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Classical Genetics — Lecture I
• Dr. Steven J. Pittler
• VH375B
• Office 4-6744
• Cell 612-9720
Suggested Reading: Lewis 2nd
Edition
Chapter on Mendelian
Inheritance
4-1
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Inheritance
• Parents and offspring
often share observable
traits.
• Grandparents and
grandchildren may share
traits not seen in parents.
• Why do traits disappear
in one generation and
reappear in another?
4-2
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Gregor Mendel: The father of modern genetics
Combined
• plant breeding
• Statistics
• Careful recordkeeping
Described hypothesis of transmission of traits
now considered the laws of inheritance
4-3
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Mendel studied pea traits
with two distinct forms
4-4
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True breeding plants
Plants which
consistently have
offspring with same
trait as parent are true
breeding plants.
4-5
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Monohybrid cross
• What happens
when true breeding
plants with two
distinct forms of a
trait are crossed?
Progeny show only one form
of the trait.
The observed trait is called dominant.
The masked trait is called recessive.
4-6
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Test cross
Is a plant showing the dominant trait true-breeding or not?
Test by crossing with a plant showing the recessive trait.
4-7
All tall offspring indicate
parent is true-breeding
Mixed offspring indicate
parent is hybrid
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Crossing hybrids to each other
Hybrid parents show the
dominant trait (tall).
Offspring:
• Dominant trait (tall) and
true breeding (1/4 total)
• Dominant trait (tall) and
NOT true breeding (1/2 total)
• Recessive trait (short) and
always true breeding
(1/4 total)
Mendel concluded that among the hybrid parents the
short trait (recessive) was hidden but not absent
4-8
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Mendel’s data
1. Crossed true-breeding plants differing at one of seven traits.
2. Crossed hybrid offspring to each other (all show dominant trait).
3. Counted offspring of hybrid crosses.
Offspring
Seed form
7,324
5,474
1,850
2.96 : 1
Seed color
8,023
6,022
2,001
3.01 : 1
929
705
224
3.15 : 1
Pod form
1,181
882
299
2.95 : 1
Pod color
580
428
152
2.82 : 1
Flower position
858
651
207
3.14 : 1
1,064
787
277
2.84 : 1
Trait
Seed coat color
Stem length
4-9
Ratio
# with
dominant :
recessive trait recessive
# with
dominant
trait
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• If you know the genotype of the parents, it is
possible to determine the gametes and use
a Punnett square to determine the
phenotypic ratio among the offspring.
• When a monohybrid reproduces with a
monohybrid, the results are 3 : 1.
• This ratio is used to state the chances of a
particular phenotype.
• A 3 : 1 ratio means that there is a 75%
chance of the dominant phenotype and a
25% chance of the recessive phenotype.
4-10
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Punnett Squares
t
Tt
The genes from one parent go here.
The genes from the other parent go
here.
Tt
4-11
T
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Punnett Squares
t
T
T
Tt
Tt
F1 generation
t
4-12
Tt
Tt
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Gamete Formation (sperm and eggs)
• Because homologous pairs separate
during meiosis, a gamete has only one
allele from each pair of alleles.
• If the allelic pair is Tt, a gamete would
contain either a T or a t, but not both.
• Tt represents the genotype of an
individual.
• Gametes are represented by T or t.
4-13
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One-Trait Crosses and Probability
• Laws of probability alone can be used
to determine results of a cross.
• The laws are:
• (1) the probability that two or more
independent events will occur together
is the product of their chances
occurring separately, and
• (2) the chance that an event that can
occur in two or more independent ways
is the sum of the individual chances.
4-14
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• In the cross of Tt x Tt, what is the
chance of obtaining either a T or a t
from a parent?
• Chance of T = ½, or chance of t = ½
• The probability of these genotypes is:
• The chance of TT = ½ x ½ = ¼
• The chance of Tt = ½ x ½ = ¼
• The chance of tT = ½ x ½ = ¼
• The chance of tt = ½ x ½ = ¼
• The chance of tall plants (TT, Tt, tT) is
¼ + ¼ + ¼ = ¾ or 75%.
