Transcript Chapter 13 PATTERNS OF INHERITANCE

Chapter 13 MENDEL AND THE
GENE
Why do we look like family
members or not?
History
• It started with farmers and botanists
• Knight used pure breeding peas, one variety with
purple flowers, one variety with white flowers.
▫ Crossing the two varieties he found that the offspring all
had purple flowers.
▫ When he crossed the offspring, some had purple flowers,
some had white flowers.
▫ Conclusion: some traits have a stronger tendency to appear
than others. No Numbers
History
• Mendel, an Austrian monk, repeated Knight’s
experiments:
▫ Also used true-breeding peas and studied 7
different traits, fig 13.2.
▫ Cross-fertilized peas showing two variations of the
same trait, ex. round peas vs. wrinkled peas.
Figure 13-2
Trait
Phenotypes
Seed shape
Round
Wrinkled
Yellow
Green
Inflated
Constricted
Green
Yellow
Purple
White
Axial (on stem)
Terminal (at tip)
Seed color
Pod shape
Pod color
Flower color
Flower
and pod
position
Stem length
Tall
Dwarf
Figure 13-1
Self-pollination
SELFPOLLINATION
Female organ
(receives pollen)
Eggs
Male organs
(produce pollen
grains, which
produce male
gametes)
Cross-pollination
CROSSPOLLINATION
1. Remove male organs
2. Collect pollen from a
3. Transfer pollen to the
from one individual.
different individual.
female organs of the
individual whose male
organs have been
removed.
Mendel Studied a Single Trait
• Mendel cross-fertilized two plants, one with
white flowers with one with purple flowers. The
hybrids, the F1 generation, all had purple
flowers.
• Studying one trait through cross-fertilization is
termed a monohybrid cross.
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Pollen transferred from
white flower to stigma
of purple flower
Anthers
removed
All purple flowers result
Mendel Studied a Single Trait
• Mendel’s experiments cont’d
▫ Mendel allowed F1 generation plants to selffertilize.
▫ Their offspring, the F2 generation, expressed
(demonstrated) both purple and white flowers.
The ratio of plants with purple to white flowers
was always 3:1.
▫ Where did these white flowered plants come
from?
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P (parental)
generation
Crossfertilize
Purple
F1 generation
White
Self-fertilize
F2 generation
Purple
Purple
3
Purple
White
:
1
Mendel Studied a Single Trait
• Mendel cont’d
▫ The F1 generation plants all resembled only
parent plant; i.e. one variation of the trait is
dominant.
▫ The F2 generation showed plants with both
variations of the character, purple and white. The
variation of the trait that was only seen in the F2
generation (white flowers) is recessive.
Mendel Studied a Single Trait
• Mendle cont’d
▫ The F2 generations were allowed to self-fertilize. Looking
at the F3 generation, Mendel discovered that the F2
generation actually consisted of 3 different types of plants:
 Pure breeding purple
 Not pure breeding purple (produced both purple and white
flowered plants.
 Pure breeding white.
 The ratio was actually 1:2:1.
Mendel Studied a Single Trait
• Conclusions (cross involving 1 trait)
Genes and Mendel’s Findings
• Traits are carried by genes.
• An individual has 2 genes or alleles for each
trait, 1 on each homologous chromosome.
• Meiosis results in separation of the homologous
chromosomes and the alleles so that each is
carried by a different gamete.
Genes and Mendel’s Findings
• An individual with 2 identical alleles is said to be
homozygous, while an individual with 2 different
alleles is said to be heterozygous.
• The genetic make-up of an individual is its
genotype. The appearance or expression of the
genotype is called its phenotype.
Genes and Mendel’s Findings
• Mendel’s results can be predicted using Punnett
squares.
▫ Dominant genes are represented by uppercase
letters, ex. round peas (R) . Expressed when there
is 1 or 2 dominant alleles present.
▫ Recessive genes are represented by lowercase
letters, ex. wrinkled peas (r). Only expressed
when there are 2 recessive alleles present.
A cross between two homozygotes
Homozygous
mother
Meiosis
Female gametes
Homozygous
father
Meiosis
Offspring genotypes: All Rr (heterozygous)
Offspring phenotypes: All round seeds
A cross between two heterozygotes
Heterozygous
mother
Female gametes
Heterozygous
father
Male gametes
Figure 13-4
Offspring genotypes: 1/4 RR : 1/2 Rr : 1/4 rr
Offspring phenotypes: 3/4 round : 1/4 wrinkled
Genes and Mendel’s Findings
• Mendels’ Principle of Segregation, fig 13.7:
Figure 13-7
Rr parent
Dominant allele
for seed shape
Recessive allele
for seed shape
Chromosomes replicate
Meiosis I
Alleles segregate
Meiosis II
Principle of segregation: Each gamete carries only
one allele for seed shape, because the alleles have
segregated during meiosis.
Mendel Studied 2 Traits
• Mendel then looked at two traits simultaneously –
dihybrid cross. Ex. plants that produced round
(R), yellow (Y) peas and plants that produced
wrinkled (r), green (y) peas.
• The pure breeding parents’ genotypes were RRYY
and rryy, fig 13.5.
• What is the genotype and phenotype of the F1
generation? The F2 generation?
Figure 13-5a
Hypothesis of independent assortment:
Alleles of different genes don’t stay together when gametes form.
Female parent
F1 PUNNET SQUARE
Female gametes
Male parent
F1 offspring all RrYy
F2 female parent
Alleles at R gene and Y gene
go to gametes independently
of each other
F2 PUNNET SQUARE
Female gametes
F2 male
parent
F2 offspring genotypes: 9/16 R–Y– : 3/16 R–yy : 3/16 rrY– : 1/16 rryy
F2 offspring phenotypes: 9/16
: 3/16
: 3/16
: 1/16
Genes and Mendel’s Findings
• Mendel’s Principle of Independent Assortment:,
fig 13.8.
Figure 13-8
R
y
y R
r
Replicated chromosomes
prior to meiosis
r
Y
Y
R
R r
r
R R r
Alleles for seed shape
Alleles for seed color
r
Chromosomes can line up in
two ways during meiosis I
Y Y y y
R
Meiosis I
R
Y
y yY Y
r
r
R
y y
Y
Meiosis I
R
Y
Y
1/4 RY
Y
Meiosis II
r
R
r
Y
y y
Meiosis II
R
r
r
y
y
1/4 ry
r
R
R
y
y
1/4 Ry
r
Y
Y
1/4 rY
Principle of independent assortment: The genes for seed shape and seed color
assort independently, because they are located on different chromosomes.
Peas are Easy


