Transcript Mendel and the Gene Idea

Mendel and the
Gene Idea
Chapter 14
1.
2.
Historical view
Work of Mendel
A.
B.
C.
3.
Extending Mendel
C.
D.
Intermidiate inheritance
Multiple Alles
Pleiotrophy
Polygenic inheritance
A.
B.
C.
D.
Human Pedigree
Recessive Disorders
Dominant Disorders
Genetic testing
A.
B.
4.
Principle of Segregation
Independent Assortment
Probability
Mendelian Inheritance in Humans
The History Of Genetics
Pythagoras 500
B.C., a Greek
philosopher,
stated that
human life began
with male and
female fluids
The History Of Genetics
Aristotle furthered
this idea and
suggested that
these fluids, or
“semens,” were
actually purified
blood—therefore,
blood must be part
of heredity.
Historical View
 Hippocrates
and Aristotle proposed
the idea of what they called
pangenes, which they thought were
tiny pieces of body parts.
 They thought that pangenes came
together to make up the
homunculus, a tiny pre-formed
human that people thought grew into
a baby
Antonie van Leeuwenhoek
In the 1600s, the development of the
microscope brought the discovery of
eggs and sperm
 thought he saw the homunculus curled up
in a sperm cell.
 His followers believed that the
homunculus was in the sperm, the father
“planted his seed,” and the mother just
incubated and nourished the homunculus
so it grew into a baby

One Bizarre Theory
•The theory of Homunculus
-17th century
•sex cells contained a
complete miniature adult,
perfect in form
•This statement was
popular way into the 18th
century
Small
individual
Regnier de Graaf
He and his followers thought that they
saw the homunculus in the egg, and the
presence of semen just somehow
stimulated its growth
 In the 1800s, a very novel, “radical” idea
arose: both parents contribute to the
new baby, but people (even Darwin, as he
proposed his theory) still believed that
these contributions were in the form of
pangenes

Genetic Principles


Drawing from the Deck of Genes?
What accounts for the transmission of
traits from parents to offspring?
“Blending” Hypothesis
Possible explanation of heredity is a
“blending” hypothesis
 The idea that genetic material
contributed by two parents mixes in a
manner analogous to the way blue and
yellow paints blend to make green

“Particulate” Hypothesis

An alternative  the gene idea

Parents pass on discrete heritable units,
genes
Modern Genetics




Traces the beginnings to Gregor Mendel, an
Austrian monk, who grew peas in a monastery
garden.
Was unique among biologists of his time
because he quantifiable data, and actually
counted the results of his crosses.
Published his findings in 1865  people didn’t
know about mitosis and meiosis, so his
conclusions seemed unbelievable
His work was ignored until it was rediscovered
in 1900 by a couple of botanists who were
doing research on something else.
Gregor Mendel
Figure 14.1



Documented a particulate mechanism of inheritance
through his experiments with garden peas
He discovered the basic principles of heredity
breeding garden peas in carefully planned
experiments
Chose to track


Only those characters that varied in an “either-or”
manner
Made sure that

He started his experiments with varieties that were
“true-breeding”
Genetic Vocabulary



Character: a heritable feature, such as flower
color
Trait: a variant of a character, such as purple
or white flowers
The true-breeding parents


The hybrid offspring of the P generation


Are called the P generation
Are called the F1 generation
When F1 individuals self-pollinate

The F2 generation is produced
 Phenotype
Physical appearance
 Genotype Genetic makeup
 Dominant trait that is easily
observed
 Recessive  trait that is often
masked
 Homozygous2 alleles for a trait are
identical  TT or tt
 Heterozygous – 2 alleles for a trait
are not identical  Tt
Allele
Alternative forms of a gene
which determines a trait.
Alleles cont.
 Uppercase
(Capital) letters for
dominant traits
 Lowercase letters for recessive
traits
 Ex: tall = T short = t,
expressed in pairs TT, Tt, tt
Why peas?
Rapid reproduction
rate
 Presence of
distinctive traits
 Closed structure of
flowers (each pea plant

has male (stamens) and
female (carpal) sexual
allows selffertilization
organs)
Seven Traits Studied by Mendel
The Law of Segregation

When Mendel crossed contrasting, truebreeding white and purple flowered pea
plants


All of the offspring were purple
When Mendel crossed the F1 plants

Many of the plants had purple flowers, but
some had white flowers
Mendel’s Experiment


Crossed pure purple
and a pure white
flower (P
generation) =F1
generation
All F1 plants
(purple) are
crossed by self
pollination = F2
generation yields
¾ purple and ¼
yellow
The Testcross
 In
pea plants with purple flower
 The genotype is not immediately
obvious
 Allows us to determine the
genotype of an organism with the
dominant phenotype, but unknown
genotype
The Testcross
 Individual
with dominant phenotype
 not possible to predict the
genotype run a test cross with
individual with recessive phenotype
to determine the allele
 Dominant Phenotype purple flower
(genotype PP or Pp)
 Recessive Phenotype  white flower
( genotype pp)
Test Cross
The Law of Independent
Assortment

Mendel derived the law of segregation


The F1 offspring produced in this cross


By following a single trait
Were monohybrids, heterozygous for one
character
How are two characters transmitted from
parents to offspring?


