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Patterns of
Inheritance
Chapter 13
Mendel and his principles
Gregor Mendel


The Father of Genetics
It was not until the midnineteenth century that
Gregor Mendel, an
Austrian monk, carried
out important studies of
heredity—the passing
on of characteristics
from parents to
offspring.
Why he succeeded.

Mendel was the first person to succeed in
predicting how traits are transferred from one
generation to the next.
 Characteristics that are inherited are called
traits.

A complete explanation requires the careful
study of genetics—the branch of biology that
studies heredity
Mendel’s Experiment

Mendel chose to use garden peas in his
experiments because they possessed 7
characteristics.
 Each characteristic occurred in 2 contrasting
traits
 Garden pea plants reproduce sexually, and
produce male and female
sex cells, called gametes
The 7 Characteristics Mendel
studied
1.
2.
3.
4.
5.
6.
7.
Plant height: long vs short
Flower position along stem: axial &
terminal
Pod appearance: inflated vs constricted
Pod color: green vs yellow
Seed texture: round vs wrinkled
Seed color: yellow vs green
Flower color: purple vs white
Why did he choose the Garden
Pea?
Mendel was able to observe how traits
were passed from 1 generation to the
next by controlling how pea plants were
pollinated
 Pollination is the transfer of pollen
grains from a male (anther), to a female
(stigma) reproductive organ in a plant

Types of Pollination
Self-pollination: occurs when pollen is
transferred from the anthers of a flower
to the stigma of either that flower or
another flower on the same plant
 Cross-pollination: occurs between
flowers of 2 plants. Also can be done
manually

Fertilization

The male gamete forms in the pollen
grain, which is produced in the male
reproductive organ & the female gamete
forms in the female reproductive
organs.

This process is called fertilization and
the result is a fertilized seed called a
zygote which forms into a seed.
Mendel’s experiment
He initially studied each characteristic &
its contrasting traits.
 He grew plants that were true-breeding
(pure) for each trait and allowed them to
self-pollinate

Mendel opened the petals of a flower and removed the male
organs.
He then dusted the female organ with pollen from the plant he
wished to cross it with by cross-pollination
By using this technique, Mendel could be sure of the parents
in his cross
Pollen
grains
Transfer pollen
Male
Female
parts
part
Cross-pollination
Mendel was a careful researcher
He studied only one trait at a time to control variables,
and he analyzed his data mathematically.
The tall pea plants he worked with were from
populations of plants that had been tall for many
generations and had always produced tall offspring.
Such plants are said to be true breeding for tallness.
Likewise, the short plants he worked with were true
breeding for shortness
Mendel’s Monohybrid Crosses
A hybrid is the offspring of parents that have different
forms of a trait, such as tall and short height.
Mendel’s first experiments are called monohybrid
crosses because mono means “one” and the two
parent plants differed from each other by a single
trait—height.
The First Generation - F1
Mendel selected a six-foot-tall pea plant (Parent
Plants or P1) that came from a population of pea
plants, all of which were over six feet tall.
He cross-pollinated this tall pea plant with pollen
from a short pea plant (Parent Plants or P1).
All of the offspring grew to be as tall as the
taller parent.
The Second Generation F2
Mendel allowed the tall plants in this first generation to
self-pollinate.
After the seeds formed, he planted them and counted
more than 1000 plants in this second generation.
Three-fourths of the plants were as tall as the tall
plants in the parent and first generations.
The Second Generation
One-fourth of the offspring
were as short as the short
plants in the parent
generation.
In the second generation, tall
and short plants occurred in a
ratio of about three tall plants
to one short plant.
P1 generation: True-breeding
Parents
F1 generation: 1st Offspring
Two F1 plants: F2 generation
P1
Short pea plant
Tall pea plant
F1
All tall pea plants
F2
3 tall: 1 short
Mendel’s Rules of
Heredity
Mendel Rules of Heredity
The rule of unit factor:

