9.2 Experimental genetics began in an abbey garden
Gregor Mendel Augustinian monk who taught natural
science to high school students
Mendel's brilliant performance at school as a youngster encouraged his family to support his pursuit of a higher education, but their resources were limited, so Mendel entered an Augustinian monastery
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Gregor Mendel discovered principles of genetics in experiments with the garden pea
• Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes)
9.2 Experimental genetics began in an abbey garden
Crosses meticulously documented
Crosses numerically/statistically analyzed
Scientists of 1860s did not understand and appreciate
Work lost in journals for 50 years
Rediscovered in 1900s independently by 3 scientists
Terms to know and understand
• The phenotype is the appearance or expression of a trait• Genes are found in alternative versions called alleles; a
genotype is the listing of alleles an individual carries for a specific gene
• If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect
• Homozygous individuals have the same allele on both homologues
• Heterozygous individuals have a different allele on each homologue
• Mating of plants can be controlled• Each pea plant has sperm-
producing organs (stamens) and egg-producing organs (carpels)
• Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
• In a typical experiment, Mendel mated two contrasting, true-breeding varieties
• The true-breeding parents are the P generation
• The hybrid offspring of the P generation are called the F1 generation
• When F1 individuals self-pollinate, the F2 generation is produced
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Transferredpollen from stamens of whiteflower to carpel of purple flower
StamensCarpel
Parents(P)
Purple
2
White
Removedstamens frompurple flower
1
Pollinated carpelmatured into pod
3
Offspring(F1)
Planted seedsfrom pod
4
Mendel’s crosses led to questions
• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
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Fig. 14-3-1
EXPERIMENT
P Generation
(true-breeding parents) Purple
flowers Whiteflowers
Fig. 14-3-2
EXPERIMENT
P Generation
(true-breeding parents) Purple
flowers Whiteflowers
F1 Generation
(hybrids) All plants hadpurple flowers
Mendel’s crosses led to questions
• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
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Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding parents) Purple
flowers Whiteflowers
F1 Generation
(hybrids) All plants hadpurple flowers
F2 Generation
705 purple-floweredplants
224 white-floweredplants
Questions are . . .
Questions that arise
– Why does one trait seemed to disappear in the F1 generation?
– Why does that trait reappear in one quarter of the F2 offspring?
Answers?
• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
• These inherited factors are what we now call a genes
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Table 14-1
Mendel’s Model
• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
• Four related concepts make up this model
• These concepts can be related to what we now know about genes and chromosomes
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1. The first concept is that alternative versions of genes account for variations in inherited characters
• For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
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2. The second concept is that for each character an organism inherits two alleles, one from each parent
• Mendel made this deduction without knowing about the role of chromosomes
• The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation - homozygous
• Alternatively, the two alleles at a locus may differ, as in the F1 hybrids - heterozygous
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3. The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
• In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant
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4. The fourth concept, now known as the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
• Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
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Let’s look at this example again Before we were only
looking at phenotype just like Gregor Mendel
Let’s apply his hypotheses and look at genotype as well
Draw a Punnett square that would represent true breeding purple flowers crossed with true breeding white flowers
To make a Punnett Square . . .
• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
• A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele
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Let’s look at this example again
Before we were only looking at phenotype just like Gregor Mendel
Let’s apply his hypotheses and look at genotype as well
PP pp
Pp
P P
p
p
Pp Pp
Pp Pp
Flowers will all be purple because of dominance
Let’s look at this example again Before we were only
looking at phenotype just like Gregor Mendel
Let’s apply his hypotheses and look at genotype as well
Draw the Punnett square that would represent two of the F1 generation breedingt
PP pp
Pp
P P
p
p
Pp Pp
Pp Pp
This cross
gives us t
he F 1 generatio
n
Let’s look at this example again
Allow F1 generation to self-pollinate and fill out the next Punnett square PP pp
Pp
PP Pp Pp pp
P p
p
P
Let’s look at this example again
Allow F1 generation to self-pollinate and fill out the next Punnett square PP pp
Pp
PP Pp Pp pp
P p
p
PP Pp
Pp pp
¾ of flowers will all be purple because of dominance. ¼ will be white.
P
Let’s look at this example again
PP pp
Pp
PP Pp Pp pp
P p
p
PP Pp
Pp pp
P
Genotypic ratio1 PP : 2 Pp : 1 pp
Phenotypic ratio3 purple : 1 white
Single trait cross
9.3 Mendel’s law of segregation describes the inheritance of a single character
Four Hypotheses
1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene
2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different
– A homozygous genotype has identical alleles
– A heterozygous genotype has two different alleles
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9.3 Mendel’s law of segregation describes the inheritance of a single character
Four Hypotheses
3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect
– The phenotype is the appearance or expression of a trait
– The same phenotype may be determined by more than one genotype
4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene
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Freckles
Widow’s peak
Free earlobe
No freckles
Straight hairline
Attached earlobe
Dominant Traits Recessive Traits
Three traits that are determined by single genes with alternate alleles
The dominant allele determines the phenotype when at least one copy is present
Humans show dominant and recessive traits
PTC taste strips
Some Genetic Disorders/Anomalies
Polydactyly - dominant
Tay Sachs - recessive
Cystic Fibrosis - recessive
Huntington’s Disease - dominant
Sickle Cell Anemia - recessive
– Cause?