As a package?
Independently?
Dihybrid Cross

Mendel identified his second law of
inheritance


By following two characters at the same
time
Crossing two, true-breeding parents
differing in two characters

Produces dihybrids in the F1 generation,
heterozygous for both characters



The Y and R alleles and
y and r alleles stay
together
F1 offspring would
produce yellow, round
seeds.
The F2 offspring would
produce two
phenotypes
in a 3:1 ratio, just like a
monohybrid cross.



If the two pairs of
alleles segregate
independently of each
other
Four classes of
gametes (YR, Yr, yR,
and yr) would be
produced in equal
amounts.
These combinations
produce four distinct
phenotypes in a
9:3:3:1 ratio

Using the information from a dihybrid
cross, Mendel developed the law of
independent assortment
 Each
pair of alleles segregates
independently during gamete formation
Punnett Square
 Diagram
which
shows the
possible
outcome of a
cross
Probability

Fractions or ratios that will predict that an
event will occur
Rr
Rr

Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
1⁄
Eggs
r
1⁄
2
r
1⁄
4
R
1⁄
Figure 14.9
r
R
R
2
r
2
R
R
1⁄
1⁄
4
r
4
r
1⁄
4
The Multiplication and Addition Rules
Applied to Monohybrid Crosses

The multiplication rule


States that the probability that two or
more independent events will occur
together is the product of their individual
probabilities
The rule of addition

States that the probability that any one
of two or more exclusive events will occur
is calculated by adding together their
individual probabilities
Extending Mendel

The inheritance of characters by a
single gene
 May
deviate from simple Mendelian
patterns
The Spectrum of Dominance

Complete dominance

Occurs when the phenotypes of the
heterozygote and dominant homozygote
are identical
Incomplete Dominance
100% Dominant/0%
recessive
Codominant
100% Dominant/100%
recessive
Incomplete Dominance


The phenotype of F1 hybrids is
somewhere between the
phenotypes of the two parental
varieties
Hheterozygotes show a distinct
intermediate phenotype, not seen
in homozygotes.



This is not blended inheritance
because the traits are separable
(particulate) as seen in further
crosses.
Offspring of a cross between
heterozygotes will show three
phenotypes: both parentals and
the heterozygote.
The phenotypic and genotypic
ratios are identical, 1:2:1.
P Generation
White
CW C W

Red
CRCR
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes
Eggs
F2 Generation
1⁄
2
1⁄
2
1⁄
2
CR
1⁄
2
CR
1⁄
2
CR
1⁄
2
CR
CR
Cw
CR CR
CR CW
CR CW
CW CW
Sperm
Codominance


Two dominant alleles affect the phenotype in
separate, distinguishable ways
The human blood group MN




Is an example of codominance
The M, N, and MN blood groups of humans 
presence of two specific molecules on the surface
of red blood cells
Both the M & N molecules are expressed in the
heterozygous individual
People of group M (genotype MM) have one type
of molecule on their red blood cells, people of
group N (genotype NN) have the other type, and
people of group MN (genotype MN) have both
molecules present
Dominance/Recessiveness
Relationships
1.
2.
3.
They range from complete dominance, through
various degrees of incomplete dominance, to
codominance
They reflect the mechanisms by which
specific alleles are expressed in the
phenotype and do not involve the ability of one
allele to subdue another at the level of DNA
They do not determine or correlate with the
relative abundance of alleles in a population
The dominance/recessiveness relationships
depend on the level at which we examine
the phenotype

Tay-Sachs disease lack a functioning
enzyme to metabolize gangliosides (a lipid)
which accumulate in the brain, harming brain
cells, and ultimately leading to death.
 Children
with two Tay-Sachs alleles have the
disease.
 Heterozygotes with one working allele and
homozygotes with two working alleles are “normal”
at the organismal level, but heterozygotes produce
less functional enzyme.
 However, both the Tay-Sachs and functional
alleles produce equal numbers of enzyme molecules,
codominant at the molecular level
Frequency of Dominant Alleles
A dominant allele does not necessarily
mean that it is more common in a
population than the recessive allele.
 Polydactyly individuals are born with
extra fingers or toes, is due to an allele
dominant to the recessive allele for five
digits per appendage.

 However,
the recessive allele is far more
prevalent than the dominant allele in the
population.

399 individuals out of 400 have five digits per
appendage.
Multiple Alleles


Most genes exist in
populations in more
than two allelic forms
The ABO blood group
in humans

Is determined by
multiple alleles
Table 14.2
Human Blood Groups
The ABO blood groups in humans are
determined by three alleles, IA, IB, and
I
 Both the IA and IB alleles are dominant
to the i allele
 The IA and IB alleles are codominant to
each other
 Because each individual carries two
alleles, there are six possible genotypes
and four possible blood types

Human Blood Groups
Pleiotropy
A gene has multiple phenotypic effects
 The wide-ranging symptoms of cystic
fibrosis & sickle-cell disease are due to
a single gene

Extending Mendelian Genetics
for Two or More Genes



Some traits may be determined by
two or more genes
Epistasis
Polygenic inheritance
Epistasis





A gene at one locus alters the phenotypic
expression of a gene at a second locus
In mice and many other mammals, coat color
depends on two genes.
One, the epistatic gene, determines whether
pigment will be deposited in hair or not.