Most phenotypes (expression of genes) are the
result of the action of more than one gene.
Continuous variation:
Number of individuals
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30
20
10
0
5'0''
5'6'’
Height
6'0''
Peas are Easy

Pleiotropic effects: An individual gene
may have effects on many traits.
• Example:
• A dominant gene causes yellow hair in mice.
• If the mouse is homozygous for the gene it dies = a
lethal defect. (In this case the gene was acting as
if it was a recessive gene).
• Other examples:
Marfan’s syndrome.
Peas are Easy

Incomplete dominance,fig 13.17.
Figure 13-17
Flower color is variable in four-o’clocks.
Incomplete dominance in flower color
Parental
generation
F1 generation
Self-fertilization
F2 generation
Purple
Lavender
White
Peas Are Easy
• Co dominance: Some phenotypes represent
both alleles, ex. blood types.
• ABO Blood groups - CoDominance
▫ 2 dominant alleles, A and B, one recessive allele, i.
▫ Alleles code for different RBC membrane proteins.
These protein act as antigens (can cause an
immune response).
▫ Immune response = antibodies.
Peas Are Easy
• ABO Blood Groups, cont’d
▫
▫
▫
▫
Type A blood type has IA,IA or IA,i alleles
Type B blood type has IB, IB or IB,i alleles
Type AB blood type has IA, IB alleles
Type O blood type has i, i alleles (recessive form, no
antigens on their RBCs).
• Rh blood group: the Rh factor consists of 8
different antigens. A person that has even one of
these antigens is Rh+ while those having none of
the antigens is Rh-.
Peas are Easy

Environmental effects:
Human Genetics


Random mutation of genes occurs
constantly, but most do not produce
changes in phenotype or disease
symptoms.
Most gene disorders are rare, i.e. the
frequency of occurrence of a defective
allele is low. Exceptions: “closed
societies”, ex. Tay Sachs, Sickle Cell
Human Genetics

Most gene disorders are recessive and
only expressed when both alleles are
recessive forms of the gene.
Exceptions: Huntington disease is
caused by a dominant gene.
Figure 13-21
Pedigree of a family with an autosomal recessive disease
I
Carrier male
Carriers
(heterozygotes)
are indicated
with half-filled
symbols
II
III
Affected
male
IV
Affected
female
Carrier female
Figure 13-22
Pedigree of a family with Huntington’s
Disease
I
Affected female
Unaffected male
II
III
IV
If a child shows the trait,
then one of the parents
shows the trait as well
Patterns of Inheritance in
Humans


Controlled mating is not practical.
Solution: Pedigrees – constructed from the
progeny of matings over many generations.
Ex. – hemophilia in family of Queen Victoria.
•
•
The defective gene is recessive and occurs on the X
chromosome. Heterozygous females are carriers.
Because the male Y chromosome does not express
many of its genes, the defective gene is expressed in
males, i.e. it is sex-linked.
A Pedigree of an X-Linked
Recessive Disease
I
Queen Victoria
Prince Albert
Female carrier of
hemophilia allele
II
Affected
male
III
IV
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
Patterns of Inheritance in
Humans

Some genetic disorders arise from the
mutation of a single base on the DNA.
This can alter one amino acid of a single
protein and have lethal effects. Ex.
Sickle cell anemia.
Gene Therapy


Replacing a defective gene with a functional
gene. In the past, this type of therapy has
worked in some isolated instances.
Problems:
•
•
•
The functional gene is carried as part of the DNA of an
adenovirus (cold virus), the vector.
The virus can cause a strong immune response
causing 1) the destruction of the virus and the gene
destroyed or 2) death of the patient.
The gene may also be incorporated into the patient’s
DNA at random and cause lethal mutations.
Gene Therapy


Gene therapy was banned for several
years but a new vector AAV (paravirus
with only 2 genes of its own) is showing
promise.
Animal trials have shown positive results
with few problems. Clinical trials are
underway for cystic fibrosis, etc.