chapter 13 mendel and the gene why do we look like family members or not?
TRANSCRIPT
Chapter 13 MENDEL AND THE GENEWhy 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
Seed color
Pod shape
Pod color
Flower color
Flower and pod position
Stem length
Tall Dwarf
Terminal (at tip)Axial (on stem)
Purple White
YellowGreen
Inflated Constricted
GreenYellow
Round Wrinkled
Figure 13-1
Self-pollination
Female organ(receives pollen)
Eggs
Male organs(produce pollengrains, whichproduce malegametes)
Cross-pollination
CROSS-POLLINATION
3. Transfer pollen to thefemale organs of theindividual whose maleorgans have beenremoved.
2. Collect pollen from adifferent individual.
1. Remove male organsfrom one individual.
SELF-POLLINATION
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
Anthersremoved
All purple flowers result
Mendel Studied a Single Trait
•Mendel’s experiments cont’d▫Mendel allowed F1 generation plants to
self- fertilize.▫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
Cross-fertilize
F1 generation
Purple White
3
Self-fertilize
White
Purple
Purple
Purple
: 1
F2 generation
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.
Figure 13-4
Homozygousmother
Meiosis
Offspring genotypes: All Rr (heterozygous)
Homozygousfather
Mal
e g
amet
es
Female gametes
Meiosis
Offspring phenotypes: All round seeds
A cross between two homozygotes
A cross between two heterozygotes
Female gametes
Heterozygousmother
Heterozygousfather
Offspring genotypes: 1/4 RR : 1/2 Rr : 1/4 rr
Mal
e g
amet
es
Offspring phenotypes: 3/4 round : 1/4 wrinkled
Genes and Mendel’s Findings
•Mendels’ Principle of Segregation, fig 13.7:
Figure 13-7
Meiosis IAlleles segregate
Recessive allelefor seed shape
Ga
me
tes
Chromosomes replicate
Rr parent
Dominant allelefor seed shape
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.
Mal
e g
amet
esMale parent
F1 PUNNET SQUARE
F1 offspring all RrYy
F2 female parent
Female parent
Female gametes
Mal
e g
amet
es
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
F2 maleparent
Female gametesF2 PUNNET SQUARE
Alleles at R gene and Y gene go to gametes independently of each other
Genes and Mendel’s Findings
•Mendel’s Principle of Independent Assortment:, fig 13.8.
Figure 13-8
Meiosis I
Replicated chromosomesprior to meiosis
Gam
etes
Alleles for seed shape
Meiosis II
Principle of independent assortment: The genes for seed shape and seed colorassort independently, because they are located on different chromosomes.
Meiosis II
Meiosis I
Alleles for seed color
1/4 RY 1/4 ry 1/4 Ry 1/4 rY
Chromosomes can line up in two ways during meiosis I
R
Ry yr
YY
R R
R R
R R
r r
r r
r r
r
Y Y
Y Y
Y Y
y y
y y
y yy y
y y
y y
R R
R R
R R
r r
r r
r r
Y Y
Y Y
YY
Peas are Easy
Most phenotypes (expression of genes) are the result of the action of more than one gene.
Continuous variation:
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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 13-17Flower color is variable in four-o’clocks.
Incomplete dominance in flower color
Parentalgeneration
F2 generation
F1 generation
Self-fertilization
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
Carrier male Carrier female
Affectedmale
Affectedfemale
I
II
III
IV
Eac
h r
ow
rep
rese
nts
a g
ener
atio
n
Carriers (heterozygotes) are indicated with half-filled symbols
Pedigree of a family with an autosomal recessive disease
Figure 13-22
Affected femaleI
II
III
IV
Unaffected male
If a child shows the trait, then one of the parents shows the trait as well
Pedigree of a family with Huntington’s Disease
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
Queen Victoria Prince Albert
Female carrier of hemophilia allele
I
II
III
IV
Affected male
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George III
EdwardDuke of Kent
Louis IGrand Duke of HesseI
KingEdward VII
Duke ofWindsor
QueenElizabeth II
PrincePhilip
Margaret
PrincessDiana
PrinceCharles
Anne Andrew Edward
William Henry
KingGeorge VI
KingGeorge V
Earl ofMountbatten
ViscountTremation
Alfonso Jamie GonzaloPrinceSigismond
PrussianRoyalHouse
British Royal House
Spanish Royal House
RussianRoyalHouse
Henry Anastasia Alexis? ?
? ?
? ?
?
Waldemar
Queen VictoriaPrince Albert
FrederickIII
I
II
III
IV
V
VI
VII
Victoria Alice Alfred Arthur Leopold Beatrice PrinceHenry
HelenaDuke ofHesse
No hemophilia No hemophilia
GermanRoyalHouse
Juan
King JuanCarlos
No evidenceof hemophilia
No evidenceof hemophilia
Irene CzarNicholas II
CzarinaAlexandra
Earl ofAthlone
PrincessAlice
QueenEugenie
AlfonsoKing ofSpain
Maurice Leopold
Gen
erat
ion
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.