Gregor Mendel Garden Pea Monohybrid Cross

Gregor Mendel
Patterns of Inheritance
Chapter 12
• Founder of genetics
• Augustinian monk
(1822-1884)
• First to use
scientific method to
study inheritance
Removed stamens
from purple flower
Garden Pea
Monohybrid Cross
Transferred spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
Carpel
Stamens
Pollinated carpel
matured into pod
Mendel
used traits
with only 2
variants
(alleles)
P Generation
(true-breeding
parents)
White
flowers
F1 Generation
(hybrids)
Planted seeds
from pod
First
generation
offspring
(F1)
Purple
flowers
Examined
offspring:
all purple
flowers
All plants had
purple flowers
Monohybrid Cross
Punnett Square
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
P Generation
(true-breeding
parents)
P Generation
Purple
flowers
White
flowers
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
p
2
F1 Generation
(hybrids)
All plants had
purple flowers
F1 sperm
P
p
F2 Generation
P
F1 eggs
F2 Generation
• First filial generation
• Offspring produced by crossing 2 truebreeding strains
• For every trait Mendel studied, all F1
plants resembled only 1 parent
– Referred to this trait as dominant
– Alternative trait was recessive
• No plants with characteristics intermediate
between the 2 parents were produced
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Pp
Pp
pp
p
3
F1 generation
PP
:1
F2 generation
• Second filial generation
• Offspring resulting from the selffertilization of F1 plants
• Although hidden in the F1 generation, the
recessive trait had reappeared among
some F2 individuals
• Counted proportions of traits
– Always found about 3:1 ratio
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Pea Characters
3:1 is really 1:2:1
3
Phenotype
Genotype
Purple
PP
(homozygous
Purple
Pp
(heterozygous
1
Mendel
produced truebreeding pea
plants with 7
different traits
2
1
Purple
Pp
(heterozygous
White
pp
(homozygous
Ratio 3:1
Ratio 1:2:1
1
Pea Characters
Mendel’s First Hypothesis
• Genes for genetic characters occur in
pairs
– One gene inherited from each parent
– Alleles are different versions of a gene
• Diploid: two copies of each gene
– Homozygous – 2 of the same allele
– Heterozygous – different alleles
Mendel’s Second Hypothesis
Mendel’s Third Hypothesis
• If two alleles of a gene are different, one
allele is dominant over the other
• Two alleles of a gene segregate
(separate) and enter gametes singly
– Dominant allele is expressed
– Recessive allele is masked
– Half the gametes carry one allele, half carry
the other allele (haploid)
– Principle of Segregation
• Recessive alleles only expressed when
two copies of the allele present
• Genotype – total set of alleles an
individual contains
• Phenotype – physical appearance
• Two gametes fuse to produce a zygote
that contains two alleles (diploid)
Punnett square
• Cross purple-flowered plant with white-flowered
plant
• P is dominant allele – purple flowers
• p is recessive allele – white flowers
• True-breeding white-flowered plant is pp
– Homozygous recessive
• True-breeding purple-flowered plant is PP
– Homozygous dominant
• Pp is heterozygote purple-flowered plant
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Human traits
• Some human traits are controlled by a
single gene
– Some of these exhibit dominant and recessive
inheritance
• Pedigree analysis is used to track
inheritance patterns in families
• Dominant pedigree – juvenile glaucoma
– Disease causes degeneration of optic nerve
leading to blindness
– Dominant trait appears in every generation
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• Recessive pedigree – albinism
– Condition in which the pigment melanin is not
produced
– Pedigree for form of albinism due to a
nonfunctional allele of the enzyme tyrosinase
– Males and females affected equally
– Most affected individuals have unaffected
parents
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Dihybrid crosses
• Examination of 2 separate traits in a single
cross
• Produced true-breeding lines for 2 traits
• RR YY x rryy
• The F1 generation of a dihybrid cross
(RrYy) shows only the dominant
phenotypes for each trait
• Allow F1 to self-fertilize to produce F2
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• F1 self-fertilizes
• RrYy x RrYy
• The F2 generation shows all four possible
phenotypes in a set ratio
– 9:3:3:1
– R_Y_:R_yy:rrY_:rryy
– Round yellow:round green:wrinkled
yellow:wrinkled green
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Mendel’s Fourth Hypothesis
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Probability
• Principle of Independent Assortment
• In a dihybrid cross, the alleles of each
gene assort independently
• The segregation of different allele pairs is
independent
• Independent alignment of different
homologous chromosome pairs during
metaphase I leads to the independent
segregation of the different allele pairs
• Rule of addition
– Probability of 2 mutually exclusive events
occurring simultaneously is the sum of their
individual probabilities
• When crossing Pp x Pp, the probability of
producing Pp offspring is
– probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)
–! + ! = "
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Testcross
• Rule of multiplication
– Probability of 2 independent events occurring
simultaneously is the product of their individual
probabilities
• When crossing Pp x Pp, the probability of
obtaining pp offspring is
– Probability of obtaining p from father = "
– Probability of obtaining p from mother = "
– Probability of pp= " x " = !
• Cross used to determine the genotype of an
individual with dominant phenotype
• Cross the individual with unknown genotype
(e.g. P_) with a homozygous recessive (pp)
• Phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent
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Extensions to Mendel
• Mendel’s model of inheritance assumes
that
– Each trait is controlled by a single gene
– Each gene has only 2 alleles
– There is a clear dominant-recessive
relationship between the alleles
• Most genes do not meet these criteria
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Polygenic inheritance
• Occurs when multiple genes are involved
in controlling the phenotype of a trait
• The phenotype is an accumulation of
contributions by multiple genes
• These traits show continuous variation and
are referred to as quantitative traits
– For example – human height
– Histogram shows normal distribution
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Pleiotropy
Multiple alleles
• Refers to an allele which has more than
one effect on the phenotype
• Pleiotropic effects are difficult to predict,
because a gene that affects one trait often
performs other, unknown functions
• This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia
• May be more than 2 alleles for a gene in a
population
• ABO blood types in humans
– Multiple symptoms can be traced back to one
defective allele
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– 3 alleles
• Each individual can only have 2 alleles
• Number of alleles possible for any gene is
constrained, but usually more than two
alleles exist for any gene in an
outbreeding population
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• Incomplete dominance
– Heterozygote is intermediate in phenotype
between the 2 homozygotes
– Red flowers x white flowers = pink flowers
• Codominance
– Heterozygote shows some aspect of the
phenotypes of both homozygotes
– Type AB blood
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Human ABO blood group
• The system demonstrates both
– Multiple alleles
• 3 alleles of the I gene (IA, IB, and i)
– Codominance
• IA and IB are dominant to i but codominant to each
other
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Environmental influence
Epistasis
• Coat color in
Himalayan
rabbits and
Siamese cats
– Allele
produces an
enzyme that
allows
pigment
production
only at
temperatures
below 30oC
• Behavior of gene products can change the
ratio expected by independent assortment,
even if the genes are on different
chromosomes that do exhibit independent
assortment
• R.A. Emerson crossed 2 white varieties of
corn
– F1 was all purple
– F2 was 9 purple:7 white – not expected
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