1:
Ss x SsSs x Ss
R r R rR r R r
½ S½ S
½ s½ s
½ S ½ s½ S ½ s
¼ SS¼ SS ¼ Ss¼ Ss
¼ Ss¼ Ss ¼ ss¼ ss
SsSs SsSs
SsSs SsSs
Genotypic RatioGenotypic Ratio: ¼ RR + ½ Rr + ¼ rr or 1:2:1: ¼ RR + ½ Rr + ¼ rr or 1:2:1
Phenotypic RatioPhenotypic Ratio: ¾ Round + ¼ Wrinkled: ¾ Round + ¼ Wrinkled

Law of Independent
Assortment
-states that alleles of different genes are
distributed randomly to the gametes and
fertilization occurs at random
- To explain this law, a dihybrid cross will
be used. This type of cross uses two
traits.

Dihybrid crosses
• Dihybrid crosses are made when
phenotypes and genotypes composed of
2 independent alleles are analyzed.
• Process is very similar to monohybrid
crosses.
• Example:
– 2 traits are being analyzed
– Plant height (Tt) with tall being dominant to
short,
– Flower color (Ww) with Purple flowers being
dominant to white.

Dihybrid cross example
• The cross with a pure-breeding (homozygous)
Tall, Purple plant with a pure-breeding Short,
white plant should look like this.
F1 generationF1 generation

Dihybrid cross example continued
• Take the offspring and cross them since they are
donating alleles for 2 traits, each parent in the F1
generation can give 4 possible combination of alleles.
TW, Tw, tW, or tw. The cross should look like this.
(The mathematical “foil” method can often be used
here)
F2 GenerationF2 Generation
F1 generationF1 generation

Dihybrid cross example continued
• Note that there is a
9:3:3:1 phenotypic ratio.
9/16 showing both
dominant traits, 3/16 &
3/16 showing one of the
recessive traits, and 1/16
showing both recessive
traits.
• Also note that this also
indicates that these
alleles are separating
independently of each
other. This is evidence of
Mendel's Law of
independent assortment

Mendel’s Principle of
Independent Assortment
When gametes are formed, the
alleles of one gene segregate
independently of the alleles of
another gene producing equal
proportions of all possible gamete
types.

Genetic Segregation +
Independent Assortment
RRYY x rryyRRYY x rryy
RY RY RY RY ry ry ry ryRY RY RY RY ry ry ry ry
ryry
RYRY RrYyRrYy
F1: 100% RrYy, round, yellowF1: 100% RrYy, round, yellow

F1 x F1:
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
¼ RY ¼ Ry ¼ rY ¼ ry
¼ RY
¼ Ry
¼ rY
¼ ry
1/16 RRYY 1/16 RRYy 1/16 RrYY 1/16 RrYy
1/16 RRYy 1/16 RRyy 1/16 RrYy 1/16 Rryy
1/16 RrYY 1/16 RrYy 1/16 rrYY 1/16 rrYy
1/16 RrYy 1/16 Rryy 1/16 rrYy 1/16 rryy
1 432
9
8765
16151413
10 11 13

F2 Genotypes and PhenotypesF2 Genotypes and Phenotypes
PhenotypesPhenotypes GenotypesGenotypes
RoundRound
YellowYellow
1/16 RRYY + 2/16 RRYy +1/16 RRYY + 2/16 RRYy +
2/16 RrYY + 4/16 RrYy2/16 RrYY + 4/16 RrYy
Total = 9/16 R_Y_Total = 9/16 R_Y_
RoundRound
GreenGreen
1/16 RRyy+ 2/16 Rryy1/16 RRyy+ 2/16 Rryy
Total = 3/16 R_yyTotal = 3/16 R_yy
Wrinkled YellowWrinkled Yellow 1/16 rrYY+ 2/16 rrYy1/16 rrYY+ 2/16 rrYy
Total = 3/16 rrY_Total = 3/16 rrY_
Wrinkled GreenWrinkled Green 1/16 rryy1/16 rryy

Refer to figure above:
Start with the shape of the seed
List down your answers on a piece of paper
Note: Mendel explained these observations as follows:
-The determiners for seed shape are inherited
separately from
the determiners for seed colors
-That is Mendel’s Law of Independent Assortment of
determiners. The inheritance of seed coats has no
effect on
the inheritance of seed color, each character is
inherited as
a unit.
Suppose you describe the appearance of the offspring
in the table above , what are their phenotypes? What
are their Genotypes?

Meiotic Segregation explainsMeiotic Segregation explains Independent AssortmentIndependent Assortment

THE NON-MENDELIAN LAWS OF
INHERITANCE
•Refer to hereditary patterns
that are not in accordance with
Mendel’s principles or those
that are not attributed to single
autosomal genes.

A. Law of Incomplete Dominance
- this principle states that a cross
between homozygous dominant
and recessive genes will result in
a progeny of heterozygous
dominant and recessive trait.
Both alleles exert an effect and
jointly produce an intermediate
phenotype.

See book: A cross using colors of the
petals of flower (red and white)
RR x rrRR x rr
R R r rR R r r
RR
RR
r rr r
RrRr RrRr
RrRr RrRr
Genotype of offspringGenotype of offspring: 100% Rr: 100% Rr
Phenotype of offspringPhenotype of offspring: 100% pink: 100% pink
Genotypic offspring: Rr; Rr; Rr; Rr
Phenotype of offspring: pink; pink;
pink; pink
The resulting genotypes of the offspring are all heterozygous (Rr)
just like in the first law, however the phenotype that is expressed is
not the dominant trait red, rather, an intermediate color between red
and white which is PINK

B.LAW OF CO-DOMINANCE
o This principle states that a cross
between homozygous dominant
genes will result in a progeny of
heterozygous genes determining a
phenotype where a mix of the
dominant recessive traits is
expressed.

Look at the diagram in your book to show this kind of cross.
This time the trait used is the color of coat of cattle
Parentals:Parentals: CR =
red color coatred color coat;; Cr =
white color coatwhite color coat
CR
CR
xx Cr
Cr
CR
CR
Cr
Cr
CR
CR
Cr
Cr
CR
Cr
Genotype of offspringGenotype of offspring: 100%: 100% CR
Cr
Phenotype of offspringPhenotype of offspring: 100% roan: 100% roan
Genotype of offspring:
CR
Cr -
CR
Cr –
CR
Cr-
CR
Cr
Phenotype of offspring: roan; roan;
roan; roan
CR
Cr
CR
Cr
CR
Cr

The roan phenotype appears pink in color, but with
respect to the hair composition, some are red and
some are white. The blending of red and white hair
will appear pink. The difference of this cross is that
the dominant trait (red hair) is expressed equally
with the recessive trait (white hair). There is the
absence of the intermediate trait that was present
in then previous cross of incomplete dominance

SEAT WORK: Open
Notes
• DO AT LEAST THREE OTHER TRAITS OF THE
PEA PLANT FOLLOWING THE PROCEDURE
DONE BY MENDEL:
• FIND F1 AND F2
• FIND GENOTYPES PHENOTYPES RATIO

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