Transcript Mendel and Inheritance - University of Missouri

Mendel and Inheritance
MUPGRET Workshop
December 4, 2004
Genetic variation
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In the beginning geneticists studied
differences they could see in plants.
These differences are called
morphological differences.
Individual variants are referred to as
phenotypes, ex. tall vs. short plants or
red vs. white flowers.
Trait
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A broad term encompassing a
distribution of phenotypic variation.
Example:
Trait: Disease resistance
• Phenotype: resistant vs. susceptible
• Morphological differences associated with
the trait might include fungal infection,
fungal growth, sporulation, etc.
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Mendel
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Monk at the St. Thomas monastery in
the Czech Republic.
Performed several experiments between
1856 and 1863 that were the basis for
what we know about heredity today.
Used garden peas for his research.
Published his work in 1866.
Mendel
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Results are remarkably accurate and
some have said they were too good to
be unbiased.
His papers were largely ignored for
more than 30 years until other
researchers appreciated its significance.
Garden Pea
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Pisum sativum
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Diploid
Differed in seed shape, seed color,
flower color, pod shape, plant height,
etc.
Each phenotype Mendel studied was
controlled by a single gene.
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Terms
Wild-type is the phenotype that would
normally be expected.
• Mutant is the phenotype that deviates from
the norm, is unexpected but heritable.
• This definition does not imply that all mutants
are bad; in fact, many beneficial mutations
have been selected by plant breeders.
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Advantages of plants
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Can make controlled hybrids.
Less costly and time consuming to
maintain than animals.
Can store their seed for long periods of
time.
One plant can produce tens to hundreds
of progeny.
Advantages of plants
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Can make inbreds in many plant species
without severe effects that are typically
seen in animals.
Generation time is often much less than
for animals.
Fast plants (Brassica sp.)
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Allele
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One of two to many alternative forms of
the same gene (eg., round allele vs.
wrinkled allele; yellow vs. green).
Alleles have different DNA sequences
that cause the different appearances we
see.
Principle of Segregation
(Mendel’s First Law)
X
Round
Parental Lines
Wrinkled
All round F1 progeny
Self-pollinate
3 Round : 1 Wrinkled
Round
5474
Wrinkled
1850
Important Observations
F1 progeny are heterozygous but express only
one phenotype, the dominant one.
• In the F2 generation plants with both
phenotypes are observedsome plants have
recovered the recessive phenotype.
• In the F2 generation there are approximately
three times as many of one phenotype as the
other.
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Mendel’s Results
Parent Cross
F1 Phenotype
F2 data
Round x wrinkled
Round
5474 : 1850
Yellow x green
Yellow
6022 : 2001
Purple x white
Purple
705 : 224
Inflated x
constricted pod
Inflated
882 : 299
Green x yellow
pod
Green
428 : 152
Axial x terminal
flower
Axial
651 : 207
Long x short stem
Long
787 : 277
3 : 1 Ratio
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The 3 : 1 ratio is the key to interpreting
Mendel’s data and the foundation for
the the principle of segregation.
The Principle of Segregation
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Genes come in pairs and each cell has
two copies.
Each pair of genes can be identical
(homozygous) or different
(heterozygous).
Each reproductive cell (gamete)
contains only one copy of the gene.
Mendel’s Principle of
Segregation
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In the formation of gametes, the paired
hereditary determinants separate (segregate)
in such a way that each gamete is equally
likely to contain either member of the pair.
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One male and one female gamete combine to
generate a new individual with two copies of
the gene.
Round vs. Wrinkled
X
Round
Parental Lines
Wrinkled
All round F1 progeny
Self-pollinate
3 Round : 1 Wrinkled
Round
5474
Wrinkled
1850
Round vs. wrinkled
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The SBEI causes the round vs. wrinkled
phenotype.
SBEI = starch-branching enzyme
Wrinkled peas result from absence of
the branched form of starch called
amylopectin.
When dried round peas shrink uniformly
and wrinkled do not.
Round vs. wrinkled
The non-mutant or wild-type round allele is
designated W.
• The mutant, wrinkled allele is designated w.
• Seeds that are Ww have half the SBEI of
wild-type WW seeds but this is enough to
make the seeds shrink uniformly.
• W is dominant over w.
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Round vs. wrinkled
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An extra DNA sequence is present in
the wrinkled allele that produces a nonfunctional SBEI and blocks the starch
synthesis pathway at this step resulting
in a lack of amylopectin.
A Molecular View
Parents
WW
ww
F1
F2 Progeny
Ww
¼WW ¼Ww ¼wW ¼ww
1: 2 : 1 Genotype = 3: 1 Phenotype
Dihybrid crosses reveal Mendel’s
law of independent assortment
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A dihybrid is an individual that is heterozygous
at two genes
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Mendel designed experiments to determine if
two genes segregate independently of one
another in dihybrids
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First constructed true-breeding lines for both traits,
crossed them to produce dihybrid offspring, and
examined the F2 for parental or recombinant types (new
combinations not present in the parents).
Mendel and two genes
Round
Yellow
Wrinkled
Green
x
All F1 Round, Yellow
Round
Yellow
315
Round
Green
108
Wrinkled
Yellow
101
Wrinkled
Green
32
Dihybrid cross produces a
predictable ratio of phenotypes
genotype
Parent
• Recombinant
• Recombinant
• Parent
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phenotype
number phenotypic ratio
Y_R_
315
9/16
yyR_
108
3/16
Y_rr
101
3/16
yyrr
32
1/16
Ratio of yellow (dominant) to green (recessive)=3:1 (12:4)
Ratio of round (dominant) to wrinkled (recessive)=3:1 (12:4)
Ratio for a cross with 2 genes
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Crosses with two genes are called
dihybrid.
Dihybrid crosses have genetic ratios of
9:3:3:1.
Mendel and two genes
Round
Yellow
315
Round
Green
108
Wrinkled
Yellow
101
Round = 423
Wrinkled = 133
Each gene has a 3 : 1 ratio.
Wrinkled
Green
32
Yellow = 416
Green = 140
Summary of Mendel's work
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Inheritance is particulate - not blending
There are two copies of each trait in a germ cell
Gametes contain one copy of the trait
Alleles (different forms of the trait) segregate
randomly
Alleles are dominant or recessive - thus the
difference between genotype and phenotype
Different traits assort independently
Rules of Probability
Independent events - probability of two events occurring together
What is the probability that both A and B will occur?
Solution = determine probability of each and multiply
them together.
Mutually exclusive events - probability of one or another event
occurring.
What is the probability of A or B occurring?
Solution = determine the probability of each and add
them together.
PRODUCT RULE
From James Birchler
SUM
RULE
Mutually exclusive ways!
From James Birchler
From James Birchler
All Dominant
Dominant
Recessive
All Recessive
From James Birchler
Punnett Square method - 24 = 16 possible gamete
combinations for each parent
Thus, a 16  16 Punnett Square with 256 genotypes
That’s one big Punnett Square!
Loci (Genes) Assort Independently - So we can look at
each locus independently to get the answer.
Branch diagrams are also convenient tools