4-15
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Law of segregation
Why do traits “disappear” in one generation only
to reappear in a subsequent generation?
• Each plant has two distinct separable
units (alleles) for each trait inherited from
each parent. (sister chromatids that together make a chromosome)
• Gametes contain ONE allele for each trait.
• Only one version is observed in an individual.
The unit (allele) does not disappear.
It may be present but hidden.
4-16
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Gene locus (locus = location)
4-17
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Law of segregation
4-18
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Alleles
Mendel’s units (or “elementen”) are called alleles.
• versions of the same gene or DNA sequence.
• differ in DNA sequence at one or more sites.
4-19
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Genotype
indicates the combination of alleles present
•
Homozygous alleles are the same
•
Heterozygous alleles differ
Phenotype
indicates the trait observed.
Terms distinguish the observed form
and the underlying alleles present.
4-20
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Genotype and phenotype
Phenotype
Genotype
Tall plant
Homozygous dominant
“tall-associated” alleles
TT
Heterozygous
Tt
Homozygous recessive
“short-associated” alleles
tt
Short plant
4-21
Abbreviation
of genotype
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Modern terms for Mendel’s crosses
• Mendel’s true-breeding plants were
homozygous for the alleles of a trait.
• Mendel’s hybrid plants were
heterozygous for the alleles of a trait.
4-22
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Wildtype
most common version in the general population
• wildtype phenotype
•most common phenotype
• mutant phenotype
• wildtype allele
•phenotype different from the
wildtype
•most frequent allele
associated with the
common phenotype
• mutant allele
•allele associated with the
mutant phenotype.
4-23
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Law of segregation: the monohybrid cross
Two heterozygous parents produce gametes
with T or t allele equally frequently.
Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt
Offspring phenotypes
4-24
3/4 tall
: 1/4 short
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Mode of inheritance
indicates the patterns with which the mutant
phenotype is associated.
Autosomal recessive
Autosomal dominant
X-linked recessive
X-linked dominant
Y-linked (holandric)
mitochondrial
4-25
Most common
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Autosomal dominant inheritance
• Heterozygotes exhibit
the affected phenotype.
• Males and females are
equally affected and
may transmit the trait.
• Affected phenotype
does not skip
generations.
4-26
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Autosomal recessive inheritance
• Heterozygotes carry
the recessive allele
but exhibit the
wildtype phenotype.
• Males and females are
equally affected and
may transmit the trait.
• May skip generations.
4-27
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Comparison of autosomal dominant
and autosomal recessive inheritance
Autosomal Autosomal
dominant recessive
4-28
Males and females
affected?
Yes
Yes
Males and females
transmit the trait?
Yes
Yes
Trait skips generations?
No
Yes
At least one parent of
affected child must be
affected?
Yes
No
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Law of independent assortment
• Two genes on different chromosomes
segregate their alleles independently.
• The inheritance of an allele of one gene
does not influence which allele is
inherited at a second gene.
4-29
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Law of independent assortment
4-30
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Independent assortment of two traits
• In a dihybrid cross, parents with two
differing traits are crossed.
• Which allele is dominant?
Heterozygous peas are round and yellow.
Therefore
4-31
round is dominant to wrinkled
yellow is dominant to green
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Two traits segregating independently
4-32
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Pedigrees
symbolic representations of family relationships
and inheritance of a trait
4-33
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Autosomal dominant inheritance of brachydactyly
Heterozygotes exhibit the phenotype.
4-34
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Autosomal recessive inheritance of albinism
Heterozygotes carry the recessive allele
but exhibit the wildtype phenotype
4-35
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Genetic predictions
Ellen’s brother Michael has
sickle cell anemia, an
autosomal recessive
disease.
What is the chance that
Ellen’s child has a sickle cell
anemia allele (a)?
Ellen and Michael’s parents
must be carriers.
A
a
A
AA Aa
a
Aa
aa
Ellen is not affected and
cannot carry aa genotype
chance Ellen is a carrier = 2/3
chance child inherits sickle
cell allele = 1/2
Ellen Michael
4-36
?
Overall chance child carries
sickle cell allele from Ellen =
2/3 x 1/2 = 1/3