Mendel concluded that each organism has
two factors that control each of its traits.
 These factors are genes and that they are
located on chromosomes in alternative forms.
 We call these different gene forms alleles
 An organism’s two alleles are located on
different copies of a chromosome—one
inherited from the female parent and one
from the male parent
Mendel Rule of Heredity
The Rule of Dominance:
•Mendel called the observed
traits as dominant and the
disappeared traits as recessive
•Tall plants are dominant
(Uppercase letters) and short
plants are recessive (lowercase
letters).
•The dominant allele is always
written first
Short plant
Tall plant
t
T T
t
t
T
F1
All tall plants
T t
Dominant & Recessive Traits
Seed Seed
shape color
Flower
color
Flower
position
Pod
color
Pod
shape
Plant
height
purple
axial
(side)
green
inflated
tall
white
terminal
(tips)
yellow
constricted
short
Dominant
trait
round yellow
Recessive
trait
wrinkled
green
Mendel 1st Law of Heredity
The Law of Segregation:
•The law of segregation states that every individual
has two alleles of each gene and when gametes are
produced, each gamete receives one of these alleles.
• During fertilization, these gametes randomly pair to
produce four combinations of alleles
•The way an organism looks and behaves is called its
phenotype
•The allele combination an organism contains is
known as its genotype
•An organism’s genotype can’t always be known by its
phenotype
Homozygous vs Heterozygous


Homozygous: An
organism is
homozygous for a trait if
its two alleles for the
trait are the same
TT or tt would be the
only possibilities

Heterozygous: An

organism is
heterozygous for a trait
if its two alleles for the
trait differ from each
other
Tt
Genotype & Phenotype

Genotype refers to
“gene type” and
describes the
possible gene
combinations.
 ¼ TT, 2/4 Tt, ¼ tt

Phenotype refers to
“gene expression”
and describes in
words the gene
combinations
 ¾ Tall plants
 ¼ Short plants
Law of Segregation
Tt ´ Tt cross
F1
Tall plant
Tall plant
T
T
t
t
F2
Tall
Tall
T T
T
Tall
t
3
T
Short
t
t
t
1
Two organisms can look
alike but have different
underlying allele
combinations.
Genotype:
G=1/4 TT, 2/4
Tt, 1/4tt
Phenotype:
P=3/4 Tall,
¼ Short
Mendel 2nd Law of Heredity

The Law of Independent Assortment:

Mendel’s second law states that genes for
different traits—for example, seed shape and
seed color—are inherited independently of
each other
Punnett Square

In 1905, Reginald Punnett, an English
biologist, devised a shorthand way of finding
the expected proportions of possible
genotypes in the offspring of a cross called
the Punnett Square
 If you know the genotypes of the parents, you
can use a Punnett square to predict the
possible genotypes of their offspring.
Monohybrid crosses
Heterozygous
tall parent
T
t
T
t
Tt
T
T
TT
Tt
t
t
Tt
Tt
tt
tt
A Punnett square for
this cross is two
boxes tall and two
boxes wide because
each parent can
produce two kinds of
gametes for this trait.
P
Gametes
p
P
Gametes
p
P
p
P
p
pp
P
P
p
P
Pp
p
pp
p
P
p
P
Pp
P PP Pp
p
Pp pp
p Pp pp
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White
(pp)
Purple
(Pp)
Gametes
p
p
P
Gametes
Gametes
p
P
P
Pp
Pp
Gametes
p
P
Purple
(PP)
Pp
Pp
F1 generation
Purple
(Pp)
PP
Pp
Pp
pp
F2 generation
Simple Recessive Heredity

Most genetic disorders are caused by
recessive alleles
1. Cystic Fibrosis

Cystic fibrosis (CF) is a fairly common genetic
disorder among white Americans
 About 1 in 28 white Americans carries the
recessive allele, & 1 in 2500 children born to
white Americans inherits the disorder.

Due to a defective protein in the plasma
membrane, cystic fibrosis results in the
formation and accumulation of thick mucus in
the lungs and digestive tract
2. Tay Sachs Disease

Tay-Sachs (tay saks) disease is a recessive
disorder of the central nervous system among
the Jewish people in the U.S. & eastern
European descent
 A recessive allele results in the absence of an
enzyme that normally breaks down a lipid
produced and stored in tissues of the central
nervous system
 Because this lipid fails to break down
properly, it accumulates in the cells
3. PKU

Phenylketonuria (fen ul kee tun YOO ree uh),
also called (PKU), is a recessive disorder that
results from the absence of an enzyme that
converts one amino acid, phenylalanine, to a
different amino acid, tyrosine.