– Single substitution of an amino acid in hemoglobin
Polydactyly
Alfredo Alfonseca "The Six Shooter", former Chicago Cubs relief pitcher
Six fingers and toes on each hand, all functional
9.8 Genetic traits in humans can be tracked through family pedigrees
A pedigree
– Shows the inheritance of a trait in a family through multiple generations
– Demonstrates dominant or recessive inheritance
– Can also be used to deduce genotypes of family members
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Ff
Female Male
Marriage
Unaffected
First generation(grandparents)
Second generation(parents, aunts,and uncles)
Third generation(two sisters)
Ff Ff
Ff
Ff Ff
Ff
ff
ff ff ff
ff
FF
FF
or
or
Siblings
Affected
Pedigree for a family inheritance of a recessive trait such as deafness
Recessive disease
Rr
?
rr
rr
rr
Dominant disease
Dominant disease
dd
dd
dd
dd
Dd
Huntington’s Disease - handout
Huntington’s Disease
Questions What are the chances of a parent with HD passing with HD
passing the gene on to any one of his or her children?
Thinking of all the ways in which the number of repeats may be passed on to children, discuss the possibility that John may get an HD gene
– from his mother
– from his father
From which parent did Dink get the gene for HD? Explain how you can tell.
Do you expect John's mother to show symptoms of HD? Why or why not.
Huntington’s Disease
Questions John's Aunt Barbara refused to have the test for HD.
Discuss some reasons that she may have had for coming to that decision.
When she applied for a job as an airline pilot, the company, knowing of the occurrence of HD in the family, insisted that she be tested. Does the company have the right to require the test? Explain why or why not.
When John reaches the age of 18, he may make the decision of whether or not to be tested for the HD gene. Consider this question from the perspective of John, John's future children, John's future wife, John's employer, John's medical insurer.
– Explain to John why he should have the test done.
– Explain to John why he should not have the test done.
VARIATIONS ON MENDEL’S LAWS
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9.11 Incomplete dominance results in intermediate phenotypes
Incomplete dominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual
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P generation
1–2
1–2
1–2
1–2
1–2
1–2
F1 generation
F2 generation
RedRR
Gametes
Gametes
Eggs
Sperm
RR rR
Rr rr
R
r
R r
R r
PinkRr
R r
Whiterr
HHHomozygous
for ability to makeLDL receptors
hhHomozygous
for inability to makeLDL receptors
HhHeterozygous
LDL receptor
LDL
CellNormal Mild disease Severe disease
Genotypes:
Phenotypes:
9.12 Many genes have more than two alleles in the population
Multiple alleles
– More than two alleles are found in the population
– A diploid individual can carry any two of these alleles
– The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood
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9.12 Many genes have more than two alleles in the population
Codominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as a distinct phenotype in the heterozygous individual
– Observed for type AB blood
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BloodGroup(Phenotype) Genotypes
O
A
ii
IAIA
orIAi
Red Blood Cells
Carbohydrate A
BIBIB
orIBi
Carbohydrate B
AB IAIB
Blood Type
Phenotype Genotype
A IA IA , IA i
B IB IB , IB i
AB IA IB
O i i
Blood Genetics
IA IB IB i
IA i i i
IA i
IB
i
Given a mom with blood type AB and child with blood type A, what are the possible blood types of the father?
IAIA
IA i
IA IB
? ____
? ____
Blood Typing Questions
IA
iIA IAIA
Dominance Worksheet
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9.13 A single gene may affect many phenotypic characters
Pleiotropy
– One gene influencing many characteristics
– The gene for sickle cell disease
– Affects the type of hemoglobin produced
– Affects the shape of red blood cells
– Causes anemia
– Causes organ damage
– Is related to susceptibility to malaria
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Clumping of cellsand clogging of
small blood vessels
Pneumoniaand otherinfections
Accumulation ofsickled cells in spleen
Pain andfever
Rheumatism
Heartfailure
Damage toother organs
Braindamage
Spleendamage
Kidneyfailure
Anemia
ParalysisImpairedmental
function
Physicalweakness
Breakdown ofred blood cells
Individual homozygousfor sickle-cell allele
Sickle cells
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped
9.14 A single character may be influenced by many genes
Polygenic inheritance
– Many genes influence one trait
– Skin color is affected by at least three genes
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P generation
1–8
F1 generation
F2 generation
Fra
ctio
n o
f p
op
ula
tio
n
Skin color
Eggs
Sperm1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
1–8
aabbcc(very light)
AABBCC(very dark)
AaBbCc AaBbCc
1––64
15––64
6––64
1––64
15––64
6––64
20––64
1––64
15––64
6––64
20––64
9.15 The environment affects many characters
Phenotypic variations are influenced by the environment
– Skin color is affected by exposure to sunlight
– Susceptibility to diseases, such as cancer, has hereditary and environmental components
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Environment—It can fool you
Environment—It can fool you !
The hydrangea flower color is controlled first by flower color genes similar to those in the pea.
Purple vs. white has complete dominance. But, pink vs. blue is controlled by the acidity of the soil
Environment—It can fool you !
The hydrangea flower color is controlled first by flower color genes similar to those in the pea.
Purple vs. white has complete dominance. But, pink vs. blue is controlled by the acidity of the soil