Mendelian Genetics
1862
1868
1880
Genetic Terminology:
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Trait - any characteristic that can be passed from parent to
offspring
Heredity - passing of traits from parent to offspring
Genetics - study of heredity
Alleles - two forms of a gene (dominant & recessive)
Dominant - stronger of two genes expressed in the hybrid;
represented by a capital letter (R)
Recessive - gene that shows up less often in a cross; represented by a
lowercase letter (r)
Genotype - gene combination for a trait (e.g. RR, Rr, rr)
Phenotype - the physical feature resulting from a genotype (e.g. tall,
short)
Homozygous genotype - gene combination involving 2 dominant or 2
recessive genes (e.g. RR or Rr); also called pure
Heterozygous genotype - gene combination of one dominant & one
recessive allele (e.g. Rr); also called hybrid
Monohybrid cross - cross involving a single trait
Dihybrid cross - cross involving two traits
Punnett Square - used to solve genetics problems
Blending Concept of Inheritance:
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Accepted before Mendel's experiments
Theory stated that offspring would have traits intermediate between
those of its parents such as red & white flowers producing pink
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The appearance of red or white flowers again was consider
instability in genetic material
Blending theory was of no help to Charles Darwin's theory of
evolution
Blending theory did not account for variation and could not explain
species diversity
Particulate theory of Inheritance, proposed by Mendel, accounted
for variation in a population generation after generation
Mendel's work was unrecognized until 1900
Gregor Mendel:
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Austrian monk
Studied science & math at the University of Vienna
Formulated the laws of heredity in the early 1860's
Did a statistical study of traits in garden peas over an eight year
period
Why peas, Pisum sativum?
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Can be grown in a small area
Produce lots of offspring
Produce pure plants when allowed to self-pollinate several
generations
Can be artificially cross-pollinate
GARDEN PEA
Mendel's Experiments:
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Mendel studied simple traits from 22 varieties of pea plants (seed
color & shape, pod color & shape, etc.)
Mendel traced the inheritance of individual traits & kept careful
records of numbers of offspring
He used his math principles of probability to interpret results
Mendel studied pea traits, each of which had a dominant & a
recessive form (alleles)
The dominant (shows up most often) gene or allele is represented
with a capital letter, & the recessive gene with a lower case of that
same letter (e.g. B, b)
Mendel's traits included:
a. Seed shape --- Round (R) or Wrinkled (r)
b. Seed Color ---- Yellow (Y) or Green (y)
c. Pod Shape --- Smooth (S) or wrinkled (s)
d. Pod Color --- Green (G) or Yellow (g)
e. Seed Coat Color --- Gray (G) or White (g)
f. Flower position --- Axial (A) or Terminal (a)
g. Plant Height --- Tall (T) or Short (t)
h. Flower color --- Purple (P) or white (p)
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Mendel produced pure strains by allowing the plants to selfpollinate for several generations
These strains were called the Parental generation or P1 strain
Mendel cross-pollinated two strains and tracked each trait through
two
generations (e.g. TT x tt )
Trait - plant height
Alleles - T tall, t short
P1 cross TT x ttgenotype -- Tt
t
t
phenotype -- Tall
T Tt Tt
genotypic ratio --all alike
T Tt Tt
phenotypic ratio- all alike
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The offspring of this cross were all hybrids showing only the
dominant trait & were called the First Filial or F1 generation
Mendel then crossed two of his F1 plants and tracked their traits;
known as an F1 cross
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Trait - plant height
Alleles - T tall, t short
F1 cross Tt x
Tt
T
T TT
t Tt
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t
Tt
tt
genotype
-- TT, Tt, tt
phenotype -- Tall &
short
genotypic ratio --1:2:1
phenotypic ratio- 3:1
When 2 hybrids were crossed, 75% (3/4) of the offspring showed the
dominant trait & 25% (1/4) showed the recessive trait; always a 3:1
ratio
The offspring of this cross were called the F2 generation
Mendel then crossed a pure & a hybrid from his F2 generation;
known as an F2 or test cross
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F2 cross
T
T
T
TT
TT
Trait - Plant Height
Alleles - T tall, t short
TT x Tt
F2 cross
t
Tt
Tt
genotype - TT, Tt
phenotype - Tall
genotypic ratio - 1:1
phenotypic ratio - all
alike
t
t
T
Tt
Tt
tt x Tt
t
tt
tt
genotype - tt, Tt
phenotype - Tall &
short
genotypic ratio - 1:1
phenotypic ratio - 1:1
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50% (1/2) of the offspring in a test cross showed the same genotype
of one parent & the other 50% showed the genotype of the other
parent; always a 1:1 ratio
Problems: Work the P1, F1, and both F2 crosses for all of the other pea
plant traits & be sure to include genotypes, phenotypes, genotypic &
phenotypic ratios.