Presence (C) is dominant to absence (c).

The black allele is dominant to the brown allele.
The second determines whether the pigment
to be deposited is black (B) or brown (b).
An individual that is cc has a white (albino)
coat regardless of the genotype of the
second gene.


A cross between two
black mice that are
heterozygous (BbCc)
will follow the law of
independent
assortment.
However, unlike the
9:3:3:1 offspring ratio
of an normal Mendelian
experiment, the ratio is
nine black, three
brown,
and four white.
Polygenic Inheritance


Many human characters the additive
effects of two or more genes on a single
phenotypic character.
For example, skin color in humans is controlled
by at least three different genes.



Each gene has two alleles, one light and one dark,
that demonstrate incomplete dominance.
An AABBCC individual is dark and aabbcc is light.
Vary in the population along a continuum and
are called quantitative characters Do not
fit the either-or basis that Mendel studied
An additive effect of two or more genes
on a single phenotype
Nature and Nurture
Mendelian Inheritance in Humans

Humans are not convenient subjects for
genetic research
 However,
the study of human genetics
continues to advance
Pedigree Analysis

Is a family tree that describes the
interrelationships of parents and
children across generations
 Can
also be used to make predictions about
future offspring
Pedigree Analysis
Recessively Inherited Disorders



Many genetic disorders are inherited in a recessive
manner
These range from the relatively mild (albinism) to
life-threatening (cystic fibrosis).
The recessive behavior of the alleles occurs because
the allele codes for either a malfunctioning protein or
no protein at all.




Heterozygotes have a normal phenotype because one
“normal” allele produces enough of the required protein.
Individuals who lack the disorder are either homozgyous
dominant or heterozygotes.
heterozygotes no clear phenotypic effects carriers
who may transmit a recessive allele to their offspring.
heterozygotes may have no clear phenotypic effects, they
are carriers who may transmit a recessive allele to their
offspring.
Cystic Fibrosis


Caucasians, 1in 25 are carriers
Symptoms of cystic fibrosis include
Mucus buildup in the some internal organs
 Abnormal absorption of nutrients in the
small intestine


Treatment
untreated  death by the age of 5
 Treated  live to 30yrs antibiotics
forced mucus discharge
 Gene therapy/?

Sickle-Cell Disease

Sickle-cell disease
Affects one out of 400 African-Americans
 Is caused by the substitution of a single
amino acid in the hemoglobin protein in red
blood cells


Symptoms include

Physical weakness, pain, organ damage, and
even paralysis
Mating of Close Relatives

Matings between relatives
Can increase the probability of the
appearance of a genetic disease
 Are called consanguineous matings

Prevalence of CF, SC & TS
Exist b/c they have benefits
 Sickle Cell 1 recessive gene gives
protection against malaria
 Tay Sachs prevention from
tuberculosis
 Cyctic Fibrosis protection against
cholera/diarrhea driven diseases

Dominantly Inherited
Disorders

Some human disorders


Are due to dominant
alleles
Achondroplasia a
form of dwarfism that
is lethal when
homozygous for the
dominant allele
Figure 14.15
Huntington’s Disease




A degenerative disease of the nervous system
Has no obvious phenotypic effects until about
35 to 40 years of age
Any child born to a parent who has the allele
for Huntington’s disease has a 50% chance of
inheriting the disease and the disorder.
Molecular geneticists have used pedigree
analysis of affected families to track down
the Huntington’s allele to a locus near the tip
of chromosomes 4.
Pedigree Analysis tracking
Diseases
Multifactorial Disorders

Many human diseases


Have both genetic and environment
components
Examples
Heart disease
 Cancer

Genetic Testing and
Counseling

Genetic counselors

Can provide information to prospective
parents concerned about a family history
for a specific disease
Counseling Based on Mendelian
Genetics and Probability Rules

Using family histories

Genetic counselors help couples determine
the odds that their children will have
genetic disorders
Tests for Identifying Carriers

For a growing number of diseases

Tests are available that identify carriers
and help define the odds more accurately
Amniocentesis

The liquid that bathes the fetus is
removed and tested
Chorionic Villus Sampling (CVS)

A sample of the placenta is removed and
tested
Newborn Screening

Some genetic disorders can be detected at
birth


By simple tests that are now routinely performed in
most hospitals in the United States
Phenyketonuria (PKU) test can detect the
presence of a recessively inherited disorder,




This disorder occurs in one in 10,000 to 15,000
births.
Individuals accumulate the amino acid
phenylalanine and its derivative phenypyruvate in
the blood to toxic levels.
This leads to mental retardation.
If the disorder is detected, a special diet low in
phenyalalanine usually promotes normal
development.