Transcript Les 3 Mendelian Genetics

Mendelian Genetics:
The Laws of
Inheritance
By Lisa Marie Meffert, PhD
Morpho plelides
Mark T. Hark, MD
BioEd Online
Blending Model of Inheritance vs.
Particulate Model of Inheritance
 Blending
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model:
Offspring are a blend of the parents.
The blend of characteristics is passed on to the next
generation.
Variation is washed out over time.
 Particulate
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model:
Offspring are a combination of the parents.
Parental contributions are passed to the next
generation as separate entities.
Variation is maintained over time.
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Some Terminology
Genotype: the combination of alleles that an organism
possesses in regard to a trait. Eg. Pp
 Genotypic ratio: the mathematical ratio of the allele
pairs.
 Phenotype: the outward appearance of the trait in the
organism (analogous to a photograph). Eg. Purple
flower
 Phenotypic ratio: the mathematical ratio of the
appearance of the traits.
Note: Some genes may have three or more alleles,
however, each individual can have a maximum of two
alleles per gene.
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More Terminology
P1 – Parental Generation
 F1 – The First Generation
 F2 – The Second Generation
 Pure Breeding Line
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Homozygous dominant (TT) or
Homozygous recessive (tt)
Hybrid
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Heterozygous (Tt)
Mendel’s Research on
Particulate Inheritance
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Made pure breeding (parental) lines for
different phenotypes of pea plants:
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Gregor Mendel
Crossed the pure lines to get F1 generation
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Plant height – tall (T) vs. short (t)
Pea color – yellow (Y) vs. green (y)
Pea texture – round (R) vs. wrinkled (r), etc.
All of the F1 individuals looked the same
Crossed F1 lines to get F2 generation
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¾ of the F2 individuals looked like one of the
parental lines
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¼ of the F2 individuals looked like the other
parental line
Pisum sativum
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Punnett Squares (Text p 260)
A bookkeeping tool devised to represent the
independent assortment and segregation of the
alleles.
 A chart that shows all the possible combinations
of the alleles that can result when two organisms
are crossed (bred).
 It allows geneticists to predict the probability of
occurrence of a particular trait.
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Terminology (cont’d)
Mendel’s Monohybrid Cross
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“Mono” means “one” and the parent plants differ from
each other by a single trait – height.
Dihybrid Cross
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“Di” means “two” and the parent plants differ from
each other by two different traits.
 In a dihybrid cross, will the two traits stay together
in the next generation or will they be inherited
independently of each other?
Working out Mendelian Genetics –
Monohybrid Cross
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The allele is the specific
form of hereditary particle
passed on through male or
female gametes.
Allele capital letter T confers
tallness. In this case, the tall
parental line has all T’s.
Allele lower case letter t
confers shortness. The short
parental line has all t’s.
Cross a tall male (top) with a
short female (side).
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Crossing the Parental Lines:
Male Contribution
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First, the male passes
on his alleles, one per
gamete (reproductive
cell).
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Crossing the Parental Lines:
Female Contribution
 Then,
the female
passes on her alleles,
one per egg.
 Crossing these two
parents yields all Tt
offspring.
 Since T is dominant to
t, all of the offspring
are tall pea plants.
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Crossing the F1 Generation
Crossing two homozygous
individuals, TT and tt, yields
all heterozygous offspring,
Tt. This generation is
referred to as F1.
 Now, cross two individuals
from the F1 generation
(Tt x Tt).
 First, the male pea plant
passes on its alleles, one
per gamete.
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Crossing the F1 Generation
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Now, the female passes on her alleles,
one per egg.
Genotypes of the F1
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Phenotypes of the F1
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¼ of the offspring are TT.
½ of the offspring are heterozygotes (one T
and one t).
¼ of the offspring are tt.
Genotypic ratio is 1:2:1
¾ are tall (at least one T).
¼ are short (homozygous
for t).
Phenotypic ratio is 3:1
Because there are four boxes, the ratios
become easy—these are expressed as
fractions &/or percentages.
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–
Dihybrid Cross
Keep T and t for tall
and short plants,
respectively.
 Add R and r for round
and wrinkled seeds,
respectively.
 A double heterozygote
male produces four
types of gametes.
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Cross with a Female Double
Heterozygote
 A double
heterozygote
female produces
four types of
eggs.
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Crossing the Double Heterozygotes
 The
male
passes
on his alleles,
two per gamete.
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Crossing the Double Heterozygotes
 The
female
passes on her
alleles, two per
egg.
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Double Heterozygote Cross:
Assessing the Resultant Genotypes
At least one T yields
tall stature.
 At least one R yield
round seeds.
 Thus, there are 9
ways (gray) to
produce tall, roundseeded offspring.
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Double Heterozygote Cross:
Assessing the Resultant Genotypes
Again, at least one T
yields tall stature.
 And one R is required
for round seeds, so rr
must produce wrinkled
seeds.
 Thus, there are 3 ways
(lighter gray) to produce
tall, wrinkly seeded
offspring
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The Third Phenotype
in a Double Heterozygote Cross
Two t alleles yield
short stature.
 At least one R
produces round seeds.
 Thus, there are 3 ways
(lightest gray) to
produce short
offspring with round
seeds.
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The Fourth Phenotype
in a Double Heterozygote Cross
Two t alleles yield
short stature.
 Two r alleles
produce wrinkled
seeds.
 There is only 1 way
to produce short
offspring with
wrinkled seeds.
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Assignment
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HEREDITY PROBLEM SET WORKSHEET
Gene diagram – Flower colour
parent
gamete
R
Offspring
genotype
Rr
phenotype
red
Male
female
Rr
rr
r
rr
r
rr
yellow
yellow
Red 50%
yellow 50%
r
Rr
red