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Mendel also crossed plants that differed in two characteristics
(Dihybrid Crosses)
such as seed shape & seed color
In the P1 cross, RRYY x rryy, all of the F1 offspring showed only
the dominant form for both traits; all hybrids, RrYy
Traits:
Alleles:
P1 Cross:
Seed Shape & Seed Color
R round
Y yellow
r wrinkled
y green
RRYY
x
r r yy
ry
Genotype:
RrYy Phenotype:
RY
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RrYy
Round yellow
seed
Genotypic
ratio:
All alike
Phenotypic
ratio:
All Alike
When Mendel crossed 2 hybrid plants (F1 cross), he got the following
results
Traits:
Alleles:
Seed Shape & Seed Color
R round
Y yellow
r wrinkled
y green
F1 Cross:
RrYy
RrYy
x
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
r rYY
r rYy
RrYy
Rryy
r rYy
r ryy
RY
Ry
rY
ry
Genotypes
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
Genotypic
Phenotypic
Phenotypes
Ratios
Ratios
1
Round
2
yellow seed
9
2
4
1
Round
green seed
3
2
r rYY
1
r rYy
2
r ryy
1
Wrinkled
yellow seed
Wrinkled
green seed
3
1
Problems: Choose two other pea plant traits and work the P1 and F1
dihybrid crosses. Be sure to show the trait, alleles, genotypes, phenotypes,
and all ratios.
Results of Mendel's Experiments:
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Inheritable factors or genes are responsible for all heritable
characteristics
Phenotype is based on Genotype
Each trait is based on two genes, one from the mother and the other
from the father
True-breeding individuals are homozygous ( both alleles) are the
same
Law of Dominance states that when different alleles for a
characteristic are inherited (heterozygous), the trait of only one (the
dominant one) will be expressed. The recessive trait's phenotype
only appears in true-breeding (homozygous) individuals
Trait: Pod Color
Genotypes:
Phenotype:
Green Pod
GG
Green Pod
Gg
Yellow Pod
gg
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Law of Segregation states that each genetic trait is produced by a
pair of alleles which separate (segregate) during reproduction
Rr
R
r
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Law of Independent Assortment states that each factor (gene) is
distributed (assorted) randomly and independently of one another in
the formation of gametes
RrYy
RY Ry rY ry
Other Patterns of Inheritance:
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Incomplete dominance occurs in the heterozygous or hybrid
genotype where the 2 alleles blend to give a different phenotype
Flower color in snapdragons shows incomplete dominance whenever
a red flower is crossed with a white flower to produce pink flowers
In some populations, multiple alleles (3 or more) may determine a
trait such as in ABO Blood type
Alleles A & B are dominant, while O is recessive
Genotype Phenotype
IOIO
Type O
IAIO
Type A
IAIA
Type A
IB IO
Type B
IBIB
Type B
IAIB
Type AB
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Polygenic inheritance occurs whenever many variations in the
resulting phenotypes such as in hair, skin, & eye color
The expression of a gene is also influenced by environmental factors
(example: seasonal change in fur color)