Because phenylalanine cannot be broken
down, it and its by-products accumulate in the
body and result in severe damage to the
central nervous system
Phenylketonuria

A PKU test is normally performed on all infants a few
days after birth
 Infants affected by PKU are given a diet that is low in
phenylalanine until their brains are fully developed
Phenylketonurics: Contains Phenylalanine
Simple Dominant


Many traits are inherited just as the rule of
dominance predicts
Remember that in Mendelian inheritance, a single
dominant allele inherited from one parent is all that is
needed for a person to show the dominant trait
A cleft chin
Widow’s peak hairline

Hitchhiker’s thumb

Almond shaped eyes

Thick lips

Presence of hair on the middle
section of your fingers


Huntington’s Disease
Huntington’s disease is a lethal genetic
disorder caused by a rare dominant allele
 Is progressive, degenerative disease that
results in a breakdown of certain areas of the
brain
 the onset of Huntington’s disease usually
occurs between the ages of 30 and 50, an
individual may already have had children
before knowing whether he or she is
affected.
 Famous person with Huntington's disease
folk singer Woody Guthrie

Dihybrid Test
Crosses
Using more than one set of
variables to determine
genetics
Mendel’s Dihybrid Crosses
Mendel performed another set of crosses in which he
used peas that differed from each other in two traits
rather than only one.
Such a cross involving two different traits is called
a dihybrid cross.
The First Generation
Mendel took true-breeding pea plants that had round
yellow seeds (RRYY) and crossed them with truebreeding pea plants that had wrinkled green seeds
(rryy).
He already knew the round-seeded trait was dominant
to the wrinkled-seeded trait.
He also knew that yellow was dominant to green.
Gametes from RrYy parent
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRYy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Dihybrid Cross
Ry
rY
ry
Gametes from RrYy parent
RY
A Punnett square
for a dihybrid cross
will need to be four
boxes on each side
for a total of 16
boxes.
The first generation
round yellow x wrinkled green
Dihybrid Cross
P1
Wrinkled green
Round yellow
All
round yellow
F1
9
3
3
1
F2
Round yellow
Round green
Ratio: 9:3:3:1
Wrinkled yellow
Wrinkled green
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P generation
Round, yellow Wrinkled, green
(RRYY)
(rryy)
Meiosis
Meiosis
Round, yellow
crossed with
wrinkled, green
X
Fertilization
(RY)
(ry)
on
F1 generation
All round, yellow
(RrYy)
Meiosis
(chromosomes
assort independently
into four types of
gametes)
(RY)
(Ry)
RY Ry
RY
Ry
rY
ry
(rY)
rY
ry
(ry)
F2 generation
9/16
round, yellow
3/16
round, green
3/16
wrinkled, yellow
1/16
wrinkled, green
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
INCOMPLETE DOMINANCE
BLENDING INHERITANCE
 TRAIT IN WHICH ONE GENE IS NOT
DOMINANT OVER THE OTHER AND
THE HETEROZYGOTE IS A BLEND
 EXAMPLE: JAPANESE FOUR
O’CLOCKS; RED AND WHITE MAKE
PINK

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Eggs
CR
CRCR
CW
CR
Sperm
CRCR
CRCW
CRCW
C WC W
CW
F1 generation
All CRCW
C WC W
F2 generation
1:2:1
CRCR:CRCW:CWCW
CODOMINANCE
ONE GENE IS NOT DOMINANT OVER
THE OTHER AND THE
HETEROZYGOTE EXHIBITS BOTH
FORMS OF THE GENE
 EXAMPLE: A black chicken crossed
with a white chicken results in a
checkered chicken (black & white
feathers)

Sex-Linked Inheritance





The pattern of sex-linked inheritance is
explained by the fact that males, who are XY,
pass an X chromosome to each daughter and
a Y chromosome to each son
Two traits that are governed by X-linked
recessive inheritance in humans are
1. red-green color blindness
2. Hemophilia
3. Muscular Distrophy
Hemophilia
Hemophilia A is an X-linked disorder that causes
a problem with blood clotting.
About one male in every 10 000 has hemophilia,
but only about one in 100 million females inherits
the same disorder
Males inherit the allele on the X chromosome
from their carrier mothers. One recessive allele
for hemophilia will cause the disorder in males
Females would need two recessive alleles to
inherit hemophilia
Red-Green Color
Blindness
People who have red-green
color blindness can’t
differentiate these two colors.
Color blindness is caused by
the inheritance of a recessive
allele at either of two gene
sites on the X chromosome.
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Y chromosome
X chromosome with X chromosome with red-eye gene
white-eye gene
Parents
Male
Female
F1 generation
Male
Female
F2 generation
Males
Females
Epistasis:
Interactions among two or more gene
pairs. For example, 2 alleles of one
gene can mask alleles of another, and
some expected phenotypes may not
occur
 Example: Fur color in labs or rats

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X
White
(AAbb)
White
(aaBB)
F1 generation
All purple
(AaBb)
Eggs
AB
Ab
aB
ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
Sperm
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
F2 generation
9/16 purple
7/16 white
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ee
No dark pigment in fur
E_
Dark pigment in fur
Yellow Lab
E_bb
eebb
eeB_
Yellow fur;
Yellow fur;
brown nose, black nose,
lips, eye rims lips, eye rims
E_B_
Chocolate Lab Black Lab
Brown fur,
nose, lips,
eye rims
Black fur,
nose, lips,
eye rims
Pedigree

A pedigree is a graphic representation of
genetic inheritance
 It is a diagram made up of a set of symbols
that identify males and females, individuals
affected by the trait being studied, and family
relationships
Male
Parents
Female
Siblings
Affected
Known
male
heterozygotes
for recessive
Affected allele
female
Mating
Death
Males: Square
Females: Circle
A horizontal line
connecting a circle and a
square indicates that the
individuals are parents, and
a vertical line connects
parents with their offspring.
Each row shows a
generation with most recent
at the bottom
A half-shaded circle or
square represents a carrier,
a heterozygous individual.
Generation
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George III
Louis I
Grand Duke of HesseI
Edward
Duke of Kent
Queen Victoria
Prince Albert
I
II
Frederick Victoria
III
III
No hemophilia
German
Royal
House
King
Edward VII
Alice
Duke of
Hesse
Alfred
King
George V
Czar
Nicholas II
IV
Duke of King
Windsor George VI
Earl of Waldemar Prince Henry
Sigismond
Mountbatten
V
Prince
Philip
Margaret
VI
Princess
Diana
Beatrice Prince
Henry
No hemophilia
Irene
Queen
Elizabeth II
Helena Arthur Leopold
Prince Anne Andrew Edward
Charles
British Royal House
VII
William Henry
Prussian
Royal
House
Czarina
Earl of Princess Maurice Leopold Queen Alfonso
Alexandra Athlone Alice
Eugenie King of
Spain
?
?
?
Anastasia Alexis Viscount Alfonso Jamie Juan
Tremation
Russian
Royal
House
?
?
Gonzalo
?
King Juan
Carlos
?
No evidence
of hemophilia
No evidence
of hemophilia
Spanish Royal House
Other Genetic
Disorders
Downs Syndrome 21: Trisomy 21
Down syndrome is the only autosomal
trisomy in which affected individuals
survive to adulthood
 It occurs in about one in 700 live births



Down syndrome is a group of
symptoms that results from
trisomy of chromosome 21
Some degree of mental
retardation
Turners Syndrome

Turner (XO) syndrome females have only
one sex chromosome, an X.
 Turner females are short, have a broad
chest and webbed neck.
 Ovaries of Turner females never become
functional; therefore, do not undergo
puberty
Klinefelter Syndrome

Klinefelter syndrome males have one Y
chromosome and two or more X
chromosomes.
 Affected individuals are sterile males; testes
and prostate are underdeveloped.
 Individuals have large hands and feet and
long arms and legs.
Triplo-X Females

Triplo-X females have three or more X
chromosomes.
 There is no increased femininity; most lack
any physical abnormalities.
 There is an increased risk of having triplo-X
daughters or XXY sons.
 May experience menstrual irregularities,
including early onset of menopause.
Jacob Syndrome

XYY males with Jacob syndrome have two
Y chromosomes instead of one.
 Results from nondisjunction during meiosis II.
 Usually taller than average; suffer from
persistent acne; tend to have lower
intelligence.
 Earlier claims that XYY individuals were likely
to be aggressive